swscale/alphablend: Support SWS_ALPHA_BLEND_CHECKERBOARD

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/scaler.texi

@@ -130,6 +130,9 @@ Default value is @samp{none}.
 @item uniform_color
 Blend onto a uniform background color
 
+@item checkerboard
+Blend onto a checkerboard
+
 @item none
 No blending
 

libswscale/alphablend.c

@@ -32,10 +32,17 @@ int ff_sws_alphablendaway(SwsContext *c, const uint8_t *src[],
     unsigned off    = 1<<desc->comp[0].depth_minus1;
     unsigned shift  = desc->comp[0].depth_minus1 + 1;
     unsigned max    = (1<<shift) - 1;
-    int target_table[3];
+    int target_table[2][3];
 
-    for (plane = 0; plane < plane_count; plane++)
-        target_table[plane] = plane && !(desc->flags & AV_PIX_FMT_FLAG_RGB) ? 1<<desc->comp[0].depth_minus1 : 0;
+    for (plane = 0; plane < plane_count; plane++) {
+        int a = 0, b = 0;
+        if (c->alphablend == SWS_ALPHA_BLEND_CHECKERBOARD) {
+            a = (1<<desc->comp[0].depth_minus1)/2;
+            b = 3*(1<<desc->comp[0].depth_minus1)/2;
+        }
+        target_table[0][plane] = plane && !(desc->flags & AV_PIX_FMT_FLAG_RGB) ? 1<<desc->comp[0].depth_minus1 : a;
+        target_table[1][plane] = plane && !(desc->flags & AV_PIX_FMT_FLAG_RGB) ? 1<<desc->comp[0].depth_minus1 : b;
+    }
 
     av_assert0(plane_count == nb_components - 1);
     if (desc->flags & AV_PIX_FMT_FLAG_PLANAR) {
@@ -47,16 +54,15 @@ int ff_sws_alphablendaway(SwsContext *c, const uint8_t *src[],
                     const uint16_t *s = src[plane      ] + srcStride[plane] * y;
                     const uint16_t *a = src[plane_count] + srcStride[plane_count] * y;
                           uint16_t *d = dst[plane      ] + dstStride[plane] * y;
-                    unsigned target = target_table[plane];
                     if ((!isBE(c->srcFormat)) == !HAVE_BIGENDIAN) {
                         for (x = 0; x < w; x++) {
-                            unsigned u = s[x]*a[x] + target*(max-a[x]) + off;
+                            unsigned u = s[x]*a[x] + target_table[((x^y)>>5)&1][plane]*(max-a[x]) + off;
                             d[x] = av_clip((u + (u >> shift)) >> shift, 0, max);
                         }
                     } else {
                         for (x = 0; x < w; x++) {
                             unsigned aswap =av_bswap16(a[x]);
-                            unsigned u = av_bswap16(s[x])*aswap + target*(max-aswap) + off;
+                            unsigned u = av_bswap16(s[x])*aswap + target_table[((x^y)>>5)&1][plane]*(max-aswap) + off;
                             d[x] = av_clip((u + (u >> shift)) >> shift, 0, max);
                         }
                     }
@@ -64,9 +70,8 @@ int ff_sws_alphablendaway(SwsContext *c, const uint8_t *src[],
                     const uint8_t *s = src[plane      ] + srcStride[plane] * y;
                     const uint8_t *a = src[plane_count] + srcStride[plane_count] * y;
                           uint8_t *d = dst[plane      ] + dstStride[plane] * y;
-                    unsigned target = target_table[plane];
                     for (x = 0; x < w; x++) {
-                        unsigned u = s[x]*a[x] + target*(255-a[x]) + 128;
+                        unsigned u = s[x]*a[x] + target_table[((x^y)>>5)&1][plane]*(255-a[x]) + 128;
                         d[x] = (257*u) >> 16;
                     }
                 }
@@ -84,7 +89,7 @@ int ff_sws_alphablendaway(SwsContext *c, const uint8_t *src[],
                     for (x = 0; x < w; x++) {
                         for (plane = 0; plane < plane_count; plane++) {
                             int x_index = (plane_count + 1) * x;
-                            unsigned u = s[x_index + plane]*a[x_index] + target_table[plane]*(max-a[x_index]) + off;
+                            unsigned u = s[x_index + plane]*a[x_index] + target_table[((x^y)>>5)&1][plane]*(max-a[x_index]) + off;
                             d[plane_count*x + plane] = av_clip((u + (u >> shift)) >> shift, 0, max);
                         }
                     }
@@ -93,7 +98,7 @@ int ff_sws_alphablendaway(SwsContext *c, const uint8_t *src[],
                         for (plane = 0; plane < plane_count; plane++) {
                             int x_index = (plane_count + 1) * x;
                             unsigned aswap =av_bswap16(a[x_index]);
-                            unsigned u = av_bswap16(s[x_index + plane])*aswap + target_table[plane]*(max-aswap) + off;
+                            unsigned u = av_bswap16(s[x_index + plane])*aswap + target_table[((x^y)>>5)&1][plane]*(max-aswap) + off;
                             d[plane_count*x + plane] = av_clip((u + (u >> shift)) >> shift, 0, max);
                         }
                     }
@@ -105,7 +110,7 @@ int ff_sws_alphablendaway(SwsContext *c, const uint8_t *src[],
                 for (x = 0; x < w; x++) {
                     for (plane = 0; plane < plane_count; plane++) {
                         int x_index = (plane_count + 1) * x;
-                        unsigned u = s[x_index + plane]*a[x_index] + target_table[plane]*(255-a[x_index]) + 128;
+                        unsigned u = s[x_index + plane]*a[x_index] + target_table[((x^y)>>5)&1][plane]*(255-a[x_index]) + 128;
                         d[plane_count*x + plane] = (257*u) >> 16;
                     }
                 }

libswscale/options.c

@@ -81,6 +81,7 @@ static const AVOption swscale_options[] = {
     { "alphablend",      "mode for alpha -> non alpha",   OFFSET(alphablend),AV_OPT_TYPE_INT,    { .i64  = SWS_ALPHA_BLEND_NONE}, 0,       SWS_ALPHA_BLEND_NB-1, VE, "alphablend" },
     { "none",            "ignore alpha",                  0,                 AV_OPT_TYPE_CONST,  { .i64  = SWS_ALPHA_BLEND_NONE}, INT_MIN, INT_MAX,       VE, "alphablend" },
     { "uniform_color",   "blend onto a uniform color",    0,                 AV_OPT_TYPE_CONST,  { .i64  = SWS_ALPHA_BLEND_UNIFORM},INT_MIN, INT_MAX,     VE, "alphablend" },
+    { "checkerboard",    "blend onto a checkerboard",     0,                 AV_OPT_TYPE_CONST,  { .i64  = SWS_ALPHA_BLEND_CHECKERBOARD},INT_MIN, INT_MAX,     VE, "alphablend" },
 
     { NULL }
 };

libswscale/swscale_internal.h

@@ -78,6 +78,7 @@ typedef enum SwsDither {
 typedef enum SwsAlphaBlend {
     SWS_ALPHA_BLEND_NONE  = 0,
     SWS_ALPHA_BLEND_UNIFORM,
+    SWS_ALPHA_BLEND_CHECKERBOARD,
     SWS_ALPHA_BLEND_NB,
 } SwsAlphaBlend;