Use proper sRGB gamma curve

README.md

@@ -65,13 +65,13 @@ Scores for version 1.1 measured against [TID2008][1] database:
 
 TID2008 Category | Spearman correlation
 --- | ---
-Noise   | -0.866
-Noise2  | -0.882
-Safe    | -0.884
+Noise   | -0.872
+Noise2  | -0.888
+Safe    | -0.889
 Hard    | -0.903
-Simple  | -0.921
-Exotic  | -0.449
-Exotic2 | -0.620
-Full    | -0.804
+Simple  | -0.922
+Exotic  | -0.484
+Exotic2 | -0.651
+Full    | -0.818
 
 [1]: http://www.computervisiononline.com/dataset/tid2008-tampere-image-database-2008

src/dssim.c

@@ -170,10 +170,25 @@ void dssim_dealloc_image(dssim_image *img)
     free(img);
 }
 
-static void set_gamma(double gamma_lut[static 256], const double invgamma)
+static int set_gamma(double gamma_lut[static 256], const double invgamma)
 {
-    for (int i = 0; i < 256; i++) {
-        gamma_lut[i] = pow(i / 255.0, 1.0 / invgamma);
+    if (invgamma == dssim_srgb_gamma) {
+        for (int i = 0; i < 256; i++) {
+            const double s = i / 255.0;
+            if (s <= 0.04045) {
+                gamma_lut[i] = s / 12.92;
+            } else {
+                gamma_lut[i] = pow((s + 0.055) / 1.055, 2.4);
+            }
+        }
+        return 1;
+    } else if (invgamma > 0 && invgamma < 1.0) {
+        for (int i = 0; i < 256; i++) {
+            gamma_lut[i] = pow(i / 255.0, 1.0 / invgamma);
+        }
+        return 1;
+    } else {
+        return 0;
     }
 }
 
@@ -202,6 +217,7 @@ inline static dssim_lab rgb_to_lab(const double gamma_lut[static 256], const uns
     };
 }
 
+#ifndef USE_COCOA
 /*
  * Flips x/y (like 90deg rotation)
  */
@@ -267,7 +283,7 @@ static void regular_1d_blur(const dssim_px_t *src, dssim_px_t *restrict tmp1, ds
         }
     }
 }
-
+#endif
 
 /*
  * blurs (approximate of gaussian)
@@ -478,14 +494,13 @@ dssim_image *dssim_create_image(dssim_attr *attr, unsigned char *const *const ro
     dssim_row_callback *converter;
     int num_channels;
 
-    if (gamma <= 0 || gamma > 1.0) {
-        return NULL;
-    }
-
     image_data im = {
         .row_pointers = (const unsigned char *const *const )row_pointers,
     };
-    set_gamma(im.gamma_lut, gamma);
+
+    if (!set_gamma(im.gamma_lut, gamma)) {
+        return NULL;
+    }
 
     switch(color_type) {
         case DSSIM_GRAY:

src/dssim.h

@@ -43,6 +43,9 @@ typedef struct {
 dssim_attr *dssim_create_attr(void);
 void dssim_dealloc_attr(dssim_attr *);
 
+// Magic number to use in place of gamma for a true sRGB curve
+static const double dssim_srgb_gamma = -47571492;
+
 /*
     Number of scales for multiscale (1 = regular SSIM). Optional weights array contains weight of each scale.
     Set before creating any images.

src/main.c

@@ -88,8 +88,17 @@ inline static unsigned char to_byte(float in) {
     return in * 256.f;
 }
 
-double get_gamma(double gamma) {
+double get_gamma(const png24_image *image) {
+    // Assume unlabelled are sRGB too
+    if (RWPNG_NONE == image->output_color || RWPNG_SRGB == image->output_color) {
+        return dssim_srgb_gamma;
+    }
+    const double gamma = image->gamma;
     if (gamma > 0 && gamma <= 1.0) {
+        // If the gamma chunk states gamma closest to sRGB that PNG can express, then assume sRGB too
+        if (RWPNG_GAMA_ONLY == image->output_color && gamma > 0.4545499 && gamma < 0.4545501) {
+            return dssim_srgb_gamma;
+        }
         return gamma;
     }
 
@@ -137,7 +146,7 @@ int main(int argc, char *const argv[])
 
     dssim_attr *attr = dssim_create_attr();
 
-    dssim_image *original = dssim_create_image(attr, image1.row_pointers, DSSIM_RGBA, image1.width, image1.height, get_gamma(image1.gamma));
+    dssim_image *original = dssim_create_image(attr, image1.row_pointers, DSSIM_RGBA, image1.width, image1.height, get_gamma(&image1));
     free(image1.row_pointers);
     free(image1.rgba_data);
 
@@ -156,7 +165,7 @@ int main(int argc, char *const argv[])
             break;
         }
 
-        dssim_image *modified = dssim_create_image(attr, image2.row_pointers, DSSIM_RGBA, image2.width, image2.height, get_gamma(image2.gamma));
+        dssim_image *modified = dssim_create_image(attr, image2.row_pointers, DSSIM_RGBA, image2.width, image2.height, get_gamma(&image2));
         free(image2.row_pointers);
         free(image2.rgba_data);