psxrecomp/docs/DUCKSTATION_TEXTURE_FORMAT.md

DuckStation texture-pack interoperability

psxrecomp implements a bounded subset of modern DuckStation texture packs. The parser, matcher, cache and dumper are independently written here. The reference version is upstream commit 697599c47a646a6cfcc4d246018adf4904d55001, observed on 2026-10-06. This pin matters for palette-range and alpha behavior.

Provenance

At the reference pin, DuckStation's license is CC-BY-NC-ND-4.0; its texture-cache and shader files carry the same SPDX identifier. This project includes no DuckStation implementation, translated implementation, patch, or binary. Its own code retains the project's PolyForm Noncommercial terms. Reading format behavior does not provide a license to incorporate DuckStation's source. Pack artwork has its own rights; users supply it separately.

The hash implementation comes directly from xxHash v0.8.3, under BSD-2-Clause. The unmodified runtime/third_party/xxhash.h has SHA-256 17973c0dc49d9854ca26caa191f0e12f7a424b68858d9a78de3860d959d85e4b. Its license ships as runtime/licenses/xxhash-NOTICES.txt. The v0.8.0 release made XXH3 output stable across future versions. Thus the upstream v0.8.3 implementation supplies the unseeded XXH3 behavior used by DuckStation's vendored v0.8.0 without importing any emulator code.

WebP decoding uses only the static decoder from the official libwebp 1.6.0 tag, commit 4fa21912338357f89e4fd51cf2368325b59e9bd9, under BSD-3-Clause. The immutable source archive has SHA-256 923f3382a47a2af185c3240c954cf004428b237bd7317413a95146d01eb4b94b. Its copyright, COPYING, PATENTS, and AUTHORS notices ship in runtime/licenses/libwebp-NOTICES.txt. No encoder, command-line utility, or libwebp DLL is required. PNG/JPEG use the existing shared stb_image implementation.

Supported files

Choose a game/package folder containing replacements/ and dumps/, or choose replacements/ itself. Replacement PNG/JPEG/static WebP images can live in subdirectories. See the player workflow for dumping, editing and reloading. DuckStation documents its serial-based folders and authoring process in its texture replacement wiki.

Paletted filename stems follow this grammar:

<kind>-<mode>-<sourceHash>-<paletteHash>-<sourceWords>x<sourceHeight>-<offsetX>-<offsetY>-<width>x<height>-P<min>-<max>
kind = texupload | texpage
mode = P4 | P8 | STP4 | STP8

Direct stems omit the palette fields:

<kind>-<mode>-<sourceHash>-<sourceWords>x<sourceHeight>-<offsetX>-<offsetY>-<width>x<height>
mode = C16 | STC16

Append .png, .jpg/.jpeg, or .webp. Hash fields contain exactly 16 hexadecimal digits; either case is accepted. Dimensions, offsets and palette endpoints are decimal. Source width is in native VRAM words. Rectangle width and X offset are expanded texels: four per word for P4, two for P8, one for C16. Height and Y offset are rows. A page's source size is 64x256, 128x256 or 256x256 words respectively. These definitions follow the upstream filename documentation.

The loader validates nonzero sizes, native VRAM bounds, word-aligned horizontal rectangles, page bounds and palette endpoints. The replacement image's actual size controls its independent X/Y scale; integer and equal-axis scale factors are not required. The renderer applies the declared native rectangle's origin and extent when mapping UVs.

Hash compatibility

Both texture and palette identities use unseeded XXH3-64 over packed native PSX words. Words are serialized low byte first. Rectangles concatenate each row's words without stride padding. texupload hashes the complete tracked post-write source rectangle; texpage hashes its declared subrectangle. Dimensions and palette data do not enter the texture hash.

A full CLUT hashes 16 P4 or 256 P8 words, including bit 15. A P8 CLUT extending beyond VRAM's right edge hashes only the remaining words, without wrapping. For reduced Pmin-max ranges, the reference implementation hashes the first max-min+1 palette words, without advancing by min; a range whose declared maximum crosses the edge does not match. It does not revalidate used indices against the declared range during lookup. psxrecomp preserves this behavior for interoperability. Changing a palette entry outside that prefix can leave a replacement matched. Set ReducePaletteRange: false to capture full ranges when that tradeoff is preferable. These are observations of the pinned texture-cache behavior.

Alpha conventions

The decoder preserves raw RGBA bytes. Ordinary P4/P8/C16 replacements treat alpha below 128 as cutout and alpha at least 128 as occupied, including black. ST names encode PSX transparency classification: alpha at most 242 marks STP, while alpha at least 243 marks an opaque texel. Exact RGBA zero is cutout; ST black with alpha at least 243 also becomes PSX transparent zero. Alpha zero with nonzero RGB in an ST image remains a semitransparent texel. Actual blending still uses the primitive's PSX transparency mode. These rules follow the pinned replacement merge shader.

