(RWEBP) The first pixel is now an exact match with the reference! MB(0,0) is 2.6 mean diff and MB(0,1) dropped from 39.6 to 7.4. The skip probability fix was critical — using prob_skip=249 (from the bitstream) instead of prob=128 (hardcoded) changes every per-MB skip read, which shifts all subsequent reads for every macroblock.

Author: LibretroAdmin

libretro-common/formats/webp/rwebp.c

@@ -1245,6 +1245,10 @@ static uint32_t *vp8_decode(const uint8_t *data, size_t len,
          { int q2 = mb_qp + uvdc_dq; uv_dc_q = vp8_dc_qlut[q2<0?0:q2>127?127:q2]; if(uv_dc_q>132)uv_dc_q=132; }
          { int q2 = mb_qp + uvac_dq; uv_ac_q = vp8_ac_qlut[q2<0?0:q2>127?127:q2]; }
 
+         /* Skip flag (after segment, before y_mode — libvpx order) */
+         if (skip_enabled)
+            is_skip = vp8b_get(&br, prob_skip);
+
          /* Y mode */
          if      (!vp8b_get(&br, vp8_ymp[0])) ym = 4; /* B_PRED first */
          else if (!vp8b_get(&br, vp8_ymp[1])) ym = 0;
@@ -1295,10 +1299,6 @@ static uint32_t *vp8_decode(const uint8_t *data, size_t len,
          else if (!vp8b_get(&br, vp8_uvmp[2])) uvm = 2;
          else uvm = 3;
 
-         /* Skip flag */
-         if (skip_enabled)
-            is_skip = vp8b_bit(&br);
-
          /* Gather prediction context */
          if (my > 0) {
             memcpy(ay, yb+(my*16-1)*ys+mx*16, 16);