From f6a2d41fe2f3316b05dffdce96f867a0b9bf88cd Mon Sep 17 00:00:00 2001
From: Niklas Haas <git@haasn.dev>
Date: Fri, 27 Mar 2026 19:18:43 +0100
Subject: [PATCH] swscale/ops: keep track of correct dither min/max

Mostly, this just affects the metadata in benign ways, e.g.:

 rgb24 -> yuv444p:
   [ u8 +++X] SWS_OP_READ         : 3 elem(s) packed >> 0
     min: {0, 0, 0, _}, max: {255, 255, 255, _}
   [ u8 +++X] SWS_OP_CONVERT      : u8 -> f32
     min: {0, 0, 0, _}, max: {255, 255, 255, _}
   [f32 ...X] SWS_OP_LINEAR       : matrix3+off3 [...]
     min: {16, 16, 16, _}, max: {235, 240, 240, _}
   [f32 ...X] SWS_OP_DITHER       : 16x16 matrix + {0 3 2 -1}
-    min: {33/2, 33/2, 33/2, _}, max: {471/2, 481/2, 481/2, _}
+    min: {16.001953, 16.001953, 16.001953, _}, max: {235.998047, 240.998047, 240.998047, _}
   [f32 +++X] SWS_OP_CONVERT      : f32 -> u8
     min: {16, 16, 16, _}, max: {235, 240, 240, _}
   [ u8 XXXX] SWS_OP_WRITE        : 3 elem(s) planar >> 0
     (X = unused, z = byteswapped, + = exact, 0 = zero)

However, it surprisingly actually includes a semantic change, whenever
converting from limited range to monob or monow:

 yuv444p -> monow:
   [ u8 +XXX] SWS_OP_READ         : 1 elem(s) planar >> 0
     min: {0, _, _, _}, max: {255, _, _, _}
   [ u8 +XXX] SWS_OP_CONVERT      : u8 -> f32
     min: {0, _, _, _}, max: {255, _, _, _}
   [f32 .XXX] SWS_OP_LINEAR       : luma [...]
     min: {-20/219, _, _, _}, max: {235/219, _, _, _}
   [f32 .XXX] SWS_OP_DITHER       : 16x16 matrix + {0 -1 -1 -1}
-    min: {179/438, _, _, _}, max: {689/438, _, _, _}
+    min: {-0.089371, _, _, _}, max: {2.071106, _, _, _}
+  [f32 .XXX] SWS_OP_MAX          : {0 0 0 0} <= x
+    min: {0, _, _, _}, max: {2.071106, _, _, _}
   [f32 .XXX] SWS_OP_MIN          : x <= {1 _ _ _}
-    min: {179/438, _, _, _}, max: {1, _, _, _}
+    min: {0, _, _, _}, max: {1, _, _, _}
   [f32 +XXX] SWS_OP_CONVERT      : f32 -> u8
     min: {0, _, _, _}, max: {1, _, _, _}
   [ u8 XXXX] SWS_OP_WRITE        : 1 elem(s) planar >> 3
     (X = unused, z = byteswapped, + = exact, 0 = zero)

Note the presence of an extra SWS_OP_MAX, to correctly clamp sub-blacks
(values below 16) to 0.0, rather than underflowing. This was previously
undetected because the dither was modelled as adding 0.5 to every pixel value,
but that's only true on average - not always.

Signed-off-by: Niklas Haas <git@haasn.dev>
---
 libswscale/ops.c            | 5 +++++
 tests/ref/fate/sws-ops-list | 2 +-
 2 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/libswscale/ops.c b/libswscale/ops.c
index c281305766..896b0564ec 100644
--- a/libswscale/ops.c
+++ b/libswscale/ops.c
@@ -304,6 +304,7 @@ void ff_sws_op_list_update_comps(SwsOpList *ops)
         switch (op->op) {
         case SWS_OP_READ:
         case SWS_OP_LINEAR:
+        case SWS_OP_DITHER:
         case SWS_OP_SWAP_BYTES:
         case SWS_OP_UNPACK:
         case SWS_OP_FILTER_H:
@@ -364,9 +365,13 @@ void ff_sws_op_list_update_comps(SwsOpList *ops)
         case SWS_OP_DITHER:
             /* Strip zero flag because of the nonzero dithering offset */
             for (int i = 0; i < 4; i++) {
+                op->comps.min[i] = prev.min[i];
+                op->comps.max[i] = prev.max[i];
                 if (op->dither.y_offset[i] < 0)
                     continue;
                 op->comps.flags[i] = prev.flags[i] & ~SWS_COMP_ZERO;
+                op->comps.min[i] = av_add_q(op->comps.min[i], op->dither.min);
+                op->comps.max[i] = av_add_q(op->comps.max[i], op->dither.max);
             }
             break;
         case SWS_OP_UNPACK:
diff --git a/tests/ref/fate/sws-ops-list b/tests/ref/fate/sws-ops-list
index f24c95f388..0dc23800a6 100644
--- a/tests/ref/fate/sws-ops-list
+++ b/tests/ref/fate/sws-ops-list
@@ -1 +1 @@
-a2ed0581163448a2c398ee9992e8eaf6
+374319dfd2b74cb5b69dac68b627fa9b