[mapguide-trac] #1518: PNG8 color quantization uses too few colors resulting in changing colors in small pans

[MapGuide Open Source]

#1518: PNG8 color quantization uses too few colors resulting in changing colors in
small pans
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   Reporter:  gabrimonfa         |       Owner:       
       Type:  enhancement        |      Status:  new  
   Priority:  low                |   Milestone:       
  Component:  Rendering Service  |     Version:  2.2.0
   Severity:  trivial            |    Keywords:       
External_id:                     |  
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 Mapguide 2.2RC on centos5

 PNG8 visual quality is much more appealing than JPEG especially in text
 and thin lines. However actually it has one problem that may prevent its
 adoption.

 Even doing very small pans objects may change sensibly their colors, which
 is visually disturbing

 Have a look to the right upper corner in
   * map_png8_1.png
   * map_png8_2.png
 or at the darker pink polygons in the lower part of the map.

 I was wondering if this is inevitable due to the color quantization or
 not, thus I've made some tests.

 First of all, look at the PNG image corresponding to the first image
   * map_png_1.png
 Looking at the number of its colors (map_png_1_info.png) it has 16238
 different colors.

 Obviously map_png8_1.png is limited to 256 colors, but the first surprise
 is that the PNG8 version has only 135 colors (map_png8_1_info.png).

 '''First question''': why the algorithm of color quantization uses so few
 colors?

 Then I try to reduce the number of colors in the PNG version transforming
 it in a palletted image
   * map_png8_1_gimp.png (and map_png8_1_gimp_info.png)
 obtaining an image with 256 colors, which colors are much more like the
 original one w.r.t. to Mapguide produced one.

 I then force GIMP to use only 135 colors, in order to see if it is
 possible to have better results also with fewer colors
   * map_png8_1_gimp_135.png (and map_png8_1_gimp_135_info.png)

 The answer is yes, the image is a lot better that that obtained by
 Mapguide

 '''Second question''': is it possible to improve the quality of the color
 quantization?

 Even if each pan does an independent color quantization, if the obtained
 image is very similar to the PNG original one, probably the images
 obtained with small pans also look similar between them.
 And the problem would disappear.

 '''Third question''': is it possible to preserve the stylization colors of
 the objects in the map if the number of colors involved is less than 256?

 I've seen that this has been addressed for base layer in
 http://trac.osgeo.org/mapguide/wiki/MapGuideRfc60.
 Would be possible to adopt a similar solution for all the layers?

 I mean, probably it is more wise to preserve these colors and changing a
 bit the colors used for smoothing around lines.
 Even with a lot of transparencies, as in my examples, we're probably under
 256 "stylization colors".
 This claims is confirmed by the fact that even with only 135 colors GIMP
 is able to produce an image which at first sight is quite similar to the
 original one

-- 
Ticket URL: <http://trac.osgeo.org/mapguide/ticket/1518>
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