1 files changed, 75 insertions, 0 deletions
diff --git a/source/blender/gpu/shaders/common/gpu_shader_common_curves.glsl b/source/blender/gpu/shaders/common/gpu_shader_common_curves.glsl
index 8948ed77557..db8e114ec7a 100644
--- a/source/blender/gpu/shaders/common/gpu_shader_common_curves.glsl
+++ b/source/blender/gpu/shaders/common/gpu_shader_common_curves.glsl
@@ -95,6 +95,81 @@ void curves_combined_only(float factor,
   result = mix(color, result, factor);
 }
 
+/* Contrary to standard tone curve implementations, the film-like implementation tries to preserve
+ * the hue of the colors as much as possible. To understand why this might be a problem, consider
+ * the violet color (0.5, 0.0, 1.0). If this color was to be evaluated at a power curve x^4, the
+ * color will be blue (0.0625, 0.0, 1.0). So the color changes and not just its luminosity, which
+ * is what film-like tone curves tries to avoid.
+ *
+ * First, the channels with the lowest and highest values are identified and evaluated at the
+ * curve. Then, the third channel---the median---is computed while maintaining the original hue of
+ * the color. To do that, we look at the equation for deriving the hue from RGB values. Assuming
+ * the maximum, minimum, and median channels are known, and ignoring the 1/3 period offset of the
+ * hue, the equation is:
+ *
+ *   hue = (median - min) / (max - min)                                  [1]
+ *
+ * Since we have the new values for the minimum and maximum after evaluating at the curve, we also
+ * have:
+ *
+ *   hue = (new_median - new_min) / (new_max - new_min)                  [2]
+ *
+ * Since we want the hue to be equivalent, by equating [1] and [2] and rearranging:
+ *
+ *   (new_median - new_min) / (new_max - new_min) = (median - min) / (max - min)
+ *   new_median - new_min = (new_max - new_min) * (median - min) / (max - min)
+ *   new_median = new_min + (new_max - new_min) * (median - min) / (max - min)
+ *   new_median = new_min + (median - min) * ((new_max - new_min) / (max - min))  [QED]
+ *
+ * Which gives us the median color that preserves the hue. More intuitively, the median is computed
+ * such that the change in the distance from the median to the minimum is proportional to the
+ * change in the distance from the minimum to the maximum. Finally, each of the new minimum,
+ * maximum, and median values are written to the color channel that they were originally extracted
+ * from. */
+void curves_film_like(float factor,
+                      vec4 color,
+                      vec4 black_level,
+                      vec4 white_level,
+                      sampler1DArray curve_map,
+                      const float layer,
+                      float range_minimum,
+                      float range_divider,
+                      float start_slope,
+                      float end_slope,
+                      out vec4 result)
+{
+  vec4 balanced = white_balance(color, black_level, white_level);
+
+  /* Find the maximum, minimum, and median of the color channels. */
+  float minimum = min(balanced.r, min(balanced.g, balanced.b));
+  float maximum = max(balanced.r, max(balanced.g, balanced.b));
+  float median = max(min(balanced.r, balanced.g), min(balanced.b, max(balanced.r, balanced.g)));
+
+  /* Evaluate alpha curve map at the maximum and minimum channels. The alpha curve is the Combined
+   * curve in the UI. */
+  float min_parameter = NORMALIZE_PARAMETER(minimum, range_minimum, range_divider);
+  float max_parameter = NORMALIZE_PARAMETER(maximum, range_minimum, range_divider);
+  float new_min = texture(curve_map, vec2(min_parameter, layer)).a;
+  float new_max = texture(curve_map, vec2(max_parameter, layer)).a;
+
+  /* Then, extrapolate if needed. */
+  new_min = extrapolate_if_needed(min_parameter, new_min, start_slope, end_slope);
+  new_max = extrapolate_if_needed(max_parameter, new_max, start_slope, end_slope);
+
+  /* Compute the new median using the ratio between the new and the original range. */
+  float scaling_ratio = (new_max - new_min) / (maximum - minimum);
+  float new_median = new_min + (median - minimum) * scaling_ratio;
+
+  /* Write each value to its original channel. */
+  bvec3 channel_is_min = equal(balanced.rgb, vec3(minimum));
+  vec3 median_or_min = mix(vec3(new_median), vec3(new_min), channel_is_min);
+  bvec3 channel_is_max = equal(balanced.rgb, vec3(maximum));
+  result.rgb = mix(median_or_min, vec3(new_max), channel_is_max);
+  result.a = color.a;
+
+  result = mix(color, result, clamp(factor, 0.0, 1.0));
+}
+
 void curves_vector(vec3 vector,
                    sampler1DArray curve_map,
                    const float layer,