igl | opengl | Fix index offset for IndexFormat::UInt8 (#211)

Summary: Pull Request resolved: #211

Reviewed By: KyleFung

Differential Revision: D66348858

Pulled By: corporateshark

fbshipit-source-id: 2be12c122638f7ed751082c1e954a035fa4ad8d1

src/igl/opengl/RenderCommandEncoder.cpp

@@ -43,7 +43,7 @@ GLenum toGlPrimitive(PrimitiveType t) {
   return result;
 }
 
-int toGlType(IndexFormat format) {
+GLenum toGlType(IndexFormat format) {
   switch (format) {
   case IndexFormat::UInt8:
     return GL_UNSIGNED_BYTE;
@@ -55,6 +55,18 @@ int toGlType(IndexFormat format) {
   IGL_UNREACHABLE_RETURN(GL_UNSIGNED_INT)
 }
 
+uint8_t getIndexByteSize(GLenum indexType) {
+  switch (indexType) {
+  case GL_UNSIGNED_BYTE:
+    return 1u;
+  case GL_UNSIGNED_SHORT:
+    return 2u;
+  case GL_UNSIGNED_INT:
+    return 4u;
+  }
+  IGL_UNREACHABLE_RETURN(4u)
+}
+
 } // namespace
 
 RenderCommandEncoder::RenderCommandEncoder(const std::shared_ptr<CommandBuffer>& commandBuffer) :
@@ -361,8 +373,7 @@ void RenderCommandEncoder::drawIndexed(size_t indexCount,
   IGL_DEBUG_ASSERT(baseInstance == 0, "Instancing is not implemented");
   IGL_DEBUG_ASSERT(indexType_, "No index buffer bound");
 
-  const size_t indexOffsetBytes =
-      static_cast<size_t>(firstIndex) * (indexType_ == GL_UNSIGNED_INT ? 4u : 2u);
+  const size_t indexOffsetBytes = static_cast<size_t>(firstIndex) * getIndexByteSize(indexType_);
 
   if (IGL_DEBUG_VERIFY(adapter_ && indexType_)) {
     getCommandBuffer().incrementCurrentDrawCount();

src/igl/opengl/RenderCommandEncoder.h

@@ -94,7 +94,7 @@ class RenderCommandEncoder final : public IRenderCommandEncoder, public WithCont
  private:
   std::unique_ptr<RenderCommandAdapter> adapter_;
   bool scissorEnabled_ = false;
-  int indexType_ = 0;
+  GLenum indexType_ = 0;
   void* indexBufferOffset_ = nullptr;
   std::shared_ptr<igl::opengl::Framebuffer> resolveFramebuffer_;
   std::shared_ptr<igl::opengl::Framebuffer> framebuffer_;