PNG Export Bit Depth

[ssoher]

Feature/enhancement description:

Currently the exported PNG file bit depth is always 32 bits, RGBA 8 bits per channel.
This is wasteful for textures that don't utilize all the channels. Like normal maps, or diffuse maps with no alpha channel usage.
Therefore, the export node, and the Preview2D export buttons should respect the channels that are being used, and save a PNG with that bit depth. Or at least, the format should be customizable with 8 bit, 24 bit and 32 bit options.

[Calinou]

This requires Godot to add support for writing PNG with higher or lower bit depth. All Material Maker does is call Image.save_png(), so it has no control over the minutiae of the image writing process.

Writing an indexed PNG in particular could be done automatically. I don't know if libpng handles this automatically, or whether you have to manually detect in the engine code whether the image that is being saved has fewer than 256 different colors.

The same applies to opaque PNG when there is no transparent pixel to be saved.

PS: I recommend running oxipng on saved PNGs regardless, even if Godot implements this optimization in the future. It can make the files significantly smaller with no loss in quality. If the app you're exporting to supports WebP, you can even save to a lossless WebP instead, which is more efficient than an optimized PNG in almost all scenarios.

[ssoher]

I'm not familiar with Godot's API (never used Godot), but perusing the source code, it seems Image_save_png() ultimately calls PNGDriverCommon::image_to_png, and it seems to set the PNG format based on the image format here:

https://github.com/godotengine/godot/blob/a19892054e871b3db3645eacb5eae0b5e69c7957/drivers/png/png_driver_common.cpp#L141-L162

I see export_image.convert(Image.FORMAT_RGBA8) in multiple places before an export in MM source code. Is it possible that removing this behavior, and setting the image format based on the channels the texture has, this can be achieved?

I'm only interested in grayscale (8bit), RGB8 and RGBA8 formats. Not higher bit depths per channel. Imagine saving a black & white single channel noise texture as just an 8 bit PNG instead of a total of 32 bits RGBA.