Add PREMUL_ALPHA builtin

servers/rendering/renderer_rd/forward_clustered/scene_shader_forward_clustered.cpp

@@ -159,6 +159,7 @@ void SceneShaderForwardClustered::ShaderData::set_code(const String &p_code) {
 	uses_fragment_time = gen_code.uses_fragment_time;
 	uses_normal |= uses_normal_map;
 	uses_tangent |= uses_normal_map;
+	uses_premul_alpha = gen_code.uses_premul_alpha;
 
 #if 0
 	print_line("**compiling shader:");
@@ -186,6 +187,10 @@ void SceneShaderForwardClustered::ShaderData::set_code(const String &p_code) {
 
 	//blend modes
 
+	if (uses_premul_alpha) {
+		blend_mode = BLEND_MODE_PREMUL_ALPHA;
+	}
+
 	// if any form of Alpha Antialiasing is enabled, set the blend mode to alpha to coverage
 	if (alpha_antialiasing_mode != ALPHA_ANTIALIASING_OFF) {
 		blend_mode = BLEND_MODE_ALPHA_TO_COVERAGE;
@@ -244,6 +249,16 @@ void SceneShaderForwardClustered::ShaderData::set_code(const String &p_code) {
 			blend_attachment.dst_color_blend_factor = RD::BLEND_FACTOR_ONE_MINUS_SRC_ALPHA;
 			blend_attachment.src_alpha_blend_factor = RD::BLEND_FACTOR_ONE;
 			blend_attachment.dst_alpha_blend_factor = RD::BLEND_FACTOR_ZERO;
+		} break;
+		case BLEND_MODE_PREMUL_ALPHA: {
+			blend_attachment.enable_blend = true;
+			blend_attachment.alpha_blend_op = RD::BLEND_OP_ADD;
+			blend_attachment.color_blend_op = RD::BLEND_OP_ADD;
+			blend_attachment.src_color_blend_factor = RD::BLEND_FACTOR_ONE;
+			blend_attachment.dst_color_blend_factor = RD::BLEND_FACTOR_ONE_MINUS_SRC_ALPHA;
+			blend_attachment.src_alpha_blend_factor = RD::BLEND_FACTOR_ONE;
+			blend_attachment.dst_alpha_blend_factor = RD::BLEND_FACTOR_ONE_MINUS_SRC_ALPHA;
+			uses_blend_alpha = true;
 		}
 	}
 
@@ -592,6 +607,7 @@ void SceneShaderForwardClustered::init(const String p_defines) {
 		actions.renames["NORMAL_MAP_DEPTH"] = "normal_map_depth";
 		actions.renames["ALBEDO"] = "albedo";
 		actions.renames["ALPHA"] = "alpha";
+		actions.renames["PREMUL_ALPHA"] = "premul_alpha";
 		actions.renames["METALLIC"] = "metallic";
 		actions.renames["SPECULAR"] = "specular";
 		actions.renames["ROUGHNESS"] = "roughness";
@@ -669,6 +685,7 @@ void SceneShaderForwardClustered::init(const String p_defines) {
 		actions.usage_defines["COLOR"] = "#define COLOR_USED\n";
 		actions.usage_defines["INSTANCE_CUSTOM"] = "#define ENABLE_INSTANCE_CUSTOM\n";
 		actions.usage_defines["POSITION"] = "#define OVERRIDE_POSITION\n";
+		actions.usage_defines["PREMUL_ALPHA"] = "#define PREMUL_ALPHA_USED\n";
 
 		actions.usage_defines["ALPHA_SCISSOR_THRESHOLD"] = "#define ALPHA_SCISSOR_USED\n";
 		actions.usage_defines["ALPHA_HASH_SCALE"] = "#define ALPHA_HASH_USED\n";

servers/rendering/renderer_rd/forward_clustered/scene_shader_forward_clustered.h

@@ -106,7 +106,8 @@ class SceneShaderForwardClustered {
 			BLEND_MODE_ADD,
 			BLEND_MODE_SUB,
 			BLEND_MODE_MUL,
-			BLEND_MODE_ALPHA_TO_COVERAGE
+			BLEND_MODE_ALPHA_TO_COVERAGE,
+			BLEND_MODE_PREMUL_ALPHA,
 		};
 
 		enum DepthDraw {
@@ -182,6 +183,7 @@ class SceneShaderForwardClustered {
 		bool writes_modelview_or_projection = false;
 		bool uses_world_coordinates = false;
 		bool uses_screen_texture_mipmaps = false;
+		bool uses_premul_alpha = false;
 		Cull cull_mode = CULL_DISABLED;
 
 		uint64_t last_pass = 0;

servers/rendering/renderer_rd/shaders/forward_clustered/scene_forward_clustered.glsl

