libwebp-0.5.2-rc2

third_party/libwebp/utils/rescaler.c

 // Copyright 2012 Google Inc. All Rights Reserved.
 //
 // Use of this source code is governed by a BSD-style license
 // that can be found in the COPYING file in the root of the source
 // tree. An additional intellectual property rights grant can be found
 // in the file PATENTS. All contributing project authors may
 // be found in the AUTHORS file in the root of the source tree.
 // -----------------------------------------------------------------------------
 //
 // Rescaling functions
@@ skipping 30 matching lines @@
   wrk->x_add = wrk->x_expand ? (x_sub - 1) : x_add;
   wrk->x_sub = wrk->x_expand ? (x_add - 1) : x_sub;
   if (!wrk->x_expand) {  // fx_scale is not used otherwise
     wrk->fx_scale = WEBP_RESCALER_FRAC(1, wrk->x_sub);
   }
   // vertical scaling parameters
   wrk->y_add = wrk->y_expand ? y_add - 1 : y_add;
   wrk->y_sub = wrk->y_expand ? y_sub - 1 : y_sub;
   wrk->y_accum = wrk->y_expand ? wrk->y_sub : wrk->y_add;
   if (!wrk->y_expand) {
-    // this is WEBP_RESCALER_FRAC(dst_height, x_add * y_add) without the cast.
+    // This is WEBP_RESCALER_FRAC(dst_height, x_add * y_add) without the cast.
+    // Its value is <= WEBP_RESCALER_ONE, because dst_height <= wrk->y_add, and
+    // wrk->x_add >= 1;
     const uint64_t ratio =
         (uint64_t)dst_height * WEBP_RESCALER_ONE / (wrk->x_add * wrk->y_add);
     if (ratio != (uint32_t)ratio) {
-      // We can't represent the ratio with the current fixed-point precision.
+      // When ratio == WEBP_RESCALER_ONE, we can't represent the ratio with the
+      // current fixed-point precision. This happens when src_height ==
+      // wrk->y_add (which == src_height), and wrk->x_add == 1.
       // => We special-case fxy_scale = 0, in WebPRescalerExportRow().
       wrk->fxy_scale = 0;
     } else {
       wrk->fxy_scale = (uint32_t)ratio;
     }
     wrk->fy_scale = WEBP_RESCALER_FRAC(1, wrk->y_sub);
   } else {
     wrk->fy_scale = WEBP_RESCALER_FRAC(1, wrk->x_add);
     // wrk->fxy_scale is unused here.
   }
@@ skipping 67 matching lines @@
 int WebPRescalerExport(WebPRescaler* const rescaler) {
   int total_exported = 0;
   while (WebPRescalerHasPendingOutput(rescaler)) {
     WebPRescalerExportRow(rescaler);
     ++total_exported;
   }
   return total_exported;
 }
 
 //------------------------------------------------------------------------------