GLSL: Add option to emulate alpha test

Author: rdb

spirv_glsl.cpp

@@ -3563,6 +3563,11 @@ void CompilerGLSL::emit_resources()
 		fixup_implicit_builtin_block_names(execution.model);
 		break;
 
+	case ExecutionModelFragment:
+		if (options.fragment.emulate_alpha_test_func)
+			statement("uniform float SPIRV_Cross_AlphaTestRef;");
+		break;
+
 	default:
 		break;
 	}
@@ -16613,6 +16618,28 @@ void CompilerGLSL::emit_fixup()
 		if (options.vertex.flip_vert_y)
 			statement("gl_Position.y = -gl_Position.y;");
 	}
+	else if (get_entry_point().model == ExecutionModelFragment)
+	{
+		if (options.fragment.emulate_alpha_test_func == Options::Never)
+		{
+			statement("discard;");
+		}
+		else if (options.fragment.emulate_alpha_test_func != Options::Always)
+		{
+			auto *output_var = find_color_output_by_location(0);
+			if (output_var && this->get<SPIRType>(output_var->basetype).vecsize >= 4)
+			{
+				// We check for test fail, so we use the opposite operator
+				static const char *ops[Options::Always] = {
+					"", ">=", "!=", ">", "<=", "==", "<",
+				};
+				assert(options.fragment.emulate_alpha_test_func < Options::Always);
+				const char *op = ops[options.fragment.emulate_alpha_test_func];
+				statement("if (", to_expression(output_var->self), ".a ", op,
+				          " SPIRV_Cross_AlphaTestRef) discard;");
+			}
+		}
+	}
 }
 
 void CompilerGLSL::flush_phi(BlockID from, BlockID to)

spirv_glsl.hpp

@@ -164,6 +164,18 @@ class CompilerGLSL : public Compiler
 			Highp
 		};
 
+		enum CompareFunc
+		{
+			Never,
+			Less,
+			Equal,
+			LessEqual,
+			Greater,
+			NotEqual,
+			GreaterEqual,
+			Always,
+		};
+
 		struct VertexOptions
 		{
 			// "Vertex-like shader" here is any shader stage that can write BuiltInPosition.
@@ -189,6 +201,9 @@ class CompilerGLSL : public Compiler
 			// Add precision highp int in ES targets when emitting GLES source.
 			Precision default_float_precision = Mediump;
 			Precision default_int_precision = Highp;
+
+			// If this is not Always, injects an alpha test for the output with location 0.
+			CompareFunc emulate_alpha_test_func = Always;
 		} fragment;
 	};