@@ -871,6 +871,9 @@ uint cluster_get_range_clip_mask(uint i, uint z_min, uint z_max) {
 void fragment_shader(in SceneData scene_data) {
 	uint instance_index = instance_index_interp;
 
+#ifdef PREMUL_ALPHA_USED
+	float premul_alpha = 1.0;
+#endif // PREMUL_ALPHA_USED
 	//lay out everything, whatever is unused is optimized away anyway
 	vec3 vertex = vertex_interp;
 #ifdef USE_MULTIVIEW
@@ -2401,6 +2404,10 @@ void fragment_shader(in SceneData scene_data) {
 
 	motion_vector = prev_position_uv - position_uv;
 #endif
+
+#if defined(PREMUL_ALPHA_USED) && !defined(MODE_RENDER_DEPTH)
+	frag_color.rgb *= premul_alpha;
+#endif //PREMUL_ALPHA_USED
 }
 
 void main() {

servers/rendering/shader_compiler.cpp

@@ -965,6 +965,10 @@ String ShaderCompiler::_dump_node_code(const SL::Node *p_node, int p_level, Gene
 				}
 			}
 
+			if (vnode->name == premul_alpha_name) {
+				r_gen_code.uses_premul_alpha = true;
+			}
+
 			if (vnode->name == time_name) {
 				if (p_actions.entry_point_stages.has(current_func_name) && p_actions.entry_point_stages[current_func_name] == STAGE_VERTEX) {
 					r_gen_code.uses_vertex_time = true;
@@ -1564,6 +1568,7 @@ void ShaderCompiler::initialize(DefaultIdentifierActions p_actions) {
 	actions = p_actions;
 
 	time_name = "TIME";
+	premul_alpha_name = "PREMUL_ALPHA";
 
 	List<String> func_list;
 

servers/rendering/shader_compiler.h

@@ -84,6 +84,7 @@ class ShaderCompiler {
 		bool uses_screen_texture;
 		bool uses_depth_texture;
 		bool uses_normal_roughness_texture;
+		bool uses_premul_alpha;
 	};
 
 	struct DefaultIdentifierActions {
@@ -115,6 +116,7 @@ class ShaderCompiler {
 	const ShaderLanguage::FunctionNode *function = nullptr;
 	StringName current_func_name;
 	StringName time_name;
+	StringName premul_alpha_name;
 	HashSet<StringName> texture_functions;
 
 	HashSet<StringName> used_name_defines;

servers/rendering/shader_types.cpp

@@ -122,6 +122,7 @@ ShaderTypes::ShaderTypes() {
 	shader_modes[RS::SHADER_SPATIAL].functions["fragment"].built_ins["UV2"] = constt(ShaderLanguage::TYPE_VEC2);
 	shader_modes[RS::SHADER_SPATIAL].functions["fragment"].built_ins["COLOR"] = constt(ShaderLanguage::TYPE_VEC4);
 	shader_modes[RS::SHADER_SPATIAL].functions["fragment"].built_ins["ALBEDO"] = ShaderLanguage::TYPE_VEC3;
+	shader_modes[RS::SHADER_SPATIAL].functions["fragment"].built_ins["PREMUL_ALPHA"] = ShaderLanguage::TYPE_FLOAT;
 	shader_modes[RS::SHADER_SPATIAL].functions["fragment"].built_ins["ALPHA"] = ShaderLanguage::TYPE_FLOAT;
 	shader_modes[RS::SHADER_SPATIAL].functions["fragment"].built_ins["METALLIC"] = ShaderLanguage::TYPE_FLOAT;
 	shader_modes[RS::SHADER_SPATIAL].functions["fragment"].built_ins["SPECULAR"] = ShaderLanguage::TYPE_FLOAT;