avcodec/hpeldsp: Fix documentation

This commit fixes two issues in the documentation:
a) The documentation for {put,avg}_pixels_tab only mentions
widths 16 and 8, although it explicitly mentions that there
are four horizontal blocksizes. This part of the patch
basically reverts e5771f4f37.
b) The restrictions on height don't match the reality. While
most users abide by it, some do not:
i) vp56.c copies a 16x12 block.
ii) indeo3 can copy an arbitrary multiple of four lines
for block widths 4, 8 and 16.
iii) SVQ3 can use block sizes luma block sizes 16x16, 8x16,
16x8, 8x8, 4x8, 8x4 and 4x4 and the corresponding
8x8, 4x8, 8x4, 4x4, 2x4, 4x2 and 2x2 chroma block sizes.

This implies that for widths 2 and 4 height can be two
and is guaranteed to be at least even. For all other widths,
height can be a multiple of four.

Furthermore, a comment for the SVQ3 blocksizes has been added.

Reviewed-by: Lynne <dev@lynne.ee>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavcodec/hpeldsp.h

@@ -31,11 +31,12 @@
 #include <stdint.h>
 #include <stddef.h>
 
-/* add and put pixel (decoding) */
-// blocksizes for hpel_pixels_func are 8x4,8x8 16x8 16x16
-// h for hpel_pixels_func is limited to {width/2, width} but never larger
-// than 16 and never smaller than 4
-typedef void (*op_pixels_func)(uint8_t *block /*align width (8 or 16)*/,
+/**
+ * Average and put pixel
+ * Widths can be 16, 8, 4 or 2. For for widths 2 and 4, h is always a positive
+ * multiple of 2; otherwise, it is a positive multiple of 4.
+ */
+typedef void (*op_pixels_func)(uint8_t *block /* align width */,
                                const uint8_t *pixels /*align 1*/,
                                ptrdiff_t line_size, int h);
 
@@ -46,8 +47,8 @@ typedef struct HpelDSPContext {
     /**
      * Halfpel motion compensation with rounding (a+b+1)>>1.
      * this is an array[4][4] of motion compensation functions for 4
-     * horizontal blocksizes (8,16) and the 4 halfpel positions<br>
-     * *pixels_tab[ 0->16xH 1->8xH ][ xhalfpel + 2*yhalfpel ]
+     * horizontal blocksizes (2,4,8,16) and the 4 halfpel positions<br>
+     * *pixels_tab[ 0->16xH 1->8xH 2->4xH 3->2xH ][ xhalfpel + 2*yhalfpel ]
      * @param block destination where the result is stored
      * @param pixels source
      * @param line_size number of bytes in a horizontal line of block
@@ -58,8 +59,8 @@ typedef struct HpelDSPContext {
     /**
      * Halfpel motion compensation with rounding (a+b+1)>>1.
      * This is an array[4][4] of motion compensation functions for 4
-     * horizontal blocksizes (8,16) and the 4 halfpel positions<br>
-     * *pixels_tab[ 0->16xH 1->8xH ][ xhalfpel + 2*yhalfpel ]
+     * horizontal blocksizes (2,4,8,16) and the 4 halfpel positions<br>
+     * *pixels_tab[ 0->16xH 1->8xH 2->4xH 3->2xH ][ xhalfpel + 2*yhalfpel ]
      * @param block destination into which the result is averaged (a+b+1)>>1
      * @param pixels source
      * @param line_size number of bytes in a horizontal line of block
@@ -85,7 +86,7 @@ typedef struct HpelDSPContext {
      * Halfpel motion compensation with no rounding (a+b)>>1.
      * this is an array[4] of motion compensation functions for 1
      * horizontal blocksize (16) and the 4 halfpel positions<br>
-     * *pixels_tab[0][ xhalfpel + 2*yhalfpel ]
+     * *pixels_tab[ xhalfpel + 2*yhalfpel ]
      * @param block destination into which the result is averaged (a+b)>>1
      * @param pixels source
      * @param line_size number of bytes in a horizontal line of block

libavcodec/svq3.c

@@ -504,6 +504,7 @@ static inline int svq3_mc_dir(SVQ3Context *s, int size, int mode,
                               int dir, int avg)
 {
     int i, j, k, mx, my, dx, dy, x, y;
+    // 0->16x16,1->8x16,2->16x8,3->8x8,4->4x8,5->8x4,6->4x4
     const int part_width    = ((size & 5) == 4) ? 4 : 16 >> (size & 1);
     const int part_height   = 16 >> ((unsigned)(size + 1) / 3);
     const int extra_width   = (mode == PREDICT_MODE) ? -16 * 6 : 0;