[twn39]

Summary

Fixes a crash (NULL pointer dereference / SIGSEGV / Windows Access Violation 0xC0000005) when encoding monochrome images (heif_colorspace_monochrome) or images with an Alpha channel using the Kvazaar HEVC encoder.

Root Cause

In kvazaar_start_sequence_encoding_intern() (libheif/plugins/encoder_kvazaar.cc):

• When encoding a monochrome image (such as an auxiliary alpha image), the code set:

  config->input_format = KVZ_FORMAT_P400;

  However, kvz_config_init() initializes config->chroma_format to KVZ_CSP_420 by default.
• Kvazaar's internal state expects config->chroma_format to be updated (or kvz_config_parse(config, "input-format", "P400") to be invoked), which sets chroma_format = KVZ_CSP_400 and chroma_shift_w = 0, chroma_shift_h = 0.
• Because chroma_format was never changed from KVZ_CSP_420, Kvazaar's encoder was created expecting 4:2:0 input.
• Later, in kvazaar_encode_sequence_frame(), the picture was allocated via api->picture_alloc_csp(KVZ_CSP_400, ...). In Kvazaar's videoframe.c, KVZ_CSP_400 does not allocate U and V chroma planes, setting pic->u = NULL; pic->v = NULL;.
• When api->encoder_encode() ran, Kvazaar's intra mode search (search_intra.c: search_intra_chroma_rough) checked state->encoder_control->cfg.chroma_format != KVZ_CSP_400. Because cfg.chroma_format was still KVZ_CSP_420, it attempted to access pic->u and pic->v, resulting in an immediate crash dereferencing NULL.

Solution

1. In kvazaar_start_sequence_encoding_intern(), explicitly set config->chroma_format and corresponding chroma shift parameters.
2. Call api->config_parse(config, "input-format", format_str) to ensure Kvazaar's internal configuration state is fully synchronized.
3. Added a regression test kvazaar encodes monochrome image in tests/encode.cc.

[farindk]

Thanks for the report and the analysis. The crash is real, but only against kvazaar git master since July 2026 (commit cbdcc31a moved chroma_format into kvz_config). The fields your patch writes do not exist in any released kvazaar, so the PR does not compile against 2.3.x. The portable fix is to configure the input format through config_parse("input-format", ...), which is what kvazaar's own CLI does and which works on every version. I've applied that variant together with your test in d3772c0.