GPUShader: Add support for gpu_BaryCoord and fallback

This adds the possibility to use the `gpu_BaryCoord[NoPersp]`
builtin to support barycentric coordinates without geometry shader.

The `BuiltinBits::LAYER` builtin needs to be manually added
to the `GPUShaderCreateInfo` in order to use this feature.
Note: This is only available for shaders using `GPUShaderCreateInfo`.

A geometry shader fallback is generated if the extension
`AMD_shader_explicit_vertex_parameter` is not available.

`NV_fragment_shader_barycentric` was not considered because it is not
present inside the `glew.h` with use and seems to only be available
with vulkan.

1 files changed, 85 insertions, 2 deletions

diff --git a/source/blender/gpu/opengl/gl_shader.cc b/source/blender/gpu/opengl/gl_shader.cc
index 4649d32deee..815c488cf8b 100644
--- a/source/blender/gpu/opengl/gl_shader.cc
+++ b/source/blender/gpu/opengl/gl_shader.cc
@@ -424,9 +424,28 @@ std::string GLShader::resources_declare(const ShaderCreateInfo &info) const
   return ss.str();
 }
 
+static std::string main_function_wrapper(std::string &pre_main, std::string &post_main)
+{
+  std::stringstream ss;
+  /* Prototype for the original main. */
+  ss << "\n";
+  ss << "void main_function_();\n";
+  /* Wrapper to the main function in order to inject code processing on globals. */
+  ss << "void main() {\n";
+  ss << pre_main;
+  ss << "  main_function_();\n";
+  ss << post_main;
+  ss << "}\n";
+  /* Rename the original main. */
+  ss << "#define main main_function_\n";
+  ss << "\n";
+  return ss.str();
+}
+
 std::string GLShader::vertex_interface_declare(const ShaderCreateInfo &info) const
 {
   std::stringstream ss;
+  std::string post_main = "";
 
   ss << "\n/* Inputs. */\n";
   for (const ShaderCreateInfo::VertIn &attr : info.vertex_inputs_) {
@@ -442,13 +461,32 @@ std::string GLShader::vertex_interface_declare(const ShaderCreateInfo &info) con
   if (!GLContext::layered_rendering_support && bool(info.builtins_ & BuiltinBits::LAYER)) {
     ss << "out int gpu_Layer;\n";
   }
+  if (bool(info.builtins_ & BuiltinBits::BARYCENTRIC_COORD)) {
+    if (!GLContext::native_barycentric_support) {
+      /* Disabled or unsupported. */
+    }
+    else if (GLEW_AMD_shader_explicit_vertex_parameter) {
+      /* Need this for stable barycentric. */
+      ss << "flat out vec4 gpu_pos_flat;\n";
+      ss << "out vec4 gpu_pos;\n";
+
+      post_main += "  gpu_pos = gpu_pos_flat = gl_Position;\n";
+    }
+  }
   ss << "\n";
+
+  if (post_main.empty() == false) {
+    std::string pre_main = "";
+    ss << main_function_wrapper(pre_main, post_main);
+  }
   return ss.str();
 }
 
