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HTML
456 wiersze
22 KiB
HTML
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<a href="doc019.html"><img src=../icons/index.gif alt="Index"></a>
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<hr>
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<title>Well-Known Options</title>
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<h2><a name="s4.5">4.5 Well-Known Options</a></h2><a name="i114">
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<p>While most backend options are completely self-describing, there are
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cases where a user interface might want to special-case the handling
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of certain options. For example, the scan area is typically defined
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by four options that specify the top-left and bottom-right corners of
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the area. With a graphical user interface, it would be tedious to
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force the user to type in these four numbers. Instead, most such
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interfaces will want to present to the user a preview (low-resolution
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scan of the full scanner surface or a high(er) resolution scan of a subpart
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of the scanner surface) and let the user pick the scan area by
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dragging a rectangle into the desired position. For this reason, the
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SANE API specifies a small number of option names that have
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well-defined meanings.
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<p><h3><a name="s4.5.1">4.5.1 Option Number Count</a></h3><a name="i115">
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<p>Option number 0 has an empty string as its name. The value of this
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option is of type <tt>SANE_TYPE_INT</tt> and it specifies the total
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number of options available for a given device (the count includes
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option number 0). This means that there are two ways of counting the
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number of options available: a frontend can either cycle through all
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option numbers starting at one until
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<tt>sane_get_option_descriptor()</tt> returns <tt>NULL</tt>, or a
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frontend can directly read out the value of option number 0.
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<p><h3><a name="s4.5.2">4.5.2 Scan Resolution Options</a></h3><a name="i116"><a name="i117">
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<p><font color="darkgreen">
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Option <tt>resolution</tt> is used to select the resolution at which an
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image should be acquired. When the backend wants to allow different
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values for x- and y-resolution it has to define the options
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<tt>x_resolution</tt> and <tt>y_resolution</tt>. Note that only
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the option <tt>resolution</tt> <b>or</b> the options
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<tt>x_resolution</tt> <b>and</b> <tt>y_resolution</tt> may be active.
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<p>The type of this option is either <tt>SANE_TYPE_INT</tt> or
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<tt>SANE_TYPE_FIXED</tt>. The unit is <tt>SANE_UNIT_DPI</tt> (dots/inch).
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<p>This option is not mandatory, but if a backend does support it, it
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must implement it in a manner consistent with the above definition.
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</font>
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<p><h3><a name="s4.5.3">4.5.3 Preview Mode Option</a></h3><a name="i118">
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<p>The boolean option <tt>preview</tt> is used by a frontend to inform the
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backend when image acquisition should be optimized for speed, rather
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than quality (``preview mode''). When set to <tt>SANE_TRUE</tt>,
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preview mode is in effect, when set to <tt>SANE_FALSE</tt> image
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acquisition should proceed in normal quality mode. The setting of
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this option <em>must not</em> affect any other option. That is, as
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far as the other options are concerned, the preview mode is completely
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side effect free. A backend can assume that the frontend will take
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care of appropriately setting the scan resolution for preview mode
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(through option <tt>resolution</tt>). A backend is free to override the
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<tt>resolution</tt> value with its own choice for preview mode, but it
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is advised to leave this choice to the frontend wherever possible.
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<font color="darkgreen">
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When the <tt>preview</tt> option is set the backend should transfer
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the image in frame type <tt>SANE_FRAME_RAW</tt> if possible.
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</font>
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<p>This option is not mandatory, but if a backend does support it, it
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must implement it in a manner consistent with the above definition.
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<p><h3><a name="s4.5.4">4.5.4 Scan Area Options</a></h3><a name="i119">
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<p>The four most important well-known options are the ones that define
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the scan area. The scan area is defined by two points (x/y coordinate
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pairs) that specify the top-left and the bottom-right corners. This
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is illustrated in Figure <a href="doc014.html#f5">5</a>. Note that the origin of the
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coordinate system is at the top-left corner of the scan surface as
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seen by the sensor (which typically is a mirror image of the scan
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surface seen by the user). For this reason, the top-left corner is
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the corner for which the abscissa and ordinate values are
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simultaneously the <em>smallest</em> and the bottom-right corner is the
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corner for which the abscissa and ordinate values are simulatenously
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the <em>largest</em>. If this coordinate system is not natural for a
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given device, it is the job of the backend to perform the necessary
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conversions.
