Fix T80974 GPU: Wide Line emulation shader does not always works

This was caused by points well behind the near clip making the
computation of the width wrong.

The fix is to clip the line to the near clip plane.

1 files changed, 27 insertions, 10 deletions

diff --git a/source/blender/gpu/shaders/gpu_shader_3D_polyline_geom.glsl b/source/blender/gpu/shaders/gpu_shader_3D_polyline_geom.glsl
index cca94680284..fd9b7e221e6 100644
--- a/source/blender/gpu/shaders/gpu_shader_3D_polyline_geom.glsl
+++ b/source/blender/gpu/shaders/gpu_shader_3D_polyline_geom.glsl
@@ -20,7 +20,23 @@ noperspective out float smoothline;
 
 #define SMOOTH_WIDTH 1.0
 
-void do_vertex(const int i, vec2 ofs)
+/* Clips point to near clip plane before perspective divide. */
+vec4 clip_line_point_homogeneous_space(vec4 p, vec4 q)
+{
+  if (p.z < -p.w) {
+    /* Just solves p + (q - p) * A; for A when p.z / p.w = -1.0. */
+    float denom = q.z - p.z + q.w - p.w;
+    if (denom == 0.0) {
+      /* No solution. */
+      return p;
+    }
+    float A = (-p.z - p.w) / denom;
+    p = p + (q - p) * A;
+  }
+  return p;
+}
+
+void do_vertex(const int i, vec4 pos, vec2 ofs)
 {
 #if defined(UNIFORM)
   finalColor = color;
@@ -38,21 +54,22 @@ void do_vertex(const int i, vec2 ofs)
 #endif
 
   smoothline = (lineWidth + SMOOTH_WIDTH) * 0.5;
-  gl_Position = gl_in[i].gl_Position;
-  gl_Position.xy += ofs * gl_Position.w;
+  gl_Position = pos;
+  gl_Position.xy += ofs * pos.w;
   EmitVertex();
 
   smoothline = -(lineWidth + SMOOTH_WIDTH) * 0.5;
-  gl_Position = gl_in[i].gl_Position;
-  gl_Position.xy -= ofs * gl_Position.w;
+  gl_Position = pos;
+  gl_Position.xy -= ofs * pos.w;
   EmitVertex();
 }
 
 void main(void)
 {
-  vec2 p0 = gl_in[0].gl_Position.xy / gl_in[0].gl_Position.w;
-  vec2 p1 = gl_in[1].gl_Position.xy / gl_in[1].gl_Position.w;
-  vec2 e = normalize((p1 - p0) * viewportSize.xy);
+  vec4 p0 = clip_line_point_homogeneous_space(gl_in[0].gl_Position, gl_in[1].gl_Position);
+  vec4 p1 = clip_line_point_homogeneous_space(gl_in[1].gl_Position, gl_in[0].gl_Position);
+  vec2 e = normalize(((p1.xy / p1.w) - (p0.xy / p0.w)) * viewportSize.xy);
+
 #if 0 /* Hard turn when line direction changes quadrant. */
   e = abs(e);
   vec2 ofs = (e.x > e.y) ? vec2(0.0, 1.0 / e.x) : vec2(1.0 / e.y, 0.0);
@@ -62,8 +79,8 @@ void main(void)
   ofs /= viewportSize.xy;
   ofs *= lineWidth + SMOOTH_WIDTH;
 
-  do_vertex(0, ofs);
-  do_vertex(1, ofs);
+  do_vertex(0, p0, ofs);
+  do_vertex(1, p1, ofs);
 
   EndPrimitive();
 }