OpenEXR: read PIZ-compressed files (Poly Haven EXRs)

THIRD_PARTY_LICENSES.md

@@ -91,3 +91,17 @@ repository, unchanged.
 - Model: © 2014 PixelMannen, [CC0 1.0](https://creativecommons.org/publicdomain/zero/1.0/legalcode).
 - Rigging and animation: © 2014 tomkranis, [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/legalcode).
 - Conversion to glTF: © 2017 @AsoboStudio and @scurest, [CC BY 4.0](https://creativecommons.org/licenses/by/4.0/legalcode).
+
+## OpenEXR
+
+`src/ae3d/exrpiz` (the PIZ block decoder of `ae3d.openexr`) follows the structure of the OpenEXR library's PIZ
+decoder, written from the format's documentation and that library's source: the wavelet's level loop and its
+Haar steps, the code-length table's run codes and the canonical code assignment. The library is BSD-3-Clause,
+Copyright (c) Contributors to the OpenEXR Project; its notice is in that module's header. ae3d links none of it:
+`tools/exr_reference/exrref.cpp` is a development tool against the official library, outside every build, that
+wrote the PIZ test fixtures and the decodes they are held to.
+
+## Poly Haven
+
+`tests/fixtures/env/polyhaven/` holds two 1k HDRIs from [Poly Haven](https://polyhaven.com), published under
+CC0 1.0 (public domain); see `tests/fixtures/env/PIZ.md` for the assets and their URLs.

changes/2026-10-28-exr-piz.md

@@ -0,0 +1,12 @@
+### OpenEXR PIZ files load as environment maps (#875 follow-up)
+
+- `ae3d.exrpiz`, the PIZ block decoder of `ae3d.openexr`: a block's Huffman stream, the per-channel 2-D wavelet
+  (the 14-bit and the 16-bit forms) and the value table from its bitmap, back to the scanlines the other
+  compressions leave, for HALF and FLOAT channels, RGB, RGBA and a lone Y, any data window. Pure Aether, from the
+  format's documentation and the structure of the OpenEXR library's decoder (BSD-3 notice in the module header).
+  Every Poly Haven EXR is PIZ, so `engine_set_environment` and `AE3D_ENV` now take them.
+- Still refused by name: PXR24, B44, B44A, DWAA, DWAB, tiled, deep and multipart files.
+- `half_to_float` keeps a half's negative zero a negative zero (it was +0), so a decode equals OpenEXR's bit for bit.
+- Tests: `test_envimage` holds 24 PIZ files written by the OpenEXR library, and two Poly Haven 1k assets, to
+  that library's own decode, bit for bit (a hash of the texel bits), and cuts and damages the small ones at every
+  byte; `tools/exr_reference/exrref.cpp` writes and dumps them (a development tool, not built, not linked).

docs/rendering.md

@@ -446,9 +446,10 @@ place and `engine_environment_error()` says why.
 The map is an equirectangular (latitude-longitude) Radiance `.hdr` (RGBE, flat
 or run-length, the four mirror orientations of its resolution line) or OpenEXR
 (`ae3d.envimage`, from the formats' descriptions, no library): scanline files,
-HALF or FLOAT, R G B (A) or a lone Y, compression NONE, RLE, ZIPS or ZIP, lines
-in any order and any data window. PIZ, PXR24, B44, B44A, DWAA, DWAB, tiled,
-deep and multipart files are refused by name. It is placed as a painted sky
+HALF or FLOAT, R G B (A) or a lone Y, compression NONE, RLE, ZIPS, ZIP or PIZ
+(`ae3d.exrpiz`, what Poly Haven's EXRs are), lines in any order and any data
+window. PXR24, B44, B44A, DWAA, DWAB, tiled, deep and multipart files are
+refused by name. It is placed as a painted sky
 is: the top row is the zenith and the middle column looks along +x. Its light
 is the engine's radiance times `intensity`: 1 is the file's own values, and
 exposure handles the rest.