From a49108fd2923f9444af04727457d14300e96666f Mon Sep 17 00:00:00 2001
From: averne <averne381@gmail.com>
Date: Thu, 31 Jul 2025 16:42:35 +0200
Subject: [PATCH] avcodec/proresdec: Remove grayscale hack

This was introduced in commit 9c43703, to support a codec "extension"
in the prores_aw encoder.
This removes the chroma fill loop, and instead performs the inverse
transform on null coefficients, which achieves the same result and
fixes an off-by-one in the chroma values produced.

Updated test to reflect this change.
---
 libavcodec/proresdec.c     | 30 +++++++++---------------------
 tests/ref/fate/prores-gray |  4 ++--
 2 files changed, 11 insertions(+), 23 deletions(-)

diff --git a/libavcodec/proresdec.c b/libavcodec/proresdec.c
index 5f917daca1..deaf84bda0 100644
--- a/libavcodec/proresdec.c
+++ b/libavcodec/proresdec.c
@@ -572,12 +572,15 @@ static int decode_slice_chroma(AVCodecContext *avctx, SliceContext *slice,
     for (i = 0; i < blocks_per_slice; i++)
         ctx->bdsp.clear_block(blocks+(i<<6));
 
-    init_get_bits(&gb, buf, buf_size << 3);
+    /* Some encodes have empty chroma scans to simulate grayscale */
+    if (buf_size) {
+        init_get_bits(&gb, buf, buf_size << 3);
 
-    if ((ret = decode_dc_coeffs(&gb, blocks, blocks_per_slice)) < 0)
-        return ret;
-    if ((ret = decode_ac_coeffs(avctx, &gb, blocks, blocks_per_slice)) < 0)
-        return ret;
+        if ((ret = decode_dc_coeffs(&gb, blocks, blocks_per_slice)) < 0)
+            return ret;
+        if ((ret = decode_ac_coeffs(avctx, &gb, blocks, blocks_per_slice)) < 0)
+            return ret;
+    }
 
     block = blocks;
     for (i = 0; i < slice->mb_count; i++) {
@@ -638,7 +641,6 @@ static int decode_slice_thread(AVCodecContext *avctx, void *arg, int jobnr, int
     LOCAL_ALIGNED_16(int16_t, qmat_chroma_scaled,[64]);
     int mb_x_shift;
     int ret;
-    uint16_t val_no_chroma;
 
     slice->ret = -1;
     //av_log(avctx, AV_LOG_INFO, "slice %d mb width %d mb x %d y %d\n",
@@ -702,7 +704,7 @@ static int decode_slice_thread(AVCodecContext *avctx, void *arg, int jobnr, int
     if (ret < 0)
         return ret;
 
-    if (!(avctx->flags & AV_CODEC_FLAG_GRAY) && (u_data_size + v_data_size) > 0) {
+    if (!(avctx->flags & AV_CODEC_FLAG_GRAY)) {
         ret = decode_slice_chroma(avctx, slice, (uint16_t*)dest_u, chroma_stride,
                                   buf + y_data_size, u_data_size,
                                   qmat_chroma_scaled, log2_chroma_blocks_per_mb);
@@ -715,20 +717,6 @@ static int decode_slice_thread(AVCodecContext *avctx, void *arg, int jobnr, int
         if (ret < 0)
             return ret;
     }
-    else {
-        size_t mb_max_x = slice->mb_count << (mb_x_shift - 1);
-        size_t i, j;
-        if (avctx->bits_per_raw_sample == 10) {
-            val_no_chroma = 511;
-        } else { /* 12b */
-            val_no_chroma = 511 * 4;
-        }
-        for (i = 0; i < 16; ++i)
-            for (j = 0; j < mb_max_x; ++j) {
-                *(uint16_t*)(dest_u + (i * chroma_stride) + (j << 1)) = val_no_chroma;
-                *(uint16_t*)(dest_v + (i * chroma_stride) + (j << 1)) = val_no_chroma;
-            }
-    }
 
     /* decode alpha plane if available */
     if (ctx->alpha_info && pic->data[3] && a_data_size) {
diff --git a/tests/ref/fate/prores-gray b/tests/ref/fate/prores-gray
index ae7becc47a..387fecf254 100644
--- a/tests/ref/fate/prores-gray
+++ b/tests/ref/fate/prores-gray
@@ -8,8 +8,8 @@
 #codec_id 1: pcm_s16le
 #sample_rate 1: 48000
 #channel_layout_name 1: stereo
-0,          0,          0,        1,  1658880, 0x43d9c9e2
+0,          0,          0,        1,  1658880, 0x365613b9
 1,          0,          0,     1024,     4096, 0x543c9a17
 1,       1024,       1024,     1024,     4096, 0x9aedadd1
-0,          1,          1,        1,  1658880, 0xa2a72f9b
+0,          1,          1,        1,  1658880, 0x95247963
 1,       2048,       2048,     1024,     4096, 0xa0c9cd55