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<p><a name="f5"></a>
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<center>
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<img width=330 height=306 src="img004.gif">
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<p><center>Figure 5: Scan area options</center>
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</center>
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<p>
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<p>The names of the four options that define the scan area are given in
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the table below:
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<center>
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<table cellpadding=0 cellspacing=0>
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<tr valign=top>
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<td colspan=1 align=left nowrap>
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<b>Name</b> </td>
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<td colspan=1 align=left nowrap> <b>Description</b> </td></tr>
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<tr valign=top>
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<td colspan=1 align=left nowrap>
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<tt>tl-x<a name="i120"></tt> </td>
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<td colspan=1 align=left nowrap> Top-left x coordinate value </td></tr>
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<tr valign=top>
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<td colspan=1 align=left nowrap>
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<tt>tl-y<a name="i121"></tt> </td>
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<td colspan=1 align=left nowrap> Top-left y coordinate value </td></tr>
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<tr valign=top>
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<td colspan=1 align=left nowrap>
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<tt>br-x<a name="i122"></tt> </td>
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<td colspan=1 align=left nowrap> Bottom-right x coordinate value </td></tr>
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<tr valign=top>
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<td colspan=1 align=left nowrap>
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<tt>br-y<a name="i123"></tt> </td>
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<td colspan=1 align=left nowrap> Bottom-right y coordinate value </td></tr>
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<tr valign=top>
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<td colspan=1 align=left nowrap>
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</td></tr>
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</table>
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</center>
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There are several rules that should be followed by front and backends
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regarding these options:
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<ul>
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<p><li>Backends must attach a unit of either pixels
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(<tt>SANE_UNIT_PIXEL</tt>) or millimeters (<tt>SANE_UNIT_MM</tt>) to
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these options. The unit of all four options must be identical.
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<p><li>Whenever meaningful, a backend should attach a range or a
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word-list constraint to these options.
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<p><li>A frontend can determine the size of the scan surface by first
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checking that the options have range constraints associated. If a
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range or word-list constraints exist, the frontend can take the
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minimum and maximum values of one of the x and y option
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range-constraints to determine the scan surface size.
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<p><li>A frontend must work properly with any or all of these options
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missing.
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<font color="darkgreen">
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<li>A frontend may temporarily set the values in a way that
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<tt>tl-x</tt> is larger than <tt>br-x</tt> and <tt>tl-y</tt> is larger than
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<tt>br-y</tt>.
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</font>
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<p></ul>
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<p><h3><a name="s4.5.5">4.5.5 Depth option</a></h3><a name="i124">
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<font color="darkgreen">
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Option <tt>depth</tt> is used to select the image depth in bits/sample
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in multi bit mode - (this means for 24 bit RGB mode this value must be 8).
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The type of this option is <tt>SANE_TYPE_INT</tt>.
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The unit is <tt>SANE_UNIT_BIT</tt>. For 1 bit modes
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(Lineart or Halftone) this option has to be inactive.
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For selection of 1 bit modes (Lineart or Halftone) the
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backend should use the well-known option <tt>mode</tt>.<br>
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This option is not mandatory, but if a backend does support it, it
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must implement it in a manner consistent with the above definition.<br>
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</font>
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<h3><a name="s4.5.6">4.5.6 Gamma table options</a></h3><a name="i125">
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<font color="darkgreen">
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The <tt>gamma-table</tt> option defines a <tt>SANE_CONSTRAINT_RANGE</tt>
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of the type <tt>SANE_TYPE_INT</tt> which represents the gamma correction
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table for gray. In color mode the <tt>gamma-table</tt> may be used to set
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a common gamma correction for red, green and blue. The options
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<tt>red-gamma-table</tt>, <tt>green-gamma-table</tt> and <tt>blue-gamma-table</tt>
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are used in color mode to set a gamma correction for each color
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separately. In color mode the backend is free to use only the
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<tt>gamma-table</tt> option, only the <tt>red-</tt>, <tt>green-</tt> and
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<tt>blue-gamma-table</tt> or all four options. When all four options
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are used then the color tables should do a gamma correction with
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the same input and output bit depth and the gray gamma table should
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reduce (if necessary) the bit depth from the scanner internal
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bit depth to the output bit depth. This should e.g. look like this:
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<pre>
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red_value = gamma-table(red-gamma-table(value))
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green_value = gamma-table(green-gamma-table(value))
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blue_value = gamma-table(blue-gamma-table(value))
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</pre>
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<p>The backend should not use the gamma tables to emulate other functions or options
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like highlight, shadow, contrast, brightness, threshold, analog_gamma.
