Re: [FFmpeg-devel] [PATCH] avfilter/vf_chromakey: The chromakey filter preserves non-opaque alpha transparency.

[Gavin Smith]

On 10/06/2020 22:33, Gavin Smith wrote:
From: Gavin Smith <gcs...@yahoo.com>

Sorry, new to all this. A description would have been helpful. I'll work 
on getting it right.

So this patch is to address Trac ticket #8724:

https://trac.ffmpeg.org/ticket/8724


---
  libavfilter/vf_chromakey.c | 43 ++++++++++++++++++++++----------------
  1 file changed, 25 insertions(+), 18 deletions(-)

diff --git a/libavfilter/vf_chromakey.c b/libavfilter/vf_chromakey.c
index 4b1669d084..7206f44441 100644
--- a/libavfilter/vf_chromakey.c
+++ b/libavfilter/vf_chromakey.c
@@ -47,7 +47,7 @@ typedef struct ChromakeyContext {
                      int jobnr, int nb_jobs);
  } ChromakeyContext;
  
-static uint8_t do_chromakey_pixel(ChromakeyContext *ctx, uint8_t u[9], uint8_t v[9])
+static uint8_t do_chromakey_pixel(ChromakeyContext *ctx, uint8_t a, uint8_t 
u[9], uint8_t v[9])
  {
      double diff = 0.0;
      int du, dv, i;
@@ -62,13 +62,13 @@ static uint8_t do_chromakey_pixel(ChromakeyContext *ctx, 
uint8_t u[9], uint8_t v
      diff /= 9.0;
  
      if (ctx->blend > 0.0001) {
-        return av_clipd((diff - ctx->similarity) / ctx->blend, 0.0, 1.0) * 
255.0;
+        return av_clipd((diff - ctx->similarity) / ctx->blend, 0.0, 1.0) * 
(float)a;
      } else {
-        return (diff > ctx->similarity) ? 255 : 0;
+        return (diff > ctx->similarity) ? a : 0;
      }
  }
  
-static uint16_t do_chromakey_pixel16(ChromakeyContext *ctx, uint16_t u[9], uint16_t v[9])
+static uint16_t do_chromakey_pixel16(ChromakeyContext *ctx, uint16_t a, 
uint16_t u[9], uint16_t v[9])
  {
      double max = ctx->max;
      double diff = 0.0;
@@ -84,9 +84,9 @@ static uint16_t do_chromakey_pixel16(ChromakeyContext *ctx, 
uint16_t u[9], uint1
      diff /= 9.0;
  
      if (ctx->blend > 0.0001) {
-        return av_clipd((diff - ctx->similarity) / ctx->blend, 0.0, 1.0) * max;
+        return av_clipd((diff - ctx->similarity) / ctx->blend, 0.0, 1.0) * 
(float)a;
      } else {
-        return (diff > ctx->similarity) ? max : 0;
+        return (diff > ctx->similarity) ? a : 0;
      }
  }
  
@@ -131,13 +131,17 @@ static int do_chromakey_slice(AVFilterContext *avctx, void *arg, int jobnr, int
  
      for (y = slice_start; y < slice_end; ++y) {
          for (x = 0; x < frame->width; ++x) {
-            for (yo = 0; yo < 3; ++yo) {
-                for (xo = 0; xo < 3; ++xo) {
-                    get_pixel_uv(frame, ctx->hsub_log2, ctx->vsub_log2, x + xo - 1, y 
+ yo - 1, &u[yo * 3 + xo], &v[yo * 3 + xo]);
+            uint8_t *a = frame->data[3] + frame->linesize[3] * y;
+            const uint8_t ao = a[x];
+
+            if (ao != 0) {
+                for (yo = 0; yo < 3; ++yo) {
+                    for (xo = 0; xo < 3; ++xo) {
+                        get_pixel_uv(frame, ctx->hsub_log2, ctx->vsub_log2, x + xo - 
1, y + yo - 1, &u[yo * 3 + xo], &v[yo * 3 + xo]);
+                    }
                  }
+                a[x] = do_chromakey_pixel(ctx, ao, u, v);
              }
-
-            frame->data[3][frame->linesize[3] * y + x] = 
do_chromakey_pixel(ctx, u, v);
          }
      }
  
@@ -163,15 +167,18 @@ static int do_chromakey16_slice(AVFilterContext *avctx, void *arg, int jobnr, in
  
      for (y = slice_start; y < slice_end; ++y) {
          for (x = 0; x < frame->width; ++x) {
-            uint16_t *dst = (uint16_t *)(frame->data[3] + frame->linesize[3] * 
y);
-
-            for (yo = 0; yo < 3; ++yo) {
-                for (xo = 0; xo < 3; ++xo) {
-                    get_pixel16_uv(frame, ctx->hsub_log2, ctx->vsub_log2, x + xo - 1, 
y + yo - 1, &u[yo * 3 + xo], &v[yo * 3 + xo]);
+            uint16_t *a = (uint16_t *)(frame->data[3] + frame->linesize[3] * 
y);
+            const uint16_t ao = a[x];
+
+            if (ao != 0) {
+                for (yo = 0; yo < 3; ++yo) {
+                    for (xo = 0; xo < 3; ++xo) {
+                        get_pixel16_uv(frame, ctx->hsub_log2, ctx->vsub_log2, x + xo - 
1, y + yo - 1, &u[yo * 3 + xo], &v[yo * 3 + xo]);
+                    }
                  }
-            }
  
-            dst[x] = do_chromakey_pixel16(ctx, u, v);
+                a[x] = do_chromakey_pixel16(ctx, ao, u, v);
+            }
          }
      }
  
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