[Impeller] Implement framebuffer fetch support for OpenGLES.

impeller/compiler/compiler.cc

@@ -155,6 +155,10 @@ static CompilerBackend CreateGLSLCompiler(const spirv_cross::ParsedIR& ir,
                              ? source_options.gles_language_version
                              : 100;
     sl_options.es = true;
+    if (source_options.require_framebuffer_fetch &&
+        source_options.type == SourceType::kFragmentShader) {
+      gl_compiler->remap_ext_framebuffer_fetch(0, 0, true);
+    }
     gl_compiler->set_variable_type_remap_callback(
         [&](const spirv_cross::SPIRType& type, const std::string& var_name,
             std::string& name_of_type) {

impeller/compiler/impellerc_main.cc

@@ -76,6 +76,7 @@ bool Main(const fml::CommandLine& command_line) {
   options.gles_language_version = switches.gles_language_version;
   options.metal_version = switches.metal_version;
   options.use_half_textures = switches.use_half_textures;
+  options.require_framebuffer_fetch = switches.require_framebuffer_fetch;
 
   Reflector::Options reflector_options;
   reflector_options.target_platform = switches.target_platform;

impeller/compiler/source_options.h

@@ -33,6 +33,11 @@ struct SourceOptions {
   /// opengl semantics. Only used on metal targets.
   bool use_half_textures = false;
 
+  /// @brief Whether the GLSL framebuffer fetch extension will be required.
+  ///
+  /// Only used on OpenGLES targets.
+  bool require_framebuffer_fetch = false;
+
   SourceOptions();
 
   ~SourceOptions();

impeller/compiler/switches.cc

@@ -75,6 +75,7 @@ void Switches::PrintHelp(std::ostream& stream) {
   stream << "[optional] --use-half-textures (force openGL semantics when "
             "targeting metal)"
          << std::endl;
+  stream << "[optional] --require-framebuffer-fetch" << std::endl;
 }
 
 Switches::Switches() = default;
@@ -136,7 +137,9 @@ Switches::Switches(const fml::CommandLine& command_line)
           command_line.GetOptionValueWithDefault("metal-version", "1.2")),
       entry_point(
           command_line.GetOptionValueWithDefault("entry-point", "main")),
-      use_half_textures(command_line.HasOption("use-half-textures")) {
+      use_half_textures(command_line.HasOption("use-half-textures")),
+      require_framebuffer_fetch(
+          command_line.HasOption("require-framebuffer-fetch")) {
   auto language =
       command_line.GetOptionValueWithDefault("source-language", "glsl");
   std::transform(language.begin(), language.end(), language.begin(),

impeller/compiler/switches.h

@@ -37,12 +37,13 @@ struct Switches {
   std::string metal_version = "";
   std::string entry_point = "";
   bool use_half_textures = false;
+  bool require_framebuffer_fetch = false;
 
   Switches();
 
   ~Switches();
 
-  Switches(const fml::CommandLine& command_line);
+  explicit Switches(const fml::CommandLine& command_line);
 
   bool AreValid(std::ostream& explain) const;
 

impeller/entity/BUILD.gn

@@ -103,22 +103,17 @@ impeller_shaders("modern_entity_shaders") {
 
 impeller_shaders("framebuffer_blend_entity_shaders") {
   name = "framebuffer_blend"
+  require_framebuffer_fetch = true
+
+  # malioc does not support analyzing shaders that use the framebuffer fetch extension on some GPUs.
+  analyze = false
 
   if (is_mac && !is_ios) {
     # Note: this needs to correspond to the Apple7 Support family
     # for M1 and M2.
     metal_version = "2.3"
   }
 
-  # This version is to disable malioc checks.
-  if (impeller_enable_opengles) {
-    gles_language_version = 460
-  }
-
-  if (impeller_enable_vulkan) {
-    vulkan_language_version = 130
-  }
-
   shaders = [
     "shaders/blending/ios/framebuffer_blend.vert",
     "shaders/blending/ios/framebuffer_blend_color.frag",

impeller/entity/contents/framebuffer_blend_contents.cc

@@ -132,12 +132,10 @@ bool FramebufferBlendContents::Render(const ContentContext& renderer,
   FS::FragInfo frag_info;
 
