# 3091 backend: Brightness, Contrast etc.

From: Frederik Ramm (frederik@remote.org)
Date: Wed Mar 07 2001 - 13:18:05 PST

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  Hi,

  the scanner I'm writing a backend for does not support the setting
  of the usual image-altering values like contrast, brightness, gamme
  correction etc.

  Instead, you can load a so-called "LUT" translation table into the
  scanner. It's basically a mapping that assigns an output value to
  every possible input value. I.e. if I wanted a much brighter image, I
  would just download a table that maps

  0 -> 50
  1 -> 51
  ...
  204 -> 255
  205 -> 255
  206 -> 255
  ...
  255 -> 255

  or something similar. Since I don't have more than 8 bits on the input
  side, I cannot have the scanner do any translations that could not
  also be done afterwards (in fact, I could do in within the backend if
  I wanted to).

  So, should I bother to "emulate" a brightness/contrast/gamma setting
  using these LUT tables? And if I should, can anyone point me to useful
  formulas that generate such translation tables for certain values of
  contrast, brightness etc.?

  What's more, my scanner supports a few settings with funny names I
  cannot make sense of. Maybe somebody knows what they are, or what they
  might be. It would be interesting for me if it is worthwile to make
  them available through the backend.

  * Initial Slice: Specifies the initial binary threshold value when in
  floating slice mode.
  * Up value: Specifies the +UP count value used in floating slice mode.
  (Valid settings: 0-3, default: 1)
  * Down value: ... as above
  * Lower Limit Slice: Specifies the lower limit slice used in floating
  slice mode (0-15, default: 6)
  * Compensation line Interval: Specifies the compensation line interval
  slice when in floating slice mode (0-15, default 0)
  * Error dispersion upper limit slice (0-255, default 230)
  * Error dispersion lower limit slice (0-255, default 20)
  * Laplacian coefficient for enhancement mode: 0 (weak), 1, 2 or 3
  (strong)
  * Gradient coefficient for enhancement mode: as above
  * Laplacian slice: Specifies which Laplacian slice is to be used for the
  enhancement mode (0-15)
  * Gradient slice: as above for Gradient slice
  * Primary Scan Ratio Compensation: on or off.

  If anyone has the slightest idea...

  Thanks
  Frederik

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