When ordinary and ST records cover the same identity and rectangle, psxrecomp prefers the primitive's convention; if only the other convention exists it can still supply the image. Duplicate canonical identities remain ambiguous. Distinct matching parts are clipped and composed in a deterministic order.

The finalized dumper writes rounded 5-bit-to-8-bit native RGB, reduced CLUT ranges by default, alpha zero for native color zero, alpha 255 for occupied ordinary texels, and alpha 143 for occupied STP texels in ST dumps, matching the pinned upstream dump's mask-expression result. DumpTextureForceAlphaChannel instead emits ordinary names and makes every pixel's alpha 255. Native conversion was checked against the pinned GPU helpers.

Current compatibility limits

The module accepts bounded replacement parts clipped to the draw query. It supports ordinary unchanged uploads and nonwrapping page subrectangles. OpenGL composes up to 64 matching parts over a native-pixel fallback, with at most 256 cached compositions, 64 MiB of composed RGBA, and 8192 pixels per side. ReplacementScaleLinearFilter enables color interpolation when scaling parts while keeping cutout, STP, and opaque classification discrete. The default is nearest scaling. Decode or composition limits retain native rendering. Bounded copy handling, partial-overwrite splitting, and adjacent upload coalescing can be enabled with their configuration keys; their defaults are off. Splits retain the immutable original source hash and words. Exceeding a split limit finalizes observed usage and drops the remaining tracked identity. Coalescing accepts unused, unsplit uploads adjacent to the right or below, within the configured incoming-write dimensions. Copies normally invalidate their destination; ConvertCopiesToWrites can refresh an existing destination source's original extent when its active region contains the copy. It does not clone the source identity. A same-session pack reload can copy intact upload identities into the newly indexed pack. Loading a savestate clears them; page matching remains available, while upload matching needs subsequent guest uploads. The module's rectangle and query caches must be invalidated on every native VRAM write.

The following cases use native rendering or report an unsupported-file/option diagnostic:

• Wrapped UV intervals, horizontally wrapped pages and wrapped uploads.
• vram-write- XXH3-128 images and legacy unnamed layouts.
• Animated WebP and image payloads outside PNG/JPEG/static WebP.
• Unrecognized or unimplemented config.yaml behavior.

config.yaml supports root-level scalar options and a bounded Aliases: mapping with literal or quoted filename keys and relative PNG/JPEG/WebP paths under replacements/. A single-line literal/folded block value is accepted. Escaped strings, multiline values, anchors and general YAML features are outside the subset. Absolute paths and .. traversal are rejected. A direct canonical replacement takes precedence over its alias. Unknown top-level options are reported by name and are not applied.

The root-level authoring keys and defaults are:

Key	Default	Behavior
DumpTexturePages	false	Select page capture instead of upload tracking.
DumpFullTexturePages	false	Capture full 256x256 pages in page mode.
DumpTextureForceAlphaChannel	false	Force opaque alpha and ordinary names for texture dumps.
DumpC16Textures	false	Include direct-color texture captures.
ReducePaletteRange	true	Emit the used palette-index range, with the pinned prefix-hash convention.
ConvertCopiesToWrites	false	Refresh bounded existing destination-source tracking after copies.
MaxVRAMWriteSplits	0	Bound partial-overwrite source splitting; zero disables it.
MaxVRAMWriteCoalesceWidth, MaxVRAMWriteCoalesceHeight	0	Bound incoming uploads eligible for adjacent coalescing.
DumpTextureWidthThreshold, DumpTextureHeightThreshold	16	Minimum finalized texture dimensions in texels.
DumpVRAMWriteForceAlphaChannel	true	Recognized upstream background-write option; that separate dump mode is unsupported.
DumpVRAMWriteWidthThreshold, DumpVRAMWriteHeightThreshold	128	Recognized background-write thresholds; do not enable XXH3-128 dumping.
ReplacementScaleLinearFilter	false	Interpolate composition colors within the nearest pixel's PSX alpha class.
MaxHashCacheEntries	256	Rectangle-hash cache entries, capped at 4096.
MaxHashCacheVRAMUsageMB	64	Source-snapshot budget in MiB, capped at 64.
MaxReplacementCacheVRAMUsage	64	Decoded replacement budget in MiB, capped at 64.

Use these keys at the root, not inside Options:. General YAML structures are unsupported. Existing full-range replacement filenames remain valid even when new captures use reduced ranges. The Tomba example's config.yaml.example documents the defaults without activating or overwriting a user's config. The cache keys are compatibility mappings to this implementation's bounded CPU storage: MaxHashCacheVRAMUsageMB controls source snapshots, rather than reproducing DuckStation's GPU hash-cache allocation.

Bounds and lifecycle

The loader hard-fails above 262144 candidate files or entries, scans at most 1048576 directory entries and limits recursive nesting to 16 levels. Duplicate identities are marked ambiguous. Matching indexes upload hashes and page geometry, with 256 rectangle hashes and 256 positive/negative query results. Native page/CLUT writes invalidate affected cached queries. A query requiring over 4096 candidate page geometries or more than 4 MiB of new hashing reports an error and renders natively.

