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POPerformance hit from blending large quad
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  1. POPerformance hit from blending large quad
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    copied!<p>I have a game which runs pretty well (55-60fps) on a retina display. I want to add a fullscreen overlay that blends with the existing scene. However, even when using a small texture, the performance hit is huge. Is there an optimization I can perform to make this useable?</p> <p>If I use a 80x120 texture (the texture is rendered on the fly, which is why it's not square), I get 25-30FPS. If I make the texture smaller, performance increases, but quality is not acceptable. In general, though, the quality of the overlay is not very important (it's just lighting).</p> <p>Renderer utilization is at 99%.</p> <p>Even if I use a square texture from a file (.png), performance is bad.</p> <p>This is how I create the texture:</p> <pre><code> [EAGLContext setCurrentContext:context]; // Create default framebuffer object. glGenFramebuffers(1, &amp;lightFramebuffer); glBindFramebuffer(GL_FRAMEBUFFER, lightFramebuffer); // Create color render buffer and allocate backing store. glGenRenderbuffers(1, &amp;lightRenderbuffer); glBindRenderbuffer(GL_RENDERBUFFER, lightRenderbuffer); glRenderbufferStorage(GL_RENDERBUFFER, GL_RGBA8_OES, LIGHT_WIDTH, LIGHT_HEIGHT); glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, lightRenderbuffer); glGenTextures(1, &amp;lightImage); glBindTexture(GL_TEXTURE_2D, lightImage); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST); glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST); glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, LIGHT_WIDTH, LIGHT_HEIGHT, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL); glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, lightImage, 0); </code></pre> <p>And here is the rendering...</p> <pre><code>/* Draw scene... */ glBlendFunc(GL_ONE, GL_ONE); //Switch to offscreen texture buffer glBindFramebuffer(GL_FRAMEBUFFER, lightFramebuffer); glBindRenderbuffer(GL_RENDERBUFFER, lightRenderbuffer); glViewport(0, 0, LIGHT_WIDTH, LIGHT_HEIGHT); glClearColor(ambientLight, ambientLight, ambientLight, ambientLight); glClear(GL_COLOR_BUFFER_BIT); /* Draw lights to texture... */ //Switch back to main frame buffer glBindFramebuffer(GL_FRAMEBUFFER, defaultFramebuffer); glBindRenderbuffer(GL_RENDERBUFFER, colorRenderbuffer); glViewport(0, 0, framebufferWidth, framebufferHeight); glBlendFunc(GL_DST_COLOR, GL_ZERO); glBindTexture(GL_TEXTURE_2D, glview.lightImage); /* Set up drawing... */ glDrawElements(GL_TRIANGLE_FAN, 4, GL_UNSIGNED_SHORT, 0); </code></pre> <p>Here are some benchmarks I took when trying to narrow down the problem. 'No blend' means I glDisable(GL_BLEND) before I draw the quad. 'No buffer switching' means I don't switch back and forth from the offscreen buffer before drawing.</p> <pre><code>(Tests using a static 256x256 .png) No blend, No buffer switching: 52FPS Yes blend, No buffer switching: 29FPS //disabled the glClear, which would artificially speed up the rendering No blend, Yes buffer switching: 29FPS Yes blend, Yes buffer switching: 27FPS Yes buffer switching, No drawing: 46FPS </code></pre> <p>Any help is appreciated. Thanks!</p> <p><strong>UPDATE</strong></p> <p>Instead of blending the whole lightmap afterward, I ended up writing a shader to do the work on the fly. Each fragment samples and blends from the lightmap (kind of like multitexturing). At first, the performance gain was minimal, but then I used a lowp sampler2d for the light map, and then I got around 45FPS.</p> <p>Here's the fragment shader:</p> <pre><code>lowp vec4 texColor = texture2D(tex, texCoordsVarying); lowp vec4 lightColor = texture2D(lightMap, worldPosVarying); lightColor.rgb *= lightColor.a; lightColor.a = 1.0; gl_FragColor = texColor * color * lightColor; </code></pre>
 

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