doing blur shader without alpha blending

src/render/d3d11/render_d3d11.mdesk

@@ -226,6 +226,7 @@ struct Vertex2Pixel
   float4 position        : SV_POSITION;
   float2 texcoord        : TEX;
   float2 sdf_sample_pos  : SDF;
+  nointerpolation float2 rect_half_size : RHS;
   float corner_radius    : RAD;
 };
 
@@ -270,6 +271,7 @@ vs_main(CPU2Vertex c2v)
     v2p.position = float4(vertex_position__scr.x, -vertex_position__scr.y, 0.f, 1.f);
     v2p.texcoord = vertex_position__pct;
     v2p.sdf_sample_pos = (2.f * cornercoords__pct - 1.f) * rect_half_size;
+    v2p.rect_half_size = rect_half_size - 2.f;
     v2p.corner_radius = corner_radii__px[c2v.vertex_id];
   }
   return v2p;
@@ -281,33 +283,28 @@ float4
 ps_main(Vertex2Pixel v2p) : SV_TARGET
 {
   // rjf: blend weighted texture samples into color
-  float4 color = kernel[0].x * stage_t2d.Sample(stage_sampler, v2p.texcoord);
-  color.a = kernel[0].x;
+  float3 color = kernel[0].x * stage_t2d.Sample(stage_sampler, v2p.texcoord).rgb;
   
   for(uint i = 1; i < blur_count; i += 1)
   {
     float weight = kernel[i].x;
     float offset = kernel[i].y;
-    float4 min_sample = stage_t2d.Sample(stage_sampler, v2p.texcoord - offset * direction);
-    float4 max_sample = stage_t2d.Sample(stage_sampler, v2p.texcoord + offset * direction);
-    min_sample.a = 1.f;
-    max_sample.a = 1.f;
-    color += min_sample * weight;
-    color += max_sample * weight;
+    color += weight * stage_t2d.Sample(stage_sampler, v2p.texcoord - offset * direction).rgb;
+    color += weight * stage_t2d.Sample(stage_sampler, v2p.texcoord + offset * direction).rgb;
   }
   
-  // rjf: determine SDF sample position
-  float2 rect_half_size = float2((rect.z-rect.x)/2, (rect.w-rect.y)/2);
-  float2 sdf_sample_pos = v2p.sdf_sample_pos;
-  
   // rjf: sample for corners
-  float corner_sdf_s = rect_sdf(sdf_sample_pos, rect_half_size - 2.f, v2p.corner_radius);
+  float corner_sdf_s = rect_sdf(v2p.sdf_sample_pos, v2p.rect_half_size, v2p.corner_radius);
   float corner_sdf_t = 1-smoothstep(0, 2, corner_sdf_s);
   
   // rjf: weight output color by sdf
-  color.a *= corner_sdf_t;
+  // this is doing alpha testing, leave blurring only where mostly opaque pixels are
+  if (corner_sdf_t < 0.9f)
+  {
+    discard;
+  }
   
-  return color;
+  return float4(color, 1.f);
 }
 """