BGR linesize

[Yay295]

The linesize of every mode except for the BGR modes is xsize * pixelsize. The BGR modes instead use (xsize * pixelsize + 3) & -4.

Pillow/src/libImaging/Storage.c

Lines 131 to 161 in ad7be55

	} else if (strcmp(mode, "BGR;15") == 0) {
	/* EXPERIMENTAL */
	/* 15-bit reversed true colour */
	im->bands = 1;
	im->pixelsize = 2;
	im->linesize = (xsize * 2 + 3) & -4;
	im->type = IMAGING_TYPE_SPECIAL;
	} else if (strcmp(mode, "BGR;16") == 0) {
	/* EXPERIMENTAL */
	/* 16-bit reversed true colour */
	im->bands = 1;
	im->pixelsize = 2;
	im->linesize = (xsize * 2 + 3) & -4;
	im->type = IMAGING_TYPE_SPECIAL;
	} else if (strcmp(mode, "BGR;24") == 0) {
	/* EXPERIMENTAL */
	/* 24-bit reversed true colour */
	im->bands = 1;
	im->pixelsize = 3;
	im->linesize = (xsize * 3 + 3) & -4;
	im->type = IMAGING_TYPE_SPECIAL;
	} else if (strcmp(mode, "BGR;32") == 0) {
	/* EXPERIMENTAL */
	/* 32-bit reversed true colour */
	im->bands = 1;
	im->pixelsize = 4;
	im->linesize = (xsize * 4 + 3) & -4;
	im->type = IMAGING_TYPE_SPECIAL;

The file history shows that they've been like that since the fork, but does anyone know why they use a different formula?

[radarhere]

My guess would be that it is rounding up to a multiple of 4 for the same reason that LA/La/PA are stored in 4 bytes - to use image32.

I'm not saying that should be the case, but it makes sense for those two things to be related.