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fix: BMP save/load uses BI_BITFIELDS for correct RGB565 color encoding #16

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Merged
bdbarnett merged 2 commits into from
Aug 4, 2026
Merged

fix: BMP save/load uses BI_BITFIELDS for correct RGB565 color encoding #16

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dbe979c
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Copilot Aug 4, 2026
5f4bb0f
fix: use BI_BITFIELDS with RGB565 masks in BMP writer to fix green ch…
Copilot Aug 4, 2026
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21 changes: 17 additions & 4 deletions lib/pygraphics/_bmp565.py
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Original file line number Diff line number Diff line change
Expand Up @@ -29,6 +29,9 @@ def read_bmp565_header(f):
bpp = struct.unpack("<H", f.read(2))[0]
if bpp != BMP565_BPP:
raise ValueError("Invalid color depth")
compression = struct.unpack("<I", f.read(4))[0]
if compression not in (0, 3): # BI_RGB or BI_BITFIELDS
raise ValueError("Unsupported BMP compression")
return width, height, data_offset


Expand All @@ -46,19 +49,29 @@ def load_bmp565_buffer(f, width, height, data_offset):
return buffer


_BMP565_R_MASK = 0xF800
_BMP565_G_MASK = 0x07E0
_BMP565_B_MASK = 0x001F

# BITMAPFILEHEADER (14) + BITMAPINFOHEADER (40) + three BI_BITFIELDS masks (12)
_BMP565_DATA_OFFSET = 14 + 40 + 12


def write_bmp565_header(f, width, height, data_size):
"""Write the Windows BITMAPINFOHEADER for an RGB565 image."""
"""Write the Windows BITMAPINFOHEADER for an RGB565 image using BI_BITFIELDS."""
f.write(b"BM")
f.write(struct.pack("<I", 14 + 40 + data_size))
f.write(struct.pack("<I", _BMP565_DATA_OFFSET + data_size))
f.write(b"\x00\x00\x00\x00")
f.write(struct.pack("<I", 14 + 40))
f.write(struct.pack("<I", _BMP565_DATA_OFFSET))
f.write(struct.pack("<I", 40))
f.write(struct.pack("<II", width, height))
f.write(struct.pack("<H", 1))
f.write(struct.pack("<H", BMP565_BPP))
f.write(b"\x00\x00\x00\x00")
f.write(struct.pack("<I", 3)) # BI_BITFIELDS
f.write(struct.pack("<I", data_size))
f.write(b"\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00")
# RGB565 channel masks
f.write(struct.pack("<III", _BMP565_R_MASK, _BMP565_G_MASK, _BMP565_B_MASK))


def write_bmp565_rows(f, buffer, width, height):
Expand Down
18 changes: 13 additions & 5 deletions src/gfx_bmp565.c
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Original file line number Diff line number Diff line change
Expand Up @@ -74,15 +74,20 @@ int gfx_bmp565_read_header_from_file(const char *path, int *width, int *height,
}
uint32_t header_size, w, h;
uint16_t planes, bpp;
uint32_t compression;
if (read_u32_le(f, &header_size) < 0 || read_u32_le(f, &w) < 0 || read_u32_le(f, &h) < 0
|| read_u16_le(f, &planes) < 0 || read_u16_le(f, &bpp) < 0) {
|| read_u16_le(f, &planes) < 0 || read_u16_le(f, &bpp) < 0
|| read_u32_le(f, &compression) < 0) {
fclose(f);
return -1;
}
fclose(f);
if (planes != 1 || bpp != GFX_BMP565_BPP) {
return -1;
}
if (compression != 0 && compression != 3) { /* BI_RGB or BI_BITFIELDS */
return -1;
}
*width = (int)w;
*height = (int)h;
*data_offset = off;
Expand Down Expand Up @@ -275,17 +280,20 @@ int gfx_bmp565_save(const gfx_bmp565_t *bmp, const char *path) {
fclose(f);
return -1;
}
if (write_u32_le(f, (uint32_t)(14 + 40 + data_size)) < 0
if (write_u32_le(f, (uint32_t)(14 + 40 + 12 + data_size)) < 0
|| fwrite("\x00\x00\x00\x00", 1, 4, f) != 4
|| write_u32_le(f, 14 + 40) < 0
|| write_u32_le(f, 14 + 40 + 12) < 0
|| write_u32_le(f, 40) < 0
|| write_u32_le(f, (uint32_t)bmp->width) < 0
|| write_u32_le(f, (uint32_t)bmp->height) < 0
|| write_u16_le(f, 1) < 0
|| write_u16_le(f, GFX_BMP565_BPP) < 0
|| fwrite("\x00\x00\x00\x00", 1, 4, f) != 4
|| write_u32_le(f, 3) < 0 /* BI_BITFIELDS */
|| write_u32_le(f, (uint32_t)data_size) < 0
|| fwrite("\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00", 1, 16, f) != 16) {
|| fwrite("\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00", 1, 16, f) != 16
|| write_u32_le(f, 0xF800) < 0 /* R mask */
|| write_u32_le(f, 0x07E0) < 0 /* G mask */
|| write_u32_le(f, 0x001F) < 0) { /* B mask */
fclose(f);
return -1;
}
Expand Down
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