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These functions are common for all backends and should be added to the frontend
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or a meta-backend.<br>
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This option is not mandatory, but if a backend does support it, it
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must implement it in a manner consistent with the above definition.<br>
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</font>
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<h3><a name="s4.5.7">4.5.7 Scan Mode options</a></h3><a name="i126">
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<font color="darkgreen">
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The option <tt>mode</tt> defines a <tt>SANE_CONSTRAINT_STRING_LIST</tt>
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of type <tt>SANE_TYPE_STRING</tt>.
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It is used to select the scanmode (e.g. Color or Gray).
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Well known modes are: <tt>Color</tt>, <tt>Gray</tt>, <tt>Halftone</tt>
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and <tt>Lineart</tt>. <tt>Color</tt> and <tt>Gray</tt> are multi bit
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modes (8 or 16 bits/sample), <tt>Halftone</tt> and <tt>Lineart</tt>
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are single bit modes.<br>
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This way a frontend can select e.g the mode <tt>Gray</tt>
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for scanning a fax.<br>
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This option is not mandatory, but if a backend does support it, it
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must implement it in a manner consistent with the above definition.<br>
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</font>
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<h3><a name="s4.5.8">4.5.8 Scan Source options</a></h3><a name="i127">
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<font color="darkgreen">
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The option <tt>source</tt> is used to select the scan source
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(e.g. Automatic Document Feeder).
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It defines a <tt>SANE_CONSTRAINT_STRING_LIST</tt>
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of type <tt>SANE_TYPE_STRING</tt>.
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Well known sources are: <tt>Flatbed</tt>, <tt>Transparancy Adapter</tt> and
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<tt>Automatic Document Feeder</tt>.<br>
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This option is not mandatory, but if a backend does support it, it
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must implement it in a manner consistent with the above definition.<br>
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</font>
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<p><h3><a name="s4.5.9">4.5.9 Threshold</a></h3><a name="i128">
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<font color="darkgreen">
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The option <tt>threshold</tt> is used to define the threshold
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for Lineart and maybe Halftone mode. In multi bit modes
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this option should be set inactive.
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The type of this option is <tt>SANE_TYPE_FIXED</tt>.
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The unit is <tt>SANE_UNIT_PERCENT</tt>. The value range
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should be 0.0...100.0 if possible.
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It defines the minimum intensity to get a white point / full intensity
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(image data bit = 0). The backend has to
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scale the values in the following way:<br>
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A value of 0.0 means all pixels get white /full intensity (all image data
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bits are 0). A value of 50.0 means intensities brighter than medium gray
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get white / full intensity (bit 0). A value of 100.0 means all pixels get
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black (all image data bits are 1). If the scanner is not able to
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cover the full range the backend has to define a reduced
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value range (e.g. 30...70 percent).
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<p>This option is not mandatory, but if a backend does support it, it
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must implement it in a manner consistent with the above definition.<br>
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</font>
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<p><h3><a name="s4.5.10">4.5.10 Analog gamma</a></h3><a name="i129">
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<font color="darkgreen">
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The option <tt>analog-gamma</tt> is used to define the gamma value
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for an analog gamma function of the scanner in multi bit modes.
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In 1 bit modes this option should be set inactive.
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The type of this option is <tt>SANE_TYPE_FIXED</tt>.
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The unit is <tt>SANE_UNIT_NONE</tt>. The value range
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can be defined by the backend as supported. The values
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have to be positive. A gamma value of 1.0 means that
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the gamma correction has no effect. A value larger than
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1.0 increases the brightness of the image.
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In color mode there also can be options <tt>analog-gamma-red</tt>,
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<tt>analog-gamma-green</tt> and <tt>analog-gamma-blue</tt>.
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It is not allowed to emulate an analog gamma function by
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a digital gamma table. The backend has to disable (or not
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define) this option when the scanner does not support an
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analog (hardware) gamma function.
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<p>When analog gamma, highlight and shadow functions are available
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at the same time then the backend author has to care about the order
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in which the functions are implemented in the scanner hardware.
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The SANE standard expects that changing the analog gamma value
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has no effect on the shadow and highlight function. When the
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analog gamma function is executed in the scanner hardware
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before the shadow and highlight functions then the backend
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should do a compensation. For this the shadow and highlight
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values have to be gamma corrected with the relevant analog gamma value.
