fix: FITS files written by utvio don't read back (Windows: wrong size, Linux: crash)

[mcoliver]

Summary

The new image format round-trip test (#64) shows two format problems on Windows. Both are fine on macOS.

1. HEIC/HEIF/HIF can't be written, but are advertised as writable

ERROR: OIIO: Unable to open ".../t.heic" for writing: Unknown error

AVIF writes fine, so OIIO's heif plugin is present. The likely cause is that the OpenUTVDeps libheif has no HEVC encoder (x265). OIIO still lists heif in output_format_list, so IOoiio advertises heic/heif/hif as writable. That's the same user-facing symptom as #59: a format listed as writable that fails on write.

Options:

• Build libheif with x265 in OpenUTVDeps.
• Or make IOoiio probe for a working HEVC encoder (e.g. a one-time 1×1 test write) before advertising heic/heif/hif as writable.

2. FITS doesn't read back

utvio in.ppm -o t.fits succeeds, but utvio t.fits -o back.ppm produces a 1280×720 image instead of 96×64. That looks like utvio's placeholder frame after a failed read. Needs investigation on Windows: the OIIO version in OpenUTVDeps, the FITS reader, or the half→float workaround in IOoiio::writeImage.

Evidence

Branch CI run 36648949811, job "2b. Build Windows", test image-format-roundtrip.

Test handling

Until this is fixed, format_roundtrip.py marks heic, heif, hif and fits as known failures on Windows (reported as xfail, linked to this issue). They'll show up as XPASS once fixed, at which point the entries should be removed.

Operating System

Windows 10 / 11

Code of Conduct

• I agree to follow this project's Code of Conduct

[mcoliver]

Update: HEIC/HEIF writing on Windows is unavailable by design. OpenUTV doesn't ship x265 (patent-encumbered HEVC encoder); see AGENTS.md §1.4 (#77). Follow-ups:

• fix(oiio): don't advertise HEIC/HEIF as writable without an HEVC encoder #79: stop advertising HEIC/HEIF as writable without an HEVC encoder
• feat(windows): decode HEIC via libheif's FFmpeg decoder plugin (no bundled HEVC decoder) #80: HEIC decoding via libheif's FFmpeg decoder plugin
• feat: HEIC/HEIF export via OS encoders (macOS ImageIO, Windows WIC) #81: HEIC export via OS encoders (ImageIO / WIC)

The remaining bug in this issue is FITS not reading back on Windows.

[mcoliver]

Update: FITS read-back also fails on Linux, where it's a crash: utvio t.fits -o back.ppm segfaults (exit -11) in #76's CI. On Windows it reads back as the 1280×720 placeholder frame. macOS reads it correctly. FITS is marked as an expected failure on both platforms in format_roundtrip.py until this is fixed.

[mcoliver]

Narrowing this issue to the FITS bug.

HEIC / HEIF part: resolved in 2026.10 (#84). OpenUTV intentionally ships no HEVC encoder (AGENTS.md §1.4); HEIC is no longer advertised as writable through OpenImageIO when there is no encoder, and export goes through the OS encoder instead (#79, #81).

FITS part: still open. A FITS file written by utvio does not read back:

• Windows: it reads back as 1280x720 instead of the 96x64 that was written;
• Linux: reading it back crashes (SIGSEGV).

macOS is fine. Both are marked as known failures in src/test/CliSmokeTest/format_roundtrip.py, so the test will report an unexpected pass once this is fixed.