Bug: pngToBmp threshold checks alpha instead of blue channel (pngtosvg.py:73)

Problem

pngToBmp converts to RGBA then thresholds on channels [0], [1], [3] — that's R, G, Alpha. Blue channel pix[2] is skipped and alpha is (incorrectly) treated as a color.

Cite

handwrite/pngtosvg.py line 73 (branch dev):

img = Image.open(path).convert("RGBA").resize((100, 100))
...
if pix[0] >= threshold and pix[1] >= threshold and pix[3] >= threshold:
    data.append((255, 255, 255, 0))
else:
    data.append((0, 0, 0, 1))

Steps

1. Feed a scan with any transparency, e.g. pixel (255, 255, 255, 0) (transparent white background).
2. Run handwrite sheet.png out/.

Expected

Transparent/white background stays background (white).

Actual

Alpha=0 fails pix[3] >= 200, so the pixel is classified as ink (0, 0, 0, 1). Background gets vectorized as glyph fill.

Secondary failure: bright but low-blue pixels (e.g. yellow pen (255, 255, 0, 255)) pass the check and are dropped as background even though they carry stroke.

Fix: replace pix[3] with pix[2] (check B, not A).

Environment

From setup.py: handwrite==0.3.1, Pillow (unpinned), Python >=3.7. dev branch HEAD.

Thanks for maintaining yashlamba/handwrite!

Open handwrite/pngtosvg.py at line 73 and inspect the RGBA threshold used by pngToBmp. Run handwrite sheet.png out/ with transparent and colored sample pixels, then verify that the blue channel is checked and transparent or white backgrounds remain background rather than becoming ink.