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<p>This option is not mandatory, but if a backend does support it, it
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must implement it in a manner consistent with the above definition.<br>
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</font>
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<p><h3><a name="s4.5.11">4.5.11 Shadow</a></h3><a name="i130">
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<font color="darkgreen">
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The option <tt>shadow</tt> is used to define the shadow level / black point level.
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The type of this option is <tt>SANE_TYPE_FIXED</tt>.
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The unit is <tt>SANE_UNIT_PERCENT</tt>. The value range
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should be 0.0...100.0 if possible.
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It is used to define the maximum intensity level that creates an image data value
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of 0 (black). The backend has to scale the values in the following way:<br>
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A value of 0.0 means that the sensitivity range is not reduced, only the
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minimum intensity produces an image data value of 0 (black).
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A value of 50.0 means that that a medium intensity and everything that is darker
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produces an image data value of 0 (black).
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A value of 100.0 means the sensitivity range is reduced to 0, all image
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data values are 0 (black). If the scanner is not able to
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cover the full range the backend has to define a reduced
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value range (e.g. 30...70 percent).
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In color mode there can be options <tt>shadow-red</tt>, <tt>shadow-green</tt>
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and <tt>shadow-blue</tt>, in this case the <tt>shadow</tt> function has to be disabled.
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It is not allowed to emulate a shadow function by
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a digital gamma table. The backend has to disable (or not
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define) this option when the scanner does not support an
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analog (hardware) shadow function.
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<p>This option is not mandatory, but if a backend does support it, it
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must implement it in a manner consistent with the above definition.<br>
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</font>
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<p><h3><a name="s4.5.12">4.5.12 Highlight</a></h3><a name="i131">
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<font color="darkgreen">
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The option <tt>highlight</tt> is used to define the highlight level / white point level.
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The type of this option is <tt>SANE_TYPE_FIXED</tt>.
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The unit is <tt>SANE_UNIT_PERCENT</tt>. The value range
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should be 0.0...100.0 if possible.
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It is used to define the minimum intensity level that creates the maximum possible
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image data value (white/full intensity). The backend has to scale the values in the
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following way:<br>
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A value of 0.0 means the sensitivity range is reduced to 0, all image
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data have maximum value (white / full intensity).
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A value of 50.0 means that a medium intensity and everything that is brighter
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produces the maximum possible image data value (white / full intensity).
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A value of 100.0 means that the sensitivity range is not reduced, only the
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maximum intensity produces an image data with maximum possible value (white / full intensity).
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If the scanner is not able to cover the full range the backend has to define a reduced
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value range (e.g. 30...70 percent).
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In color mode there can be options <tt>highlight-red</tt>, <tt>highlight-green</tt>
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and <tt>highlight-blue</tt>, in this case <tt>highlight</tt> has to be disabled.
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It is not allowed to emulate a highlight function by
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a digital gamma table. The backend has to disable (or not
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define) this option when the scanner does not support an
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analog (hardware) highlight function.
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<p>This option is not mandatory, but if a backend does support it, it
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must implement it in a manner consistent with the above definition.<br>
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</font>
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<p><h3><a name="s4.5.13">4.5.13 Turn lamp on and off</a></h3><a name="i132"><a name="i133">
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<font color="darkgreen">
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The option <tt>lamp-on</tt> is used to turn the lamp of the scanner on.
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The type of this option is <tt>SANE_TYPE_BUTTON</tt>.
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The unit is <tt>SANE_UNIT_NONE</tt>. When the option is set
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then the lamp of the scanner is turned on.
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<p>The option <tt>lamp-off</tt> is used to turn the lamp of the scanner off.
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The type of this option is <tt>SANE_TYPE_BUTTON</tt>.
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The unit is <tt>SANE_UNIT_NONE</tt>. When the option is set
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then the lamp of the scanner is turned off.
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<p>These options are not mandatory, but if a backend does support them, it
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must implement them in a manner consistent with the above definition.<br>
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</font>
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<p><h3><a name="s4.5.14">4.5.14 Scanner buttons</a></h3><a name="i134">
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<font color="darkgreen">
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Some scanners have buttons which state can be read by the scanner driver.
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It is necessary to implement a locking function for the buttons
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because it is possible that several frontends try to connect to the
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same backend/scanner at the same time. Imagine what could happen
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when no locking would be implemented:
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Five people have started a scanning application which is connected
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via network to the scanner you want to use. You start a frontend,
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put a paper to the scanner and press the scan-button on the scanner.
