Flesh out specs for class_type and class_type=

[Le1gh]

[mockdeep]

@dlemstra we're trying to understand what happens when changing the image class_type. On ImageMagick 7, it actually seems to compress the image some, but on ImageMagick 6 it does not? Does that sound right? Can you explain the difference?

[dlemstra]

I could check this for you but it could be possible that when we notice that the image only has a fully opaque alpha channel we remove that channel and then reduce the channels from 4 (RGBA) to 3 (RGB) when changing between class_type.

[mockdeep]

@dlemstra thanks for the response. Would that result in the pixel color values changing? For some reason when we get the actual pixel colors they have changed. Anecdotally, it seemed to be reducing the number of colors. Correction to my previous statement, this behavior seems to have changed from IM 6.8 to 6.9.

[dlemstra]

It turns out that Image_class_type_eq is calling QuantizeImage and that will dither the image by default. What happens when qinfo.dither_method = NoDitherMethod is added? This is the settings that are being used in my Magick.Native library:

number_colors = MaxColormapSize;
tree_depth = 8; // MaxTreeDepth;
dither_method = NoDitherMethod;