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POWhy does a single Vec4 multiplication slow down my ogl es 2 fragment shader so much?
 

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  1. POWhy does a single Vec4 multiplication slow down my ogl es 2 fragment shader so much?
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    <p>I'm writing a 2D OpenGL game for iOS devices. Right now, i'm looking at the performance on iPad (first gen). The game has code paths for ogl 1.1 and 2.0, and I can switch which one is being used with a <code>#define</code>.</p> <p>When using ogl 2.0, the profiler tells me that my Renderer Utilization % is a fairly steady 100%, and my framerate is about 30 fps. When using ogl 1.1, the profiler tells me that my Renderer Utilization % is ~60% and my framerate is 60 fps.</p> <p>I want to improve the performance with ogl 2.0, and being fill rate limited, I suspected the fragment shader. Here is the fragment shader that was being used:</p> <pre><code>precision highp float; uniform vec4 u_color; uniform sampler2D u_sampler0; varying vec2 v_texCoord; void main() { gl_FragColor = u_color * texture2D( u_sampler0, v_texCoord ); } </code></pre> <p>You can see that the shader is pretty simple. It's just multiplying the geometry color by the texture color. As an experiment, I removed the multiplication, so that the output color was just the texture color, like this:</p> <pre><code>precision highp float; uniform vec4 u_color; uniform sampler2D u_sampler0; varying vec2 v_texCoord; void main() { gl_FragColor = texture2D( u_sampler0, v_texCoord ); } </code></pre> <p>Profiling the code using this modified shader gave a Renderer Utilization % of ~60% and a framerate of 60fps, the same performance achieved by the ogl 1.1 codepath.</p> <p>My question:</p> <p>1) Should a simple Vec4 multiplication in the fragment shader really have this large a negative effect on the performance?</p> <p>2) I've heard it said that on ogl es 2 devices, the 1.1 functionality is implemented with shaders. Obviously these shaders manage to efficiently achieve the effect i'm going for (blending geom color into texture color). How can I efficiently achieve this effect in my ogl 2 shader?</p>
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