Disclosure: This article was generated with AI for the FrameSprite team, which has a commercial interest in FrameSprite. It received a separate AI editorial review against the cited sources. Executed synthetic fixtures are identified with their runtime and limits; no other editor, engine, production or performance test is implied.
Read an uncompressed RGB BMP's height as a signed value. A positive height describes bottom-up storage; a negative height describes top-down storage. Flipping every BMP, or interpreting the height as unsigned, can turn a valid icon upside down or create an absurd allocation request.
Key takeaway: Decide file row orientation before converting to your engine's texture convention. Do not fix two different coordinate changes with one unconditional flip.
Compare two files with the same intended pixels
The authored fixture contains a 2 by 2 RGB image stored in two ways. One header uses height +2, with bottom-up rows. The other uses height -2, with top-down rows. Their stored pixel arrays differ, but a correct decoder produces the same canonical top-down pixels.
The BITMAPINFOHEADER reference describes this signed-height meaning for uncompressed RGB. Its rules for other data families are not interchangeable, so retain the RGB profile restriction.
width, signed_height = struct.unpack_from('<ii', data, 18)
if width <= 0 or signed_height == 0:
raise ValueError('invalid fixture dimensions')
height = abs(signed_height)
# Read each stored row with the validated RGB24 stride.
canonical_rows = stored_rows if signed_height < 0 else list(reversed(stored_rows))
The code is a placement excerpt, not a complete parser. Validate the DIB header size, bit depth, compression, pixel offset and range before reaching this stage.
Keep the two flips in separate records
A file decoder may normalize all images to top-down memory. Your renderer may later require a particular texture or UV convention. Record both transformations explicitly. If you reverse rows during decoding and flip texture coordinates again without intending to, the result can look correct for one file variant and wrong for another.
The Pillow BMP parser source is a maintained implementation reference for reading file metadata and handing orientation to its decoder. Use a supported library for general untrusted BMP input rather than adapting a tiny example without its validation.
Use asymmetric pixels as the control
A symmetric icon can conceal an upside-down import. Test different colors in each corner, or use a visible top marker that is not repeated at the bottom. Preserve the intended corner identities in the fixture input rather than relying on a screenshot of a visually symmetric asset.
Python 3.12.10 executed three assertions in the checker. The recorded result confirms equal canonical pixels from positive- and negative-height files, different stored row order, and rejection of a truncated pixel array. The input preserves the two signed heights.
No GPU or engine importer was run. These results concern the authored uncompressed RGB24 files only. The Pillow format guide describes broader supported image formats; it does not turn every BMP variant into this fixture profile.
Check neighboring orientation problems separately
A photo reference can carry EXIF orientation, which is a different metadata system. An atlas importer has its own y-origin convention. A render target can introduce a readback orientation boundary.
Accept the repaired decoder only when both signed-height controls yield the same intended corner layout. Do not remove the negative-height test after one successful import.
Editorial ownership
Prepared by the workflow team, which has a commercial relationship with this site. This guide was drafted with AI assistance; independent editorial review is recorded separately. Report a technical correction with the relevant input, tool version and observed result.
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