[libpng16] Made PNG_NO_READ_GAMMA switch off interfaces that depend on

READ_GAMMA.  Prior to 1.6.0 switching off READ_GAMMA did unpredictable things
to the interfaces that use it (specifically, png_do_background in 1.4 would
simply display composite for grayscale images but do composition
with the incorrect arithmetic for color ones). In 1.6 the semantic
of -DPNG_NO_READ_GAMMA is changed to simply disable any interface that
depends on it; this obliges people who set it to consider whether they
really want it off if they happen to use any of the interfaces in
question (typically most users who disable it won't).

contrib/libtests/pngvalid.c

@@ -244,6 +244,7 @@ random_mod(unsigned int max)
    return x % max; /* 0 .. max-1 */
 }
 
+#ifdef PNG_READ_RGB_TO_GRAY_SUPPORTED
 static int
 random_choice(void)
 {
@@ -253,6 +254,7 @@ random_choice(void)
 
    return x & 1;
 }
+#endif
 #endif /* PNG_READ_SUPPORTED */
 
 /* A numeric ID based on PNG file characteristics.  The 'do_interlace' field
@@ -1714,6 +1716,7 @@ white_point(PNG_CONST color_encoding *encoding)
    return white;
 }
 
+#ifdef PNG_READ_RGB_TO_GRAY_SUPPORTED
 static void
 normalize_color_encoding(color_encoding *encoding)
 {
@@ -1733,6 +1736,7 @@ normalize_color_encoding(color_encoding *encoding)
       encoding->blue.Z /= whiteY;
    }
 }
+#endif
 
 static size_t
 safecat_color_encoding(char *buffer, size_t bufsize, size_t pos,
@@ -1967,6 +1971,7 @@ modifier_init(png_modifier *pm)
  * to a calculation - not a digitization operation - unless the following API is
  * called directly.
  */
+#ifdef PNG_READ_RGB_TO_GRAY_SUPPORTED
 static double digitize(PNG_CONST png_modifier *pm, double value,
    int sample_depth, int do_round)
 {
@@ -1991,7 +1996,10 @@ static double digitize(PNG_CONST png_modifier *pm, double value,
    if (do_round) value += .5;
    return floor(value)/digitization_factor;
 }
+#endif
 
+#if (defined PNG_READ_GAMMA_SUPPORTED) ||\
+   (defined PNG_READ_RGB_TO_GRAY_SUPPORTED)
 static double abserr(PNG_CONST png_modifier *pm, int in_depth, int out_depth)
 {
    /* Absolute error permitted in linear values - affected by the bit depth of
@@ -2003,6 +2011,7 @@ static double abserr(PNG_CONST png_modifier *pm, int in_depth, int out_depth)
    else
       return pm->maxabs8;
 }
+#endif
 
 #ifdef PNG_READ_GAMMA_SUPPORTED
 static double calcerr(PNG_CONST png_modifier *pm, int in_depth, int out_depth)
@@ -2174,6 +2183,7 @@ modification_init(png_modification *pmm)
    modification_reset(pmm);
 }
 
+#ifdef PNG_READ_RGB_TO_GRAY_SUPPORTED
 static void
 modifier_current_encoding(PNG_CONST png_modifier *pm, color_encoding *ce)
 {
@@ -2185,6 +2195,7 @@ modifier_current_encoding(PNG_CONST png_modifier *pm, color_encoding *ce)
 
    ce->gamma = pm->current_gamma;
 }
+#endif
 
 static size_t
 safecat_current_encoding(char *buffer, size_t bufsize, size_t pos,
@@ -6004,6 +6015,7 @@ image_transform_default_ini(PNG_CONST image_transform *this,
    this->next->ini(this->next, that);
 }
 
+#ifdef PNG_READ_BACKGROUND_SUPPORTED
 static int
 image_transform_default_add(image_transform *this,
     PNG_CONST image_transform **that, png_byte colour_type, png_byte bit_depth)
@@ -6016,6 +6028,7 @@ image_transform_default_add(image_transform *this,
 
    return 1;
 }
+#endif
 
 #ifdef PNG_READ_EXPAND_SUPPORTED
 /* png_set_palette_to_rgb */
@@ -7694,6 +7707,7 @@ gamma_component_compose(int do_background, double input_sample, double alpha,
 {
    switch (do_background)
    {
+#ifdef PNG_READ_BACKGROUND_SUPPORTED
       case PNG_BACKGROUND_GAMMA_SCREEN:
       case PNG_BACKGROUND_GAMMA_FILE:
       case PNG_BACKGROUND_GAMMA_UNIQUE:
@@ -7711,6 +7725,7 @@ gamma_component_compose(int do_background, double input_sample, double alpha,
                input_sample = background;
          }
          break;
+#endif
 
 #ifdef PNG_READ_ALPHA_MODE_SUPPORTED
       case ALPHA_MODE_OFFSET + PNG_ALPHA_STANDARD:
@@ -7743,6 +7758,9 @@ gamma_component_compose(int do_background, double input_sample, double alpha,
          /* Standard cases where no compositing is done (so the component
           * value is already correct.)
           */
+         UNUSED(alpha)
+         UNUSED(background)
+         UNUSED(compose)
          break;
    }
 
@@ -8142,11 +8160,13 @@ gamma_component_validate(PNG_CONST char *name, PNG_CONST validate_info *vi,
              */
             switch (do_background)
             {
-            case PNG_BACKGROUND_GAMMA_SCREEN:
-            case PNG_BACKGROUND_GAMMA_FILE:
-            case PNG_BACKGROUND_GAMMA_UNIQUE:
-               use_background = (alpha >= 0 && alpha < 1);
-               /*FALL THROUGH*/
+#           ifdef PNG_READ_BACKGROUND_SUPPORTED
+               case PNG_BACKGROUND_GAMMA_SCREEN:
+               case PNG_BACKGROUND_GAMMA_FILE:
+               case PNG_BACKGROUND_GAMMA_UNIQUE:
+                  use_background = (alpha >= 0 && alpha < 1);
+                  /*FALL THROUGH*/
+#           endif
 #           ifdef PNG_READ_ALPHA_MODE_SUPPORTED
                case ALPHA_MODE_OFFSET + PNG_ALPHA_STANDARD:
                case ALPHA_MODE_OFFSET + PNG_ALPHA_BROKEN:
@@ -9065,7 +9085,9 @@ perform_gamma_test(png_modifier *pm, int summary)
    /* Save certain values for the temporary overrides below. */
    unsigned int calculations_use_input_precision =
       pm->calculations_use_input_precision;
-   double maxout8 = pm->maxout8;
+#  ifdef PNG_READ_BACKGROUND_SUPPORTED
+      double maxout8 = pm->maxout8;
+#  endif
 
    /* First some arbitrary no-transform tests: */
    if (!pm->this.speed && pm->test_gamma_threshold)