utils/rescaler: add WebPRescalerGetScaledDimensions

+ use it in WebPPictureRescale()

Change-Id: I491bea8cd56f0eb1ac8bf0829b9f36c77804219a

src/enc/picture_rescale.c

@@ -211,16 +211,10 @@ int WebPPictureRescale(WebPPicture* pic, int width, int height) {
   if (pic == NULL) return 0;
   prev_width = pic->width;
   prev_height = pic->height;
-  // if width is unspecified, scale original proportionally to height ratio.
+  if (!WebPRescalerGetScaledDimensions(
-  if (width == 0) {
+          prev_width, prev_height, &width, &height)) {
-    width = (prev_width * height + prev_height / 2) / prev_height;
+    return 0;
   }
-  // if height is unspecified, scale original proportionally to width ratio.
-  if (height == 0) {
-    height = (prev_height * width + prev_width / 2) / prev_width;
-  }
-  // Check if the overall dimensions still make sense.
-  if (width <= 0 || height <= 0) return 0;
 
   PictureGrabSpecs(pic, &tmp);
   tmp.width = width;

src/utils/rescaler.c

@@ -50,6 +50,34 @@ void WebPRescalerInit(WebPRescaler* const wrk, int src_width, int src_height,
   WebPRescalerDspInit();
 }
 
+int WebPRescalerGetScaledDimensions(int src_width, int src_height,
+                                    int* const scaled_width,
+                                    int* const scaled_height) {
+  assert(scaled_width != NULL);
+  assert(scaled_height != NULL);
+  {
+    int width = *scaled_width;
+    int height = *scaled_height;
+
+    // if width is unspecified, scale original proportionally to height ratio.
+    if (width == 0) {
+      width = (src_width * height + src_height / 2) / src_height;
+    }
+    // if height is unspecified, scale original proportionally to width ratio.
+    if (height == 0) {
+      height = (src_height * width + src_width / 2) / src_width;
+    }
+    // Check if the overall dimensions still make sense.
+    if (width <= 0 || height <= 0) {
+      return 0;
+    }
+
+    *scaled_width = width;
+    *scaled_height = height;
+    return 1;
+  }
+}
+
 //------------------------------------------------------------------------------
 // all-in-one calls
 

src/utils/rescaler.h

@@ -48,6 +48,14 @@ void WebPRescalerInit(WebPRescaler* const rescaler,
                       int num_channels,
                       int32_t* const work);
 
+// If either 'scaled_width' or 'scaled_height' (but not both) is 0 the value
+// will be calculated preserving the aspect ratio, otherwise the values are
+// left unmodified. Returns true on success, false if either value is 0 after
+// performing the scaling calculation.
+int WebPRescalerGetScaledDimensions(int src_width, int src_height,
+                                    int* const scaled_width,
+                                    int* const scaled_height);
+
 // Returns the number of input lines needed next to produce one output line,
 // considering that the maximum available input lines are 'max_num_lines'.
 int WebPRescaleNeededLines(const WebPRescaler* const rescaler,