Nodes: simplify node link drawing shader

* Better separation between drawing backdrop and main line.
* Pass u and v coordinates of line to fragment shader for further processing.
* Remove `colorGradient` which can also be computed from `lineUV`.
* Simplify drawing potentially more than one parallel line as is done in #112677.

source/blender/gpu/shaders/gpu_shader_2D_nodelink_frag.glsl

@@ -2,15 +2,25 @@
  *
  * SPDX-License-Identifier: GPL-2.0-or-later */
 
-#define ANTIALIAS 1.5
-#define MINIMUM_ALPHA 0.5
+#define ANTIALIAS 0.75
+
+float get_line_alpha(float center, float relative_radius) {
+  float radius = relative_radius * lineThickness;
+  float sdf = abs(lineThickness * (lineUV.y - center));
+  return smoothstep(radius, radius - ANTIALIAS, sdf);
+}
 
 void main()
 {
-  fragColor = finalColor;
+  if (isMainLine == 0) {
+    fragColor = finalColor;
+    fragColor.a *= get_line_alpha(0.5, 0.5);
+    return;
+  }
 
-  if ((isMainLine != 0) && (dashFactor < 1.0)) {
-    float distance_along_line = lineLength * lineU;
+  float dash_frag_alpha = 1.0;
+  if (dashFactor < 1.0) {
+    float distance_along_line = lineLength * lineUV.x;
     float normalized_distance = fract(distance_along_line / dashLength);
 
     /* Checking if `normalized_distance <= dashFactor` is already enough for a basic
@@ -25,8 +35,9 @@ void main()
     float unclamped_alpha = 1.0 - slope * (normalized_distance_triangle - dashFactor + t);
     float alpha = max(dashAlpha, min(unclamped_alpha, 1.0));
 
-    fragColor.a *= alpha;
+    dash_frag_alpha = alpha;
   }
 
-  fragColor.a *= smoothstep(lineThickness, lineThickness - ANTIALIAS, abs(colorGradient));
+  fragColor = finalColor;
+  fragColor.a *= get_line_alpha(0.5, 0.5) * dash_frag_alpha;
 }

source/blender/gpu/shaders/gpu_shader_2D_nodelink_vert.glsl

@@ -42,8 +42,9 @@ void main(void)
 #endif
 
   float line_thickness = thickness;
+  bool is_outline_pass = gl_VertexID < MID_VERTEX;
 
-  if (gl_VertexID < MID_VERTEX) {
+  if (is_outline_pass) {
     /* Outline pass. */
     finalColor = colShadow;
   }
@@ -67,15 +68,15 @@ void main(void)
   }
 
   aspect = node_link_data.aspect;
+  isMainLine = expand.y == 1.0 && !is_outline_pass ? 1 : 0;
   /* Parameters for the dashed line. */
-  isMainLine = expand.y != 1.0 ? 0 : 1;
   dashLength = dash_params.x;
   dashFactor = dash_params.y;
   dashAlpha = dash_params.z;
   /* Approximate line length, no need for real bezier length calculation. */
   lineLength = distance(P0, P3);
   /* TODO: Incorrect U, this leads to non-uniform dash distribution. */
-  lineU = uv.x;
+  lineUV = uv;
 
   float t = uv.x;
   float t2 = t * t;
@@ -106,7 +107,6 @@ void main(void)
              ModelViewProjectionMatrix[1].xy * exp_axis.yy;
 
   float expand_dist = line_thickness * (uv.y * 2.0 - 1.0);
-  colorGradient = expand_dist;
   lineThickness = line_thickness;
 
   finalColor[3] *= dim_factor;

source/blender/gpu/shaders/infos/gpu_shader_2D_nodelink_info.hh

@@ -10,8 +10,7 @@
 
 GPU_SHADER_INTERFACE_INFO(nodelink_iface, "")
     .smooth(Type::VEC4, "finalColor")
-    .smooth(Type::FLOAT, "colorGradient")
-    .smooth(Type::FLOAT, "lineU")
+    .smooth(Type::VEC2, "lineUV")
     .flat(Type::FLOAT, "lineLength")
     .flat(Type::FLOAT, "lineThickness")
     .flat(Type::FLOAT, "dashLength")