From dbe979c8f963a809ad660c4aba6f8aea6b8cd8af Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Tue, 4 Aug 2026 19:06:22 +0000
Subject: [PATCH 1/2] Initial plan


From 5f4bb0f2e78abe457fc240493e30a4ca96ed032d Mon Sep 17 00:00:00 2001
From: "copilot-swe-agent[bot]" <198982749+Copilot@users.noreply.github.com>
Date: Tue, 4 Aug 2026 19:09:08 +0000
Subject: [PATCH 2/2] fix: use BI_BITFIELDS with RGB565 masks in BMP writer to
 fix green channel corruption

Co-authored-by: bdbarnett <127794626+bdbarnett@users.noreply.github.com>
---
 lib/pygraphics/_bmp565.py | 21 +++++++++++++++++----
 src/gfx_bmp565.c          | 18 +++++++++++++-----
 2 files changed, 30 insertions(+), 9 deletions(-)

diff --git a/lib/pygraphics/_bmp565.py b/lib/pygraphics/_bmp565.py
index 5605703..9c0dece 100644
--- a/lib/pygraphics/_bmp565.py
+++ b/lib/pygraphics/_bmp565.py
@@ -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
 
 
@@ -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):
diff --git a/src/gfx_bmp565.c b/src/gfx_bmp565.c
index b3138fe..0353e07 100644
--- a/src/gfx_bmp565.c
+++ b/src/gfx_bmp565.c
@@ -74,8 +74,10 @@ 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;
     }
@@ -83,6 +85,9 @@ int gfx_bmp565_read_header_from_file(const char *path, int *width, int *height,
     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;
@@ -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;
     }