[FFmpeg-devel] [PATCH] vf_colorspace: Add an option to clamp trc LUT output

* [FFmpeg-devel] [PATCH] vf_colorspace: Add an option to clamp trc LUT output
       [not found] <347845>
@ 2025-08-19 14:17 ` Drew Dunne via ffmpeg-devel
  0 siblings, 0 replies; 5+ messages in thread
From: Drew Dunne via ffmpeg-devel @ 2025-08-19 14:17 UTC (permalink / raw)
  To: ffmpeg-devel; +Cc: Drew Dunne

Add a new flag to the vf_colorspace filter which provides the user an
option to clamp the linear and delinear transfer characteristics LUT
values to the [0, 1] represented range. This helps constrain the
potential value range when converting between colorspaces.

Sorry about sending this multiple times, my comments outside my commit
message kept being removed from the patch for some reason. I was trying
to respond to the comment on confirming no additional time added.

I ran "time" with a command running just this filter on YUV frames and
confirmed that no significant time was adding with this patch. They both
averaged to the same time to the hundredth second.

Signed-off-by: Drew Dunne <asdunne@google.com>
---
 doc/filters.texi            |  3 +++
 libavfilter/vf_colorspace.c | 14 ++++++++++++--
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/doc/filters.texi b/doc/filters.texi
index 908c98a3cf..cb09d73f62 100644
--- a/doc/filters.texi
+++ b/doc/filters.texi
@@ -10403,6 +10403,9 @@ von Kries whitepoint adaptation
 identity whitepoint adaptation (i.e. no whitepoint adaptation)
 @end table
 
+@item clamptrc
+Clamps the linear and delinear transfer characteristics LUT values to the [0, 1] represented range.
+
 @item iall
 Override all input properties at once. Same accepted values as @ref{all}.
 
diff --git a/libavfilter/vf_colorspace.c b/libavfilter/vf_colorspace.c
index e1f4725f63..15b4858b24 100644
--- a/libavfilter/vf_colorspace.c
+++ b/libavfilter/vf_colorspace.c
@@ -123,6 +123,7 @@ typedef struct ColorSpaceContext {
     int fast_mode;
     enum DitherMode dither;
     enum WhitepointAdaptation wp_adapt;
+    int clamp_trc;
 
     int16_t *rgb[3];
     ptrdiff_t rgb_stride;
@@ -215,7 +216,9 @@ static int fill_gamma_table(ColorSpaceContext *s)
         } else {
             d = out_alpha * pow(v, out_gamma) - (out_alpha - 1.0);
         }
-        s->delin_lut[n] = av_clip_int16(lrint(d * 28672.0));
+        long d_rounded = lrint(d * 28672.0);
+        s->delin_lut[n] = s->clamp_trc ? av_clip64(d_rounded, 0, 28672)
+                                       : av_clip_int16(d_rounded); 
 
         // linearize
         if (v <= -in_beta * in_delta) {
@@ -225,7 +228,9 @@ static int fill_gamma_table(ColorSpaceContext *s)
         } else {
             l = pow((v + in_alpha - 1.0) * in_ialpha, in_igamma);
         }
-        s->lin_lut[n] = av_clip_int16(lrint(l * 28672.0));
+        long l_rounded = lrint(l * 28672.0);
+        s->lin_lut[n] = s->clamp_trc ? av_clip64(l_rounded, 0, 28672)
+                                     : av_clip_int16(l_rounded); 
     }
 
     return 0;
@@ -1000,6 +1005,11 @@ static const AVOption colorspace_options[] = {
     ENUM("vonkries", WP_ADAPT_VON_KRIES, "wpadapt"),
     ENUM("identity", WP_ADAPT_IDENTITY, "wpadapt"),
 
+    { "clamptrc", 
+      "Clamps the linear and delinear LUT output values to the range [0, 1].",
+      OFFSET(clamp_trc), AV_OPT_TYPE_BOOL,  { .i64 = 0    },
+      0, 1, FLAGS },
+
     { "iall",       "Set all input color properties together",
       OFFSET(user_iall),   AV_OPT_TYPE_INT, { .i64 = CS_UNSPECIFIED },
       CS_UNSPECIFIED, CS_NB - 1, FLAGS, .unit = "all" },
-- 
2.51.0.rc1.167.g924127e9c0-goog

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