One low-priority background worker performs PNG/JPEG/WebP decoding and PNG dumping. The decode queue is bounded to 32, decoded metadata to 512 entries, and decoded pixels to a default 64 MiB budget. Encoded files are limited to 64 MiB and each image dimension to 8192 before decoding. Pixel leases survive eviction and pack destruction. Complete compositions retain their source pixels and can be reused after source PNGs leave the decoded cache. OpenGL also reuses GPU-resident full replacements without decoding them again; its image cache holds at most 256 entries within the existing 128 MiB budget. Decoding failure leaves native rendering active.

Dumps accumulate the union of word-aligned used rectangles per source and palette, including semitransparent draw usage. Changed observations are checkpointed approximately once per second during draws, so long-lived backgrounds appear while capture remains enabled. Checkpoints retain the original words, captured palettes, and crop union; later usage can enlarge the crop. Unchanged captures are deduplicated. Sources also finalize when they retire or capture is flushed. Default upload mode emits texupload; optional page mode emits texpage. Original source snapshots are bounded to 64 MiB by default, with at most 8192 tracked sources/uploads and 8192 palette records; pressure retires sources and finalizes their observed usage. The write queue is bounded to eight jobs and 32 MiB of queued RGBA, applying backpressure instead of silently losing captures, and dump deduplication to a rolling 8192 identities. Exiting the game flushes remaining sources and waits for PNG writing. A temporary file is published atomically without replacing an existing final file. Write failures remove only the created temporary and are returned by capture/flush operations. All snapshots decode native VRAM rather than replacement pixels.

Texture identities describe upload/page pixels, palette ranges, source sizes, and used rectangles. A PSX sprite assembled from several uploads or palettes can still produce separate parts; palette animation can produce several IDs. Different histories, supported options, wrapping, or composition requirements can still differ from DuckStation. This is bounded interoperability, not a promise that every real game's dumps have identical filenames or reconstructed sprite images. Alias names can make editing easier without changing identity.

Independent validation

runtime/tests/test_duckstation_texture_pack.cpp contains fixed XXH3 vectors generated with the separate Python xxhash 3.6.0 native wheel. Input word i is ((i * 7919) ^ 0xa51c) & 0xffff, serialized little endian. Examples:

Word count	XXH3-64
0	2D06800538D394C2
4	3631D2A04EC85311
8	000CA76CB9B329DC
16	9CBCDEA9305D7528
256	0005D0B83A005F4E
8192	C68A00DCCA652920

The test also checks literal filenames, strided native rectangles, crop coordinates, CLUT edges and reduced-prefix behavior, positive/negative cache invalidation, reload identity copying, duplicate ambiguity, alias diagnostics, raw alpha thresholds, Unicode folders, background decoding, native dump pixels, worker shutdown and preservation of existing edited files. Renderer fixtures exercise the alpha classification and native-VRAM authority separately.

texture_image_decode_test uses synthetic literal-pixel fixtures generated by Pillow 12.3.0. Windows Clang and Linux GCC checks cover raw PNG/lossless WebP RGBA (including alpha 0, 1, 128, 254), baseline/progressive JPEG, lossy WebP, dimensions, encoded/decoded bounds, truncated input, and explicit animation rejection. Decoders accept bytes supplied by the background loader and perform no filesystem access themselves.

An official pinned DuckStation binary was also used as an external oracle: 0.1-12074-g697599c47 (dev), development ZIP SHA-256 4813f22823e6aa262c46a4a516f10b01e1e2ddf6fcea82763e359ef9ac812a9d. The owned standalone PS-X probe contains no game artwork. It uploads 10x56 words at VRAM (640,0), repeating 3210 7654 BA98 FEDC 140 times. Palette A at (0,256) is zero followed by (i*1537)&0x7fff for indices 1..15; palette B at (0,257) is zero followed by ((i*293)&0x7fff)|0x8000. It draws a 32x32 ordinary head and nested 36x32 semitransparent head at UV (0,0) with A, then a 40x24 ordinary body at UV (0,32) with B. Overwriting the complete upload retires it. The reference produces exactly these two lifetime-union files:

Reference filename	Decoded RGBA SHA-256
texupload-STP4-18DDF9B731440D20-0DAE5F73C88719B1-10x56-0-0-36x32-P0-15.png	5d1bec29956fb3979547f18c9ccc719903924bd9c9d70584913d363b67248ca7
texupload-P4-18DDF9B731440D20-AF506869B80F5CE7-10x56-0-32-40x24-P0-15.png	1a26a7b32dc147cd952364bc59290eaaa8f973fb1c2201c84b9e2756a7bc3479

psxrecomp produces the same two filenames and dimensions, with every decoded RGBA byte identical to those reference PNGs. The module regression preserves the literal oracle names; the primary audit compared the actual files independently.

This controlled oracle isolates naming, lifetime union, palette hashing, and native pixel conversion. It does not establish full Tomba gameplay identity equivalence, and no reference binary or proprietary game pixels are distributed.