swscale: Fix scaling for unscaled dithered planar convertions.

This fixes some overflow in bright areas and ensures that the maximum brightness level is
mapped to the maximum without cliping and without showing dither patterens in flat max
brightness areas.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libswscale/swscale.c

@@ -341,6 +341,24 @@ DECLARE_ALIGNED(8, const uint8_t, dithers)[8][8][8]={
   { 112, 16,104,  8,118, 22,110, 14,},
 }};
 
+uint16_t dither_scale[15][16]={
+{    2,    3,    3,    5,    5,    5,    5,    5,    5,    5,    5,    5,    5,    5,    5,    5,},
+{    2,    3,    7,    7,   13,   13,   25,   25,   25,   25,   25,   25,   25,   25,   25,   25,},
+{    3,    3,    4,   15,   15,   29,   57,   57,   57,  113,  113,  113,  113,  113,  113,  113,},
+{    3,    4,    4,    5,   31,   31,   61,  121,  241,  241,  241,  241,  481,  481,  481,  481,},
+{    3,    4,    5,    5,    6,   63,   63,  125,  249,  497,  993,  993,  993,  993,  993, 1985,},
+{    3,    5,    6,    6,    6,    7,  127,  127,  253,  505, 1009, 2017, 4033, 4033, 4033, 4033,},
+{    3,    5,    6,    7,    7,    7,    8,  255,  255,  509, 1017, 2033, 4065, 8129,16257,16257,},
+{    3,    5,    6,    8,    8,    8,    8,    9,  511,  511, 1021, 2041, 4081, 8161,16321,32641,},
+{    3,    5,    7,    8,    9,    9,    9,    9,   10, 1023, 1023, 2045, 4089, 8177,16353,32705,},
+{    3,    5,    7,    8,   10,   10,   10,   10,   10,   11, 2047, 2047, 4093, 8185,16369,32737,},
+{    3,    5,    7,    8,   10,   11,   11,   11,   11,   11,   12, 4095, 4095, 8189,16377,32753,},
+{    3,    5,    7,    9,   10,   12,   12,   12,   12,   12,   12,   13, 8191, 8191,16381,32761,},
+{    3,    5,    7,    9,   10,   12,   13,   13,   13,   13,   13,   13,   14,16383,16383,32765,},
+{    3,    5,    7,    9,   10,   12,   14,   14,   14,   14,   14,   14,   14,   15,32767,32767,},
+{    3,    5,    7,    9,   11,   12,   14,   15,   15,   15,   15,   15,   15,   15,   16,65535,},
+};
+
 static av_always_inline void yuv2yuvX16inC_template(const int16_t *lumFilter, const int16_t **lumSrc, int lumFilterSize,
                                                     const int16_t *chrFilter, const int16_t **chrSrc, int chrFilterSize,
                                                     const int16_t **alpSrc, uint16_t *dest, uint16_t *uDest, uint16_t *vDest, uint16_t *aDest,
@@ -1858,21 +1876,22 @@ static int packedCopyWrapper(SwsContext *c, const uint8_t* src[], int srcStride[
 }
 
 #define DITHER_COPY(dst, dstStride, src, srcStride, bswap)\
+    uint16_t scale= dither_scale[dst_depth-1][src_depth-1];\
+    int shift= src_depth-dst_depth + dither_scale[src_depth-2][dst_depth-1];\
     for (i = 0; i < height; i++) {\
-        int shift= src_depth-dst_depth;\
         uint8_t *dither= dithers[src_depth-9][i&7];\
         for (j = 0; j < length-7; j+=8){\
-            dst[j+0] = (bswap(src[j+0]) + dither[0])>>shift;\
-            dst[j+1] = (bswap(src[j+1]) + dither[1])>>shift;\
-            dst[j+2] = (bswap(src[j+2]) + dither[2])>>shift;\
-            dst[j+3] = (bswap(src[j+3]) + dither[3])>>shift;\
-            dst[j+4] = (bswap(src[j+4]) + dither[4])>>shift;\
-            dst[j+5] = (bswap(src[j+5]) + dither[5])>>shift;\
-            dst[j+6] = (bswap(src[j+6]) + dither[6])>>shift;\
-            dst[j+7] = (bswap(src[j+7]) + dither[7])>>shift;\
+            dst[j+0] = (bswap(src[j+0]) + dither[0])*scale>>shift;\
+            dst[j+1] = (bswap(src[j+1]) + dither[1])*scale>>shift;\
+            dst[j+2] = (bswap(src[j+2]) + dither[2])*scale>>shift;\
+            dst[j+3] = (bswap(src[j+3]) + dither[3])*scale>>shift;\
+            dst[j+4] = (bswap(src[j+4]) + dither[4])*scale>>shift;\
+            dst[j+5] = (bswap(src[j+5]) + dither[5])*scale>>shift;\
+            dst[j+6] = (bswap(src[j+6]) + dither[6])*scale>>shift;\
+            dst[j+7] = (bswap(src[j+7]) + dither[7])*scale>>shift;\
         }\
         for (; j < length; j++)\
-            dst[j] = (bswap(src[j]) + dither[j&7])>>shift;\
+            dst[j] = (bswap(src[j]) + dither[j&7])*scale>>shift;\
         dst += dstStride;\
         src += srcStride;\
     }
@@ -1919,7 +1938,7 @@ static int planarCopyWrapper(SwsContext *c, const uint8_t* src[], int srcStride[
                         dstPtr2 += dstStride[plane]/2;
                         srcPtr  += srcStride[plane];
                     }
-                } else if (src_depth < dst_depth) {
+                } else if (src_depth <= dst_depth) {
                     for (i = 0; i < height; i++) {
                         if(isBE(c->dstFormat)){
                             for (j = 0; j < length; j++)