   auto src_sampler_descriptor = src_snapshot->sampler_descriptor;
-  if (!renderer.GetDeviceCapabilities().SupportsDecalSamplerAddressMode()) {
-    // No known devices that support framebuffer fetch but not decal tile mode.
-    return false;
+  if (renderer.GetDeviceCapabilities().SupportsDecalSamplerAddressMode()) {
+    src_sampler_descriptor.width_address_mode = SamplerAddressMode::kDecal;
+    src_sampler_descriptor.height_address_mode = SamplerAddressMode::kDecal;
   }
-  src_sampler_descriptor.width_address_mode = SamplerAddressMode::kDecal;
-  src_sampler_descriptor.height_address_mode = SamplerAddressMode::kDecal;
   auto src_sampler = renderer.GetContext()->GetSamplerLibrary()->GetSampler(
       src_sampler_descriptor);
   FS::BindTextureSamplerSrc(cmd, src_snapshot->texture, src_sampler);

impeller/entity/shaders/blending/ios/framebuffer_blend.glsl

@@ -9,20 +9,13 @@
 #include <impeller/texture.glsl>
 #include <impeller/types.glsl>
 
-#ifdef IMPELLER_TARGET_METAL
 layout(set = 0,
        binding = 0,
        input_attachment_index = 0) uniform subpassInput uSub;
 
 vec4 ReadDestination() {
   return subpassLoad(uSub);
 }
-#else
-
-vec4 ReadDestination() {
-  return vec4(0);
-}
-#endif
 
 uniform sampler2D texture_sampler_src;
 
@@ -35,12 +28,20 @@ in vec2 v_src_texture_coords;
 
 out vec4 frag_color;
 
+vec4 Sample(sampler2D texture_sampler, vec2 texture_coords) {
+// gles 2.0 is the only backend without native decal support.
+#ifdef IMPELLER_TARGET_OPENGLES
+  return IPSampleDecal(texture_sampler, texture_coords);
+#else
+  return texture(texture_sampler, texture_coords);
+#endif
+}
+
 void main() {
-  f16vec4 dst_sample = f16vec4(ReadDestination());
-  f16vec4 dst = dst_sample;
-  f16vec4 src = f16vec4(texture(texture_sampler_src,  // sampler
-                                v_src_texture_coords  // texture coordinates
-                                )) *
+  f16vec4 dst = f16vec4(ReadDestination());
+  f16vec4 src = f16vec4(Sample(texture_sampler_src,  // sampler
+                               v_src_texture_coords  // texture coordinates
+                               )) *
                 frag_info.src_input_alpha;
 
   f16vec4 blended = mix(src, f16vec4(Blend(dst.rgb, src.rgb), dst.a), dst.a);

impeller/renderer/backend/gles/capabilities_gles.cc

@@ -8,6 +8,10 @@
 
 namespace impeller {
 
+// https://registry.khronos.org/OpenGL/extensions/EXT/EXT_shader_framebuffer_fetch.txt
+static std::string kFramebufferFetchExtension =
+    "GL_EXT_shader_framebuffer_fetch";
+
 CapabilitiesGLES::CapabilitiesGLES(const ProcTableGLES& gl) {
   {
     GLint value = 0;
@@ -87,6 +91,9 @@ CapabilitiesGLES::CapabilitiesGLES(const ProcTableGLES& gl) {
     num_shader_binary_formats = value;
   }
 
+  supports_framebuffer_fetch_ =
+      gl.GetDescription()->HasExtension(kFramebufferFetchExtension);
+
   if (gl.GetDescription()->HasExtension("GL_EXT_texture_border_clamp") ||
       gl.GetDescription()->HasExtension("GL_NV_texture_border_clamp") ||
       gl.GetDescription()->HasExtension("GL_OES_texture_border_clamp")) {
@@ -126,7 +133,7 @@ bool CapabilitiesGLES::SupportsTextureToTextureBlits() const {
 }
 
 bool CapabilitiesGLES::SupportsFramebufferFetch() const {
-  return false;
+  return supports_framebuffer_fetch_;
 }
 
 bool CapabilitiesGLES::SupportsCompute() const {

impeller/renderer/backend/gles/capabilities_gles.h

@@ -116,6 +116,7 @@ class CapabilitiesGLES final
   PixelFormat GetDefaultDepthStencilFormat() const override;
 
  private:
+  bool supports_framebuffer_fetch_ = false;
   bool supports_decal_sampler_address_mode_ = false;
 };
 

impeller/renderer/backend/gles/pipeline_library_gles.h

@@ -23,7 +23,7 @@ class PipelineLibraryGLES final : public PipelineLibrary {
   ReactorGLES::Ref reactor_;
   PipelineMap pipelines_;
 
