Andre Majorel on wrote...
| On 2007-12-23 22:42 +1000, Anthony Thyssen wrote:
| > Andre Majorel on wrote...
| > | Hello all. I have large 600 DPI colour scans that I would like to
| > | convert to compact PNG. To let PNG do its work, the number of
| > | colours needs to be reduced.
| > |=20
| > | The document lends itself to quantisation very well ; it is
| > | typeset text with some orange highlighter so the scan contains
| > | only black, white and shades of fluorescent orange and grey. It
| > | could be adequately rendered with just, say, 8 colours : black,
| > | white, 3 greys and 3 shades of orange.
| > |=20
| > | Unfortunately, convert's native quantisation does not work that
| > | well with it. I end up with either too many shades of light grey
| > | for good compression or light brown instead of orange.
| > |=20
| > | I would like the quantiser to pick something along the lines of
| > | #000 #444 #888 #ccc #FFF for the greys and #FF9840 #804c20 #402610
| > | for the oranges.
| > |=20
| > | Some scaling would be required, as the thresholds for logical
| > | black and white are maybe #333 and #ddd in the original.
| > | Normalisation cannot be used because many pixels have at least one
| > | component set to 0xFF.
| > |=20
| > | How would you approach this ?
| >
| > I would specify the colors you want in a small image with just thouse
| > colors, than use -map. That is avoid the color quantization entirely.
|
| Thanks but that doesn't work too well. The original image is noisy
| and it's not obvious how to choose shades that are distant enough
| to avoid the dithering but not so distant that pixels end up
| remapped to the wrong hue.
|
| And the scaling problem is still there.
|
You can just turn off dithering using +dither.
In that case -map will just set colors to the closest color found in
the given color map image.
See IM Examples Quantization and Dithering.
Anthony Thyssen ( System Programmer ) <[EMAIL PROTECTED]>
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