`bmp_file_header` fields are incorrectly set on bmp export

[Arecher]

Aseprite and System version

• Aseprite 1.3-beta21-x64
• Windows 10

Thought I would make this a separate bug report. Contributors at the Godot Engine project have looked into the bmp files not properly importing, and found that something about the bmps is in fact incorrecly set.

The fields bmp_file_header.bmp_file_size and bmp_info_header.bmp_size_image are incorrectly set

Here is the specific post, seems like something that might need investigating further:
godotengine/godot#66238 (comment)

[Gasparoken]

Thanks again. I'll review it ASAP.

[Gasparoken]

I made a patch for this and saved test.bmp. Could you try this example on your Godot engine?
test.bmp.zip

The bad calculation of bmp image size (biSizeImage) is done here:

aseprite/src/app/file/bmp_format.cpp

Lines 1215 to 1217 in ae889d4

	else {
	biSizeImage = (w*3 + filler) * h;
	if (withAlpha)

biSizeImage was calculated based on the old 24 bits (3 bytes). But now, if the BMP is inferred to have alpha content, the file will be 32 bpp (4 bytes per pixel). I forgot to change it at bdcabde

It'll be fixed in the next release.

[Arecher]

Yes, that test .bmp imports correctly into Godot!

[Gasparoken]

Thank you for your feedback! Perhaps the new feature in #3532 is no longer needed. We'll review it next week.

[Arecher]

This fix would resolve all my problems, so the new feature wouldn't be required anymore for my specific use-case. Unless there are apps that don't handle specific bmp files well, I think it could be closed. (After all, it can always be reopened if other users run into these apps!)

And thank you for all the effort you put in! ❤️