-  PipelineLibraryGLES(ReactorGLES::Ref reactor);
+  explicit PipelineLibraryGLES(ReactorGLES::Ref reactor);
 
   // |PipelineLibrary|
   bool IsValid() const override;

impeller/renderer/backend/gles/test/capabilities_unittests.cc

@@ -44,5 +44,16 @@ TEST(CapabilitiesGLES, SupportsDecalSamplerAddressMode) {
   EXPECT_TRUE(capabilities->SupportsDecalSamplerAddressMode());
 }
 
+TEST(CapabilitiesGLES, SupportsFramebufferFetch) {
+  auto const extensions = std::vector<const unsigned char*>{
+      reinterpret_cast<const unsigned char*>("GL_KHR_debug"),  //
+      reinterpret_cast<const unsigned char*>(
+          "GL_EXT_shader_framebuffer_fetch"),  //
+  };
+  auto mock_gles = MockGLES::Init(extensions);
+  auto capabilities = mock_gles->GetProcTable().GetCapabilities();
+  EXPECT_TRUE(capabilities->SupportsFramebufferFetch());
+}
+
 }  // namespace testing
 }  // namespace impeller

impeller/tools/impeller.gni

@@ -321,6 +321,10 @@ template("impellerc") {
     if (defined(invoker.use_half_textures) && invoker.use_half_textures) {
       args += [ "--use-half-textures" ]
     }
+    if (defined(invoker.require_framebuffer_fetch) &&
+        invoker.require_framebuffer_fetch) {
+      args += [ "--require-framebuffer-fetch" ]
+    }
 
     if (json) {
       args += [ "--json" ]
@@ -506,6 +510,12 @@ template("_impeller_shaders_gles") {
   assert(defined(invoker.name), "Name of the shader library must be specified.")
   assert(defined(invoker.analyze), "Whether to analyze must be specified.")
 
+  require_framebuffer_fetch = false
+  if (defined(invoker.require_framebuffer_fetch) &&
+      invoker.require_framebuffer_fetch) {
+    require_framebuffer_fetch = invoker.require_framebuffer_fetch
+  }
+
   shaders_base_name = string_join("",
                                   [
                                     invoker.name,
@@ -515,6 +525,7 @@ template("_impeller_shaders_gles") {
   impellerc(impellerc_gles) {
     shaders = invoker.shaders
     sl_file_extension = "gles"
+    require_framebuffer_fetch = require_framebuffer_fetch
     if (defined(invoker.gles_language_version)) {
       gles_language_version = invoker.gles_language_version
     }
@@ -690,11 +701,14 @@ template("_impeller_shaders_vk") {
 #
 #    The SPIR-V version required by the shaders.
 #
-# @param[options] use_half_textures
+# @param[optional] use_half_textures
 #
 #    Whether the metal shader is using half-precision textures and requires
 #    openGL semantics when compilig SPIR-V.
 #
+# @param[optional] require_framebuffer_fetch
+#
+#    Whether to require the framebuffer fetch extension for GLES fragment shaders.
 template("impeller_shaders") {
   if (defined(invoker.metal_version)) {
     metal_version = invoker.metal_version
@@ -705,9 +719,16 @@ template("impeller_shaders") {
     use_half_textures = true
   }
 
+  require_framebuffer_fetch = false
+  if (defined(invoker.require_framebuffer_fetch) &&
+      invoker.require_framebuffer_fetch) {
+    require_framebuffer_fetch = true
+  }
+
   not_needed([
                "metal_version",
                "use_half_textures",
+               "require_framebuffer_fetch",
              ])
 
   enable_opengles = impeller_enable_opengles
@@ -737,6 +758,7 @@ template("impeller_shaders") {
     gles_shaders = "gles_$target_name"
     _impeller_shaders_gles(gles_shaders) {
       name = invoker.name
+      require_framebuffer_fetch = require_framebuffer_fetch
       if (defined(invoker.gles_language_version)) {
         gles_language_version = invoker.gles_language_version
       }