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POOpenGL ES iOS drawing performance a lot slower with VBOs than without
 

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  1. POOpenGL ES iOS drawing performance a lot slower with VBOs than without
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    <p>I've recently changed drawing in my current project from standard drawing from a memory array to VBOs. To my surprise the framerate dropped significantly from 60fps to 30fps drawing a model with 1200verts 8 times. Doing further profiling showed that glDrawElements took 10 times as long when using VBOs compared to drawing from memory.</p> <p>I am really puzzled why this is happening. Does anyone know what could be the cause for a performance decrease?</p> <p>I am testing on an iPhone 5 running iOS 6.1.2.</p> <p>I've isolated my VBO handling into a single function where I create the vertex/index buffer once statically at the top of the function. I can switch between normal and VBO rendering with an #ifdef USE_VBO</p> <pre><code>- (void)drawDuck:(Toy*)toy reflection:(BOOL)reflection { ModelOBJ* model = _duck[0].model; int stride = sizeof(ModelOBJ::Vertex); #define USE_VBO #ifdef USE_VBO static bool vboInitialized = false; static unsigned int vbo, ibo; if (!vboInitialized) { vboInitialized = true; // Generate VBO glGenBuffers(1, &amp;vbo); int numVertices = model-&gt;getNumberOfVertices(); glBindBuffer(GL_ARRAY_BUFFER, vbo); glBufferData(GL_ARRAY_BUFFER, stride*numVertices, model-&gt;getVertexBuffer(), GL_STATIC_DRAW); glBindBuffer(GL_ARRAY_BUFFER, 0); // Generate index buffer glGenBuffers(1, &amp;ibo); int numIndices = model-&gt;getNumberOfIndices(); glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibo); glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(unsigned short)*numIndices, model-&gt;getIndexBuffer(), GL_STATIC_DRAW); glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0); } #endif [self setupDuck:toy reflection:reflection]; #ifdef USE_VBO // Draw with VBO glBindBuffer(GL_ARRAY_BUFFER, vbo); glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibo); glEnableVertexAttribArray(GC_SHADER_ATTRIB_POSITION); glEnableVertexAttribArray(GC_SHADER_ATTRIB_NORMAL); glEnableVertexAttribArray(GC_SHADER_ATTRIB_TEX_COORD); glVertexAttribPointer(GC_SHADER_ATTRIB_POSITION, 3, GL_FLOAT, GL_FALSE, stride, (void*)offsetof(ModelOBJ::Vertex, position)); glVertexAttribPointer(GC_SHADER_ATTRIB_TEX_COORD, 2, GL_FLOAT, GL_FALSE, stride, (void*)offsetof(ModelOBJ::Vertex, texCoord)); glVertexAttribPointer(GC_SHADER_ATTRIB_NORMAL, 3, GL_FLOAT, GL_FALSE, stride, (void*)offsetof(ModelOBJ::Vertex, normal)); glDrawElements(GL_TRIANGLES, model-&gt;getNumberOfIndices(), GL_UNSIGNED_SHORT, 0); glBindBuffer(GL_ARRAY_BUFFER, 0); glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0); #else // Draw with array glEnableVertexAttribArray(GC_SHADER_ATTRIB_POSITION); glEnableVertexAttribArray(GC_SHADER_ATTRIB_NORMAL); glEnableVertexAttribArray(GC_SHADER_ATTRIB_TEX_COORD); glVertexAttribPointer(GC_SHADER_ATTRIB_POSITION, 3, GL_FLOAT, GL_FALSE, stride, model-&gt;getVertexBuffer()-&gt;position); glVertexAttribPointer(GC_SHADER_ATTRIB_TEX_COORD, 2, GL_FLOAT, GL_FALSE, stride, model-&gt;getVertexBuffer()-&gt;texCoord); glVertexAttribPointer(GC_SHADER_ATTRIB_NORMAL, 3, GL_FLOAT, GL_FALSE, stride, model-&gt;getVertexBuffer()-&gt;normal); glDrawElements(GL_TRIANGLES, model-&gt;getNumberOfIndices(), GL_UNSIGNED_SHORT, model-&gt;getIndexBuffer()); #endif } </code></pre> <p>ModelOBJ::Vertex is just 3,2,3 float for pos, texcoord, normal. Indices are ushort.</p> <p><strong>UPDATE:</strong> I've now wrapped the draw setup (ie. the attrib binding calls) into an VAO and now performance is ok, even slightly better than drawing from main memory. So my conclusion is that VBO support without VAOs is broken on iOS. Is that assumption correct?</p>
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    1. COShouldn't model->getVertexBuffer()->position be &model->getVertexBuffer()->position?
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    2. COposition is defined as float position[3], so this is ok.
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    3. COVery interesting that the VAO fixed it! Are you sure you don't have any stray enabled vertex attributes from elsewhere in the program? If so, that could explain it, since the VAO encapsulates the set of enabled vertex attributes.
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