BMP image loading and rendering issues

[PaintYourDragon]

Certain BMP varieties fail to load or display as intended. Behavior varies depending on image type, size, and whether using displayio.OnDiskBitmap vs adafruit_imageload library. (Last image is skipped because it's known that imageload doesn't handle 24-bit BMPs -- this is NOT a bug report or feature request for that -- the other images represent the issues being raised here.)

Test platform is Adafruit MagTag running adafruit-circuitpython-adafruit_magtag_2.9_grayscale-en_US-20201117-a0d3050.uf2. Test code and data attached. Comments in code describe required libraries and walk through the issues.

circuitpy-image-test.zip

Elsewhere I mentioned an issue where 4-bit pixels might be extracted in the wrong hi/lo order. I'm not seeing that here now, so please disregard that. Will add comments/changes here if I'm able to reproduce it again.

[PaintYourDragon]

UPDATE: PR #3719 (now merged) fixes the imageload (bottom row) thresholding error; those items can be ignored.

WRT OnDiskBitmap: looking more carefully, it appears that black is transparent in all three cases: 4-bit paletted, 8-bit paletted and 24-bit RGB. (Compare the upper & lower 8-bit images with the #3719 fix, they should be identical.) Or…here’s a photo…

Looking at the outer ring in the (top center) test pattern image…the reason it appears as a dotted line is due to antialiasing in the source image…the “black” pixels seen in those sections are really near-black, while the truly black pixels in the source image instead show through to the background…the white stripe…which is why it’s not seen at the top and bottom of the image, due to the alternating black/white bars.