In the newest nightly build (26 August 2017 - Revision c29954d
) a few changes have been merged that might make a warning such as this appear:
Animation 'bla' has images that are of different sizes (currently at frame 2). Performance could be improved by making all images the same size.
The reason for that is that FSO now uses OpenGL texture arrays for storing the individual frames of an animation in a more efficient format. This should improve particle rendering performance significantly since FSO can now render all particles of an animation with a single draw call. There should be no behavioral or visual changes from this new code so if you notice something different, please let me know.
Now, the reason that warning above exists is that all images of a texture array must be the same size, same compression format, etc... Effectively, the format must be the same and only the image data may differ. If that is not the case then FSO will simply fall back to using single image arrays which is similar to how it was before. Since that is not good for performance, the warning above will be triggered to let the modders know that some data may cause unneeded performance issues. Since there is a fallback mechanism, it's only a warning since the issue can be fixed by the code.
One problem may be that the warning is shown for animations where using a texture array isn't beneficial for performance such as for interface art since such an animation is often just drawn once. Currently, the bmpman loading code simply checks the texture array conditions for every image loaded and doesn't consider if the image being loaded would actually benefit from being in a texture array. I have not encountered this warning in any mod while testing this so there may be no issue here but if you encounter this message and think it's not relevant for that image, please let me know here. I have a plan for how to change the warning logic in that case but I haven't implemented that yet since, as far as I can tell, it's not needed.
Thank you for reading, please let me know if you have any issues with these changes.