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.
Resolve each glyph's page value through the descriptor's page IDs. Do not use it as the position of a file in a sorted list. Otherwise a valid page image can be present on disk while the renderer samples another page or reports a missing glyph.
Key takeaway: File existence, page identity and array order are three different checks.
Reproduce the lookup without a GPU
The BMFont file format associates each page with an ID and each character with a page reference. The following authored descriptor uses IDs 7 and 2 in that order. It is a stress case for a general ID lookup, not a promise that every engine accepts sparse page IDs.
pages = [
{'id': 7, 'file': 'symbols.png'},
{'id': 2, 'file': 'letters.png'},
]
by_id = {page['id']: page['file'] for page in pages}
assert by_id[2] == 'letters.png'
assert by_id[7] == 'symbols.png'
An array-position lookup for 7 cannot work with a two-element list. Sorting filenames does not repair that relationship. Sorting page declarations by ID can make one input appear to work while leaving the underlying assumption unchanged.
Validate the complete reference chain
At export or import time, check these relationships in order:
- Page IDs are unique within the descriptor.
- Each glyph page reference resolves to a declared page.
- Each declared page resolves to a file beneath the intended asset root.
- The glyph rectangle fits that page's actual image dimensions.
Report a missing ID as a reference error with the glyph ID and page ID. Do not silently use page zero, because that turns an actionable import error into a wrong symbol in the game. If your chosen importer requires contiguous pages, validate that requirement explicitly and re-export a compatible descriptor.
The Cyotek BMFont consumer documents page filenames as resources the application still has to load. The UnrealBMFont maintainer changelog also records page-ID and multi-page validation work. These implementations illustrate why target compatibility must be checked separately from a general-purpose parser.
Read the result as a data-contract test
The input and result show successful resolution to letters.png and symbols.png, plus rejection of an undeclared page zero. Python 3.12.10 executed three assertions in the checker. No font texture was uploaded and no engine importer was run.
For a real font, first display one known glyph from each page. Then display a string crossing page boundaries. A renderer that works for the first page only can pass a short ASCII sample while failing punctuation, localized labels or symbols stored elsewhere.
Keep neighboring checks separate
- A missing symbol with no descriptor entry needs the font coverage guide, not a page fallback.
- Two different atlases sharing a frame key need the multi-atlas key collision check; that is a namespace problem rather than a page ID.
- Files that work locally but fail after export need a case-sensitive asset-path check.
Delivery boundary
Keep page declarations, page files and the descriptor together. Do not rename or flatten the images independently of their references. Accept the package only after the target importer can render its intended glyph set; this fixture alone cannot establish runtime support for sparse page IDs.
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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