Fix AA with large linewidth differences (#2953)

* scale AA padding with line width difference

* update NEWS [skip ci]

* optimize expression

Author: ffreyer

GLMakie/assets/shader/lines.geom

@@ -55,7 +55,8 @@ void emit_vertex(vec3 position, vec2 uv, int index)
     f_color     = g_color[index];
     gl_Position = vec4((position.xy / resolution), position.z, 1.0);
     f_id        = g_id[index];
-    f_thickness = g_thickness[index];
+    // linewidth scaling may shrink the effective linewidth
+    f_thickness = abs(uv.y) - AA_THICKNESS; 
     EmitVertex();
 }
 
@@ -77,7 +78,7 @@ void emit_vertex(vec3 position, vec2 uv, int index, vec4 color)
     f_color     = color;
     gl_Position = vec4((position.xy / resolution), position.z, 1.0);
     f_id        = g_id[index];
-    f_thickness = g_thickness[index];
+    f_thickness = abs(uv.y) - AA_THICKNESS;
     EmitVertex();
 }
 void emit_vertex(vec3 position, vec2 uv, vec4 color)
@@ -618,10 +619,6 @@ void draw_solid_line(bool isvalid[4])
     vec3 p2 = screen_space(gl_in[2].gl_Position); // end of current segment, start of next segment
     vec3 p3 = screen_space(gl_in[3].gl_Position); // end of next segment
 
-    // linewidth with padding for anti aliasing
-    float thickness_aa1 = g_thickness[1] + AA_THICKNESS;
-    float thickness_aa2 = g_thickness[2] + AA_THICKNESS;
-
     // determine the direction of each of the 3 segments (previous, current, next)
     vec3 v1 = p2 - p1;
     float segment_length = length(v1.xy);
@@ -641,6 +638,14 @@ void draw_solid_line(bool isvalid[4])
     vec2 n1 = vec2(-v1.y, v1.x);
     vec2 n2 = vec2(-v2.y, v2.x);
 
+    // determine stretching of AA border due to linewidth change
+    float temp = (g_thickness[2] - g_thickness[1]) / segment_length;
+    float edge_scale = sqrt(1 + temp * temp);
+
+    // linewidth with padding for anti aliasing (used for geometry)
+    float thickness_aa1 = g_thickness[1] + edge_scale * AA_THICKNESS;
+    float thickness_aa2 = g_thickness[2] + edge_scale * AA_THICKNESS;
+
     // Setup for sharp corners (see above)
     vec2 miter_a = normalize(n0 + n1);
     vec2 miter_b = normalize(n1 + n2);
@@ -717,6 +722,10 @@ void draw_solid_line(bool isvalid[4])
         segment_length += corner_offset;
     }
 
+    // scaling of uv.y due to different linewidths
+    // the padding for AA_THICKNESS should always have AA_THICKNESS width in uv
+    thickness_aa1 = g_thickness[1] / edge_scale + AA_THICKNESS;
+    thickness_aa2 = g_thickness[2] / edge_scale + AA_THICKNESS;
 
     // Generate line segment
     u1 *= px2uv;

NEWS.md

@@ -2,6 +2,8 @@
 
 ## master
 
+- Fix broken AA for lines with strongly varying linewidth [#2953](https://github.com/MakieOrg/Makie.jl/pull/2953)
+
 ## v0.19.5
 
 - Add `loop` option for GIF outputs when recording videos with `record` [#2891](https://github.com/MakieOrg/Makie.jl/pull/2891).