vulkan: Antialiasing for linear gradients

Shaders are complicated now...

gsk/vulkan/resources/linear.frag

@@ -4,11 +4,6 @@
 #include "clip.frag.glsl"
 #include "rect.frag.glsl"
 
-struct ColorStop {
-  float offset;
-  vec4 color;
-};
-
 layout(location = 0) in vec2 inPos;
 layout(location = 1) in flat Rect inRect;
 layout(location = 2) in float inGradientPos;
@@ -18,47 +13,116 @@ layout(location = 5) in flat int inStopCount;
 
 layout(location = 0) out vec4 color;
 
-ColorStop
-get_stop(int i)
+float
+get_offset (int i)
 {
-  ColorStop result;
+  return get_float (inStopOffset + i * 5);
+}
 
-  result.offset = get_float(inStopOffset + i * 5);
-  result.color = vec4(get_float(inStopOffset + i * 5 + 1),
-                      get_float(inStopOffset + i * 5 + 2),
-                      get_float(inStopOffset + i * 5 + 3),
-                      get_float(inStopOffset + i * 5 + 4));
+vec4
+get_color (int i)
+{
+  i = clamp (i, 0, inStopCount - 1);
+  return color = vec4 (get_float (inStopOffset + i * 5 + 1),
+                       get_float (inStopOffset + i * 5 + 2),
+                       get_float (inStopOffset + i * 5 + 3),
+                       get_float (inStopOffset + i * 5 + 4));
+}
+
+vec4
+get_color_for_range_unscaled (float start,
+                             float end)
+{
+  vec4 result = vec4 (0);
+  float offset;
+  int i;
+
+  for (i = 0; i < inStopCount; i++)
+    {
+      offset = get_offset (i);
+      if (offset >= start)
+        break;
+    }
+  if (i == inStopCount)
+    offset = 1;
+
+  float last_offset = i > 0 ? get_offset (i - 1) : 0;
+  vec4 last_color = get_color (i - 1);
+  vec4 color = get_color (i);
+  if (last_offset < start)
+    {
+      last_color = mix (last_color, color, (start - last_offset) / (offset - last_offset));
+      last_offset = start;
+    }
+  if (end <= start)
+    return last_color;
+
+  for (; i < inStopCount; i++)
+    {
+      offset = get_offset (i);
+      color = get_color (i);
+      if (offset >= end)
+        break;
+      result += 0.5 * (color + last_color) * (offset - last_offset);
+      last_offset = offset;
+      last_color = color;
+    }
+  if (i == inStopCount)
+    {
+      offset = 1;
+      color = get_color (i);
+    }
+  if (offset > end)
+    {
+      color = mix (last_color, color, (end - last_offset) / (offset - last_offset));
+      offset = end;
+    }
+  result += 0.5 * (color + last_color) * (offset - last_offset);
 
   return result;
 }
 
+vec4
+get_color_for_range (float start,
+                     float end)
+{
+  return get_color_for_range_unscaled (start, end) / (end - start);
+}
+
 void main()
 {
-  float pos;
+  vec4 c;
+  float pos_start, pos_end;
+  float dPos = 0.5 * abs (fwidth (inGradientPos));
 
   if (inRepeating != 0)
-    pos = fract (inGradientPos);
-  else
-    pos = clamp (inGradientPos, 0, 1);
-
-  ColorStop stop = get_stop (0);
-  float last_offset = stop.offset;
-  color = stop.color;
-  for (int i = 1; i < inStopCount; i++)
     {
-      stop = get_stop(i);
-      if (stop.offset < pos)
-        color = stop.color;
+      pos_start = inGradientPos - dPos;
+      pos_end = inGradientPos + dPos;
+      if (floor (pos_end) > floor (pos_start))
+        {
+          float fract_end = fract(pos_end);
+          float fract_start = fract(pos_start);
+          float n = floor (pos_end) - floor (pos_start);
+          if (fract_end > fract_start + 0.01)
+            c = get_color_for_range_unscaled (fract_start, fract_end);
+          else if (fract_start > fract_end + 0.01)
+            c = -get_color_for_range_unscaled (fract_end, fract_start);
+          c += get_color_for_range_unscaled (0.0, 1.0) * n;
+          c /= pos_end - pos_start;
+        }
       else
-        color = mix (color, stop.color, clamp((pos - last_offset) / (stop.offset - last_offset), 0, 1));
-      last_offset = stop.offset;
-      if (last_offset >= pos)
-        break;
+        {
+          c = get_color_for_range (fract (pos_start), fract (pos_end));
+        }
+    }
+  else
+    {
+      pos_start = clamp (inGradientPos - dPos, 0, 1);
+      pos_end = clamp (inGradientPos + dPos, 0, 1);
+      c = get_color_for_range (pos_start, pos_end);
     }
 
-  if (last_offset < pos)
-    color = mix (color, stop.color, clamp((pos - last_offset) / (1 - last_offset), 0, 1));
-
-  float alpha = color.a * rect_coverage (inRect, inPos);
-  color = clip_scaled (inPos, vec4(color.rgb, 1) * alpha);
+  float alpha = c.a * rect_coverage (inRect, inPos);
+  color = clip_scaled (inPos, vec4(c.rgb, 1) * alpha);
 }