 std::string GLShader::fragment_interface_declare(const ShaderCreateInfo &info) const
 {
   std::stringstream ss;
+  std::string pre_main = "";
+
   ss << "\n/* Interfaces. */\n";
   const Vector<StageInterfaceInfo *> &in_interfaces = (info.geometry_source_.is_empty()) ?
                                                           info.vertex_out_interfaces_ :
@@ -456,6 +494,32 @@ std::string GLShader::fragment_interface_declare(const ShaderCreateInfo &info) c
   for (const StageInterfaceInfo *iface : in_interfaces) {
     print_interface(ss, "in", *iface);
   }
+  if (bool(info.builtins_ & BuiltinBits::BARYCENTRIC_COORD)) {
+    if (!GLContext::native_barycentric_support) {
+      ss << "smooth in vec3 gpu_BaryCoord;\n";
+      ss << "noperspective in vec3 gpu_BaryCoordNoPersp;\n";
+    }
+    else if (GLEW_AMD_shader_explicit_vertex_parameter) {
+      /* NOTE(fclem): This won't work with geometry shader. Hopefully, we don't need geometry
+       * shader workaround if this extension/feature is detected. */
+      ss << "\n/* Stable Barycentric Coordinates. */\n";
+      ss << "flat in vec4 gpu_pos_flat;\n";
+      ss << "__explicitInterpAMD in vec4 gpu_pos;\n";
+      /* Globals. */
+      ss << "vec3 gpu_BaryCoord;\n";
+      ss << "vec3 gpu_BaryCoordNoPersp;\n";
+      ss << "\n";
+      ss << "vec2 stable_bary_(vec2 in_bary) {\n";
+      ss << "  vec3 bary = vec3(in_bary, 1.0 - in_bary.x - in_bary.y);\n";
+      ss << "  if (interpolateAtVertexAMD(gpu_pos, 0) == gpu_pos_flat) { return bary.zxy; }\n";
+      ss << "  if (interpolateAtVertexAMD(gpu_pos, 2) == gpu_pos_flat) { return bary.yzx; }\n";
+      ss << "  return bary.xyz;\n";
+      ss << "}\n";
+
+      pre_main += "  gpu_BaryCoord = stable_bary_(gl_BaryCoordSmoothAMD);\n";
+      pre_main += "  gpu_BaryCoordNoPersp = stable_bary_(gl_BaryCoordNoPerspAMD);\n";
+    }
+  }
   ss << "\n/* Outputs. */\n";
   for (const ShaderCreateInfo::FragOut &output : info.fragment_outputs_) {
     ss << "layout(location = " << output.index;
@@ -473,6 +537,11 @@ std::string GLShader::fragment_interface_declare(const ShaderCreateInfo &info) c
     ss << "out " << to_string(output.type) << " " << output.name << ";\n";
   }
   ss << "\n";
+
+  if (pre_main.empty() == false) {
+    std::string post_main = "";
+    ss << main_function_wrapper(pre_main, post_main);
+  }
   return ss.str();
 }
 
@@ -562,7 +631,7 @@ std::string GLShader::workaround_geometry_shader_source_create(
 
   const bool do_layer_workaround = !GLContext::layered_rendering_support &&
                                    bool(info.builtins_ & BuiltinBits::LAYER);
-  const bool do_barycentric_workaround = !GLContext::native_barycentrics_support &&
+  const bool do_barycentric_workaround = !GLContext::native_barycentric_support &&
                                          bool(info.builtins_ & BuiltinBits::BARYCENTRIC_COORD);
 
   shader::ShaderCreateInfo info_modified = info;
@@ -578,6 +647,10 @@ std::string GLShader::workaround_geometry_shader_source_create(
   if (do_layer_workaround) {
     ss << "in int gpu_Layer[];\n";
   }
+  if (do_barycentric_workaround) {
+    ss << "smooth out vec3 gpu_BaryCoord;\n";
+    ss << "noperspective out vec3 gpu_BaryCoordNoPersp;\n";
+  }
   ss << "\n";
 
   ss << "void main()\n";
@@ -592,6 +665,10 @@ std::string GLShader::workaround_geometry_shader_source_create(
         ss << " = " << iface->instance_name << "_in[" << i << "]." << inout.name << ";\n";
       }
     }
+    if (do_barycentric_workaround) {
+      ss << "  gpu_BaryCoordNoPersp = gpu_BaryCoord =";
+      ss << " vec3(" << int(i == 0) << ", " << int(i == 1) << ", " << int(i == 2) << ");\n";
+    }
     ss << "  gl_Position = gl_in[" << i << "].gl_Position;\n";
     ss << "  EmitVertex();\n";
   }
@@ -602,7 +679,10 @@ std::string GLShader::workaround_geometry_shader_source_create(
 bool GLShader::do_geometry_shader_injection(const shader::ShaderCreateInfo *info)
 {
   BuiltinBits builtins = info->builtins_;
-  if (!GLContext::native_barycentrics_support && bool(builtins & BuiltinBits::BARYCENTRIC_COORD)) {
+  if (!GLContext::native_barycentric_support && bool(builtins & BuiltinBits::BARYCENTRIC_COORD)) {
+    return true;
+  }
+  if (!GLContext::layered_rendering_support && bool(builtins & BuiltinBits::LAYER)) {
     return true;
   }
   return false;
@@ -665,6 +745,9 @@ static char *glsl_patch_default_get()
     STR_CONCAT(patch, slen, "#extension GL_AMD_vertex_shader_layer: enable\n");
     STR_CONCAT(patch, slen, "#define gpu_Layer gl_Layer\n");
   }
+  if (GLContext::native_barycentric_support) {
+    STR_CONCAT(patch, slen, "#extension GL_AMD_shader_explicit_vertex_parameter: enable\n");
+  }
 
   /* Fallbacks. */
   if (!GLContext::shader_draw_parameters_support) {