fix(blp): scale 1-bit and 4-bit alpha to 8-bit range#46
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danielsreichenbach merged 1 commit intomainfrom Feb 16, 2026
Merged
fix(blp): scale 1-bit and 4-bit alpha to 8-bit range#46danielsreichenbach merged 1 commit intomainfrom
danielsreichenbach merged 1 commit intomainfrom
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BLP1 images with 1-bit and 4-bit alpha channels were not correctly scaling alpha values to the 0-255 range required by RGBA image formats. - 1-bit alpha: now maps 0→0, 1→255 (previously 0→0, 1→1) - 4-bit alpha: now maps 0-15→0-255 via multiplication by 17 (previously left as 0-15 values) This matches the behavior of the reference blp-rs implementation and fixes transparency issues with War3 textures like Mountain King. Fixes #32
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Summary
Fixes incorrect transparency rendering for BLP1 textures with 1-bit and 4-bit alpha channels.
Problem
BLP1 images with 1-bit and 4-bit alpha channels were not correctly scaling alpha values to the 0-255 range required by RGBA image formats. This caused transparency to appear incorrectly in War3 textures like the Mountain King.
1-bit alpha bug: Values 0-1 were used directly as 0-1 instead of scaling to 0-255
4-bit alpha bug: Values 0-15 were used directly instead of scaling to 0-255
Solution
(nibble << 4) | nibble)This matches the behavior of the reference blp-rs implementation.
Testing
Added two new tests:
test_raw1_1bit_alpha_scaling: Verifies 1-bit alpha is scaled to 0 or 255test_raw1_4bit_alpha_scaling: Verifies 4-bit alpha is scaled to valid 8-bit valuesAll existing tests pass.
Issue
Fixes #32