From 9f226bf8c3da517c6c671badbb67518ac98b054a Mon Sep 17 00:00:00 2001
From: Pascal Massimino <pascal.massimino@gmail.com>
Date: Tue, 6 Oct 2015 00:15:18 -0700
Subject: [PATCH] rescaler: better handling of the fxy_scale=0 special case.

Change-Id: I1d1852d989519d40e672995be4f08706265271f5
---
 src/utils/rescaler.c | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/src/utils/rescaler.c b/src/utils/rescaler.c
index 78e26837..a0c5898c 100644
--- a/src/utils/rescaler.c
+++ b/src/utils/rescaler.c
@@ -48,9 +48,15 @@ void WebPRescalerInit(WebPRescaler* const wrk, int src_width, int src_height,
   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) {
-    // note the very special case where x_add = y_add = 1 cannot be represented.
-    // We special-case fxy_scale = 0 in this case, in WebPRescalerExportRow().
-    wrk->fxy_scale = WEBP_RESCALER_FRAC(dst_height, wrk->x_add * wrk->y_add);
+    // this is WEBP_RESCALER_FRAC(1, x_add * y_add) without the cast.
+    const uint64_t ratio = 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.
+      // => 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);