Add an extra backbuffer color texture that can be used when an upscaler is in use.

Fixes issue #83152. Due to how BLUR_0 is reused for multiple purposes and requires being at native resolution for some post-processing effects to work, FSR2 will use an alternate texture at internal size to use as the screen texture read by shaders instead. The rendering pipeline will prefer using this texture if it exists.

servers/rendering/renderer_rd/forward_clustered/render_forward_clustered.cpp

@@ -2137,6 +2137,12 @@ void RenderForwardClustered::_render_scene(RenderDataRD *p_render_data, const Co
 		RD::get_singleton()->draw_list_end();
 	}
 
+	if (rb_data.is_valid() && using_fsr2) {
+		// Make sure the upscaled texture is initialized, but not necessarily filled, before running screen copies
+		// so it properly detect if a dedicated copy texture should be used.
+		rb->ensure_upscaled();
+	}
+
 	if (scene_state.used_screen_texture) {
 		RENDER_TIMESTAMP("Copy Screen Texture");
 
@@ -2200,7 +2206,6 @@ void RenderForwardClustered::_render_scene(RenderDataRD *p_render_data, const Co
 
 	if (rb_data.is_valid() && (using_fsr2 || using_taa)) {
 		if (using_fsr2) {
-			rb->ensure_upscaled();
 			rb_data->ensure_fsr2(fsr2_effect);
 
 			RID exposure;

servers/rendering/renderer_rd/renderer_scene_render_rd.cpp

@@ -260,15 +260,29 @@ void RendererSceneRenderRD::_render_buffers_copy_screen_texture(const RenderData
 
 	RD::get_singleton()->draw_command_begin_label("Copy screen texture");
 
-	rb->allocate_blur_textures();
-
+	StringName texture_name;
 	bool can_use_storage = _render_buffers_can_be_storage();
 	Size2i size = rb->get_internal_size();
 
+	// When upscaling, the blur texture needs to be at the target size for post-processing to work. We prefer to use a
+	// dedicated backbuffer copy texture instead if the blur texture is not an option so shader effects work correctly.
+	Size2i target_size = rb->get_target_size();
+	bool internal_size_matches = (size.width == target_size.width) && (size.height == target_size.height);
+	bool reuse_blur_texture = !rb->has_upscaled_texture() || internal_size_matches;
+	if (reuse_blur_texture) {
+		rb->allocate_blur_textures();
+		texture_name = RB_TEX_BLUR_0;
+	} else {
+		uint32_t usage_bits = RD::TEXTURE_USAGE_SAMPLING_BIT | RD::TEXTURE_USAGE_CAN_COPY_TO_BIT;
+		usage_bits |= can_use_storage ? RD::TEXTURE_USAGE_STORAGE_BIT : RD::TEXTURE_USAGE_COLOR_ATTACHMENT_BIT;
+		rb->create_texture(RB_SCOPE_BUFFERS, RB_TEX_BACK_COLOR, rb->get_base_data_format(), usage_bits);
+		texture_name = RB_TEX_BACK_COLOR;
+	}
+
 	for (uint32_t v = 0; v < rb->get_view_count(); v++) {
 		RID texture = rb->get_internal_texture(v);
-		int mipmaps = int(rb->get_texture_format(RB_SCOPE_BUFFERS, RB_TEX_BLUR_0).mipmaps);
-		RID dest = rb->get_texture_slice(RB_SCOPE_BUFFERS, RB_TEX_BLUR_0, v, 0);
+		int mipmaps = int(rb->get_texture_format(RB_SCOPE_BUFFERS, texture_name).mipmaps);
+		RID dest = rb->get_texture_slice(RB_SCOPE_BUFFERS, texture_name, v, 0);
 
 		if (can_use_storage) {
 			copy_effects->copy_to_rect(texture, dest, Rect2i(0, 0, size.x, size.y));
@@ -279,8 +293,8 @@ void RendererSceneRenderRD::_render_buffers_copy_screen_texture(const RenderData
 
 		for (int i = 1; i < mipmaps; i++) {
 			RID source = dest;
-			dest = rb->get_texture_slice(RB_SCOPE_BUFFERS, RB_TEX_BLUR_0, v, i);
-			Size2i msize = rb->get_texture_slice_size(RB_SCOPE_BUFFERS, RB_TEX_BLUR_0, i);
+			dest = rb->get_texture_slice(RB_SCOPE_BUFFERS, texture_name, v, i);
+			Size2i msize = rb->get_texture_slice_size(RB_SCOPE_BUFFERS, texture_name, i);
 
 			if (can_use_storage) {
 				copy_effects->make_mipmap(source, dest, msize);

servers/rendering/renderer_rd/storage_rd/render_scene_buffers_rd.h

@@ -58,6 +58,7 @@
 #define RB_TEX_BLUR_1 SNAME("blur_1")
 #define RB_TEX_HALF_BLUR SNAME("half_blur") // only for raster!
 
+#define RB_TEX_BACK_COLOR SNAME("back_color")
 #define RB_TEX_BACK_DEPTH SNAME("back_depth")
 
 class RenderSceneBuffersRD : public RenderSceneBuffers {
@@ -267,7 +268,16 @@ public:
 	}
 
 	// back buffer (color)
-	RID get_back_buffer_texture() const { return has_texture(RB_SCOPE_BUFFERS, RB_TEX_BLUR_0) ? get_texture(RB_SCOPE_BUFFERS, RB_TEX_BLUR_0) : RID(); } // We (re)use our blur texture here.
+	RID get_back_buffer_texture() const {
+		// Prefer returning the dedicated backbuffer color texture if it was created. Return the reused blur texture otherwise.
+		if (has_texture(RB_SCOPE_BUFFERS, RB_TEX_BACK_COLOR)) {
+			return get_texture(RB_SCOPE_BUFFERS, RB_TEX_BACK_COLOR);
+		} else if (has_texture(RB_SCOPE_BUFFERS, RB_TEX_BLUR_0)) {
+			return get_texture(RB_SCOPE_BUFFERS, RB_TEX_BLUR_0);
+		} else {
+			return RID();
+		}
+	}
 
 	// Upscaled.
 	void ensure_upscaled();