In developing a plugin I met a problem with the toolbar icon. I followed the instructions in the help file and for 256-color icons I decided to make RGB(1, 1, 1) a transparent color. So, I designed an icon with 4 pixels in corners with this color (to make it look a bit rounded), and in the code I wrote:

However, when I try the plugin in 256-color mode, its toolbar icon becomes garbled. Here is the screenshot of how the icon looks as itself and when shown by EmEditor:



EE 9.07; OS Vista Business SP2 x32 working in true-color mode.


<EDITED>
I performed some more experiments and found the following:
1. If I return CLR_NONE instead of RGB(...), the color with index 0 from the image's index table is used for transparency.
2. The problem seems to lie in fact that my transparent color has index 0. When I move it further in the index table, the problem seems to be gone.

The questions are: what are the "official" recommendations on creating transparent images, and why is all this not described in the help?

I think the issue is the color palette with 256-color mode. Even if your plug-in bitmap looks fine by itself, it becomes ugly with other plug-ins because the color palette must be adjusted with other bitmaps. For this reason, I would recommend you not adding 256-color bitmaps. Just use true-color bitmaps and 16-color bitmaps, and omit 256-color bitmaps. In most of my standard plug-ins, I don't supply 256-color bitmaps any more for this reason. Most PCs now display true color, and 256-color mode is becoming obsolete. The 256-color mode feature might be taken out in the future.

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Yutaka Emura
Developer of EmEditor
http://www.emeditor.com/

I see, I'll keep that in mind.

But even if the problem is in adjusting color palette, why simple repositioning of the transparent color fixes the problem completely? I don't change the color itself, I don't change colors of other pixels, I just move one color in the index table, and that's all.

PS: Since I found how to eliminate the problem on the plugin's side, it no longer bothers me as itself, but I'm concerned about possible bug in EmEditor which caused such strange behaviour.