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The scanner does scan three times (because three frontends asked the
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button status when you pressed the button). For three people the
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image is saved to the harddisk, but it is not sure that your
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frontend did scan the image.<br>
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<p>A backend that does make available the scanner-buttons has to
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implement the following options:<br>
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<tt>scanner-buttons-lock</tt> is of type <tt>SANE_TYPE_BOOL</tt>, default = <tt>SANE_FALSE</tt><br>
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<tt>scanner-buttons-status</tt> is of type <tt>SANE_TYPE_INT</tt>, default = 0<br>
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<tt>scanner-buttons-status-update</tt> is of type <tt>SANE_TYPE_BUTTON</tt><br>
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When setting these options the backend does not set <tt>SANE_INFO_RELOAD_OPTIONS</tt>
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or <tt>SANE_INFO_RELOAD_PARAMS</tt> if not explictly defined.
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<p>A frontend has to disable the usage of the scanner-buttons by default. This is important
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because other frontends will not be able to use the buttons when the button-functions are locked.
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Another important thing is that some scanners do not turn off their lamp when the driver
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does frequently talk to the scanner (what is done when reading the button status from the scanner).
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<p><ul>
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<p><li>
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A frontend that wants to read the button status has to lock the
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button functions at first. For this it does set the option
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<tt>scanner-buttons-lock</tt> to <tt>SANE_TRUE</tt>.
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While setting the value of option <tt>scanner-buttons-lock</tt> to <tt>SANE_TRUE</tt>
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the backend does check if a lockfile (e.g. "backend"-buttons.lock) does exist.
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The lockfile has to be placed in a directory where every user has read and write access to.
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<p> <ul>
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<li>
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If the lockfile does not exist then the backend creates the lockfile and writes the
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process ID (PID) of the backend to the file. Button access is allowed: the value
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of option <tt>scanner-buttons-lock</tt> is set to <tt>SANE_TRUE</tt>
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<li>
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If the file does exist and the backend PID is not the file PID then the
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backend has to check if the process with the PID stored in the lockfile
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still is running. If yes then the button access is not allowed: the value of
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option <tt>scanner-buttons-lock</tt> is set to <tt>SANE_FALSE</tt>. If not then the lockfile
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is recreated and the PID of the backend is stored in the lockfile, button
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access is allowed: the value of option <tt>scanner-buttons-lock</tt> is set to <tt>SANE_TRUE</tt>
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</ul>
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<p><li>
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The frontend does read the value of option <tt>scanner-buttons-lock</tt>. If
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it is <tt>SANE_TRUE</tt> then the frontend has access to the scanner buttons.
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If it is <tt>SANE_FALSE</tt> then access has been denied.
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<p><li>
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If the button access is allowed the frontend has to do the following about
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once per second (while not scanning):
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<ul>
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<li>
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The frontend does set option <tt>scanner-buttons-status-update</tt>.
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The backend checks if access to the buttons is allowed by comparing
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the backend PID with the lockfile PID. If access is allowed it
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does read the button status from the scanner and stores it in
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the option <tt>scanner-buttons-status</tt>, each bit represents a button, a
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value of 0 means the button is not pressed, a value of 1 means that the button
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|
is pressed. When the scanner is busy the backend must not wait, it has to
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return immedeatly and the button state keeps unchanged.
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The backend has to implement a timeout function. When no button has been pressed
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within a predefined time (e.g. 15 minutes) then the access permission is lost.
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In this case the backend does set option <tt>scanner-buttons-lock</tt> to <tt>SANE_FALSE</tt>
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and does set <tt>SANE_INFO_RELOAD_OPTIONS</tt> to inform the frontend that it has
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lost permission to access the scanner-button functions.
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If access is not allowed it does set the <tt>scanner-buttons-status</tt> to 0.
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<p> <li>
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The frontend does read the value of option <tt>scanner-buttons-status</tt>
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</ul>
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<p><li>
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When the frontend does exit or it does not want to use the buttons
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it does set option <tt>scanner-buttons-lock</tt> to <tt>SANE_FALSE</tt>.
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The backend does check if the backend PID and the lockfile PID are
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the same. If this is true then it removes the lockfile and sets the value
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of <tt>scanner-buttons-lock</tt> to <tt>SANE_FALSE</tt>.
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<p><tt>sane_close()</tt> should do the same as setting option
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<tt>scanner-buttons-lock</tt> to <tt>SANE_FALSE</tt>.
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<p></ul>
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