Clear screen to neutral gray

assets/blit.slang

@@ -8,7 +8,8 @@
 struct BlitData {
 	float4 rect;   // NDC fit rect: x0, y0, x1, y1
 	uint   selected;
-	uint   pad[3];
+	uint   mode;   // 0 = sample texture, 1 = solid background
+	uint   pad[2];
 };
 
 [[vk::push_constant]]
@@ -33,5 +34,8 @@ VSOutput main(uint vertexIndex : SV_VertexID) {
 
 [shader("fragment")]
 float4 main(VSOutput input) {
+	if (blit.mode == 1) {
+		return float4(0.18, 0.18, 0.18, 1.0);
+	}
 	return textures[NonUniformResourceIndex(blit.selected)].Sample(input.UV);
 }

src/main.cpp

@@ -50,7 +50,8 @@ struct TextureResources {
 struct BlitData {
 	f32 rect[4];   // NDC fit rect: x0, y0, x1, y1
 	u32 selected;
-	u32 pad[3];
+	u32 mode;      // 0 = sample texture, 1 = solid background
+	u32 pad[2];
 };
 
 // Typedefs for wapp arrays of our types
@@ -481,12 +482,28 @@ int main() {
 			prRhiCmdBindDescriptorSets(cb, PR_RHI_PIPELINE_BIND_POINT_GRAPHICS,
 			                           app.pipeline_layout, 0, sets);
 
+			// Draw background (fullscreen grey quad)
+			BlitData bg = {};
+			bg.rect[0] = -1.0f;
+			bg.rect[1] = -1.0f;
+			bg.rect[2] =  1.0f;
+			bg.rect[3] =  1.0f;
+			bg.selected = 0;
+			bg.mode     = 1;
+			prRhiCmdPushConstants(cb, app.pipeline_layout,
+			                      (PrRhiShaderStage)(PR_RHI_SHADER_STAGE_VERTEX |
+			                                         PR_RHI_SHADER_STAGE_FRAGMENT),
+			                      0, sizeof(BlitData), &bg);
+			prRhiCmdDraw(cb, 4, 1, 0, 0);
+
+			// Draw texture (contain-fit)
 			BlitData blit = {};
 			blit.rect[0] = fit_rect[0];
 			blit.rect[1] = fit_rect[1];
 			blit.rect[2] = fit_rect[2];
 			blit.rect[3] = fit_rect[3];
 			blit.selected = app.selected;
+			blit.mode     = 0;
 			prRhiCmdPushConstants(cb, app.pipeline_layout,
 			                      (PrRhiShaderStage)(PR_RHI_SHADER_STAGE_VERTEX |
 			                                         PR_RHI_SHADER_STAGE_FRAGMENT),