anim_diff: add a -raw_comparison flag

If this flag is not used, RGB is premultiplied before comparison.
Otherwise, the raw R/G/B values are compared, which can be a problem
in transparent area (alpha=0 R/G/B=anything)

Change-Id: I131cc10ec92414ad508b81f599a60d0097cac470

examples/anim_diff.cc

@@ -37,6 +37,7 @@ bool CompareValues(T a, T b, const std::string& output_str) {
 // is OK for other aspects like offsets, dispose/blend method to vary.
 bool CompareAnimatedImagePair(const AnimatedImage& img1,
                               const AnimatedImage& img2,
+                              bool premultiply,
                               double min_psnr) {
   bool ok = true;
   ok = CompareValues(img1.canvas_width, img2.canvas_width,
@@ -68,7 +69,7 @@ bool CompareAnimatedImagePair(const AnimatedImage& img1,
     int max_diff;
     double psnr;
     GetDiffAndPSNR(rgba1, rgba2, img1.canvas_width, img1.canvas_height,
-                   &max_diff, &psnr);
+                   premultiply, &max_diff, &psnr);
     if (min_psnr > 0.) {
       if (psnr < min_psnr) {
         fprintf(stderr, "Frame #%zu, psnr = %.2lf (min_psnr = %f)\n", i,
@@ -87,7 +88,7 @@ bool CompareAnimatedImagePair(const AnimatedImage& img1,
 
 void Help() {
   printf("\nUsage: anim_diff <image1> <image2> [-dump_frames <folder>] "
-         "[-min_psnr <float>]\n");
+         "[-min_psnr <float>][-raw_comparison]\n");
 }
 
 }  // namespace
@@ -98,6 +99,7 @@ int main(int argc, const char* argv[]) {
   double min_psnr = 0.;
   bool got_input1 = false;
   bool got_input2 = false;
+  bool premultiply = true;
   const char* files[2];
 
   if (argc < 3) {
@@ -127,6 +129,8 @@ int main(int argc, const char* argv[]) {
       } else {
         parse_error = true;
       }
+    } else if (!strcmp(argv[c], "-raw_comparison")) {
+      premultiply = false;
     } else {
       if (!got_input1) {
         files[0] = argv[c];
@@ -161,7 +165,8 @@ int main(int argc, const char* argv[]) {
     }
   }
 
-  if (!CompareAnimatedImagePair(images[0], images[1], min_psnr)) {
+  if (!CompareAnimatedImagePair(images[0], images[1],
+                                premultiply, min_psnr)) {
     fprintf(stderr, "\nFiles %s and %s differ.\n", files[0], files[1]);
     return -3;
   }

examples/anim_util.cc

@@ -815,20 +815,41 @@ bool ReadAnimatedImage(const char filename[], AnimatedImage* const image,
   }
 }
 
+static void Accumulate(double v1, double v2, double* const max_diff,
+                       double* const sse) {
+  const double diff = fabs(v1 - v2);
+  if (diff > *max_diff) *max_diff = diff;
+  *sse += diff * diff;
+}
+
 void GetDiffAndPSNR(const uint8_t rgba1[], const uint8_t rgba2[],
-                    uint32_t width, uint32_t height, int* const max_diff,
-                    double* const psnr) {
+                    uint32_t width, uint32_t height, bool premultiply,
+                    int* const max_diff, double* const psnr) {
   const uint32_t stride = width * kNumChannels;
-  *max_diff = 0;
+  const int kAlphaChannel = kNumChannels - 1;
+  double f_max_diff = 0.;
   double sse = 0.;
   for (uint32_t y = 0; y < height; ++y) {
-    for (uint32_t x = 0; x < stride; ++x) {
+    for (uint32_t x = 0; x < stride; x += kNumChannels) {
       const size_t offset = y * stride + x;
-      const int diff = abs(rgba1[offset] - rgba2[offset]);
-      if (diff > *max_diff) *max_diff = diff;
-      sse += diff * diff;
+      const int alpha1 = rgba1[offset + kAlphaChannel];
+      const int alpha2 = rgba2[offset + kAlphaChannel];
+      Accumulate(alpha1, alpha2, &f_max_diff, &sse);
+      if (!premultiply) {
+        for (int k = 0; k < kAlphaChannel; ++k) {
+          Accumulate(rgba1[offset + k], rgba2[offset + k], &f_max_diff, &sse);
+        }
+      } else {
+        // premultiply R/G/B channels with alpha value
+        for (int k = 0; k < kAlphaChannel; ++k) {
+          Accumulate(rgba1[offset + k] * alpha1 / 255.,
+                     rgba2[offset + k] * alpha2 / 255.,
+                     &f_max_diff, &sse);
         }
       }
+    }
+  }
+  *max_diff = static_cast<int>(f_max_diff);
   if (*max_diff == 0) {
     *psnr = 99.;  // PSNR when images are identical.
   } else {

examples/anim_util.h

@@ -40,8 +40,10 @@ bool ReadAnimatedImage(const char filename[], AnimatedImage* const image,
                        bool dump_frames, const char dump_folder[]);
 
 // Given two RGBA buffers, calculate max pixel difference and PSNR.
+// If 'premultiply' is true, R/G/B values will be pre-multiplied by the
+// transparency before comparison.
 void GetDiffAndPSNR(const uint8_t rgba1[], const uint8_t rgba2[],
-                    uint32_t width, uint32_t height, int* const max_diff,
-                    double* const psnr);
+                    uint32_t width, uint32_t height, bool premultiply,
+                    int* const max_diff, double* const psnr);
 
 #endif  // WEBP_EXAMPLES_ANIM_UTIL_H_