CODEWITHSUNDEEP
iphoneipadopengl-esvisualizer
winterGoGoGo
winterGoGoGo

Reputation: 31

Why is the framerate of my tweaked OpenGL ES 2.0 template so slow on the iPad?

I've modified the OpenGL es 2.0 template in Xcode to render that little box to an offscreen texture (50*50), then reset the view port and render the texture to the screen using a fullscreen quad. But the FPS dropped down so much that obvious lags were seen (about 10).

I know iPad has problems concerning fillrate, but this just doesn't seem right. I used only one FBO and changed its color attachment between texture and renderBuffer in the loop. Does this have any influence?

Besides, I was writing an audio visualizer (like the one in Windows Media Player) editing pixel values in OpenGL. Any suggestions?

here goes the code: 

//implement the texture in -(id)init        
glGenTextures(1, &ScreenTex);  
glBindTexture(GL_TEXTURE_2D, ScreenTex);  
glTexImage2D(GL_TEXTURE_2D, 0, GL_RGB, texSize, texSize, 0, GL_RGB, GL_UNSIGNED_BYTE, nil);

//And in the render loop  
//draw to the texture  
glViewport(0, 0, texSize, texSize);  
glBindTexture(GL_TEXTURE_2D, ScreenTex);  
glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, ScreenTex, 0);  
glClear(GL_COLOR_BUFFER_BIT);

glVertexAttribPointer(ATTRIB_VERTEX, 2, GL_FLOAT, 0, 0, squareVertices);  
glUniform1i(Htunnel, 0);  
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);  

//switch to render to render buffer here  
glViewport(0, 0, backingWidth, backingHeight);

glBindRenderbuffer(GL_RENDERBUFFER, colorRenderbuffer);  
glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER,colorRenderbuffer);  
glClear(GL_COLOR_BUFFER_BIT);  

glVertexAttribPointer(ATTRIB_VERTEX, 2, GL_FLOAT, 0, 0, texVertices);  
glUniform1i(Htunnel, 1);  
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);  

//vertex shader   
void main()  
{  
    if (tunnel==0) {  
        gl_Position = position;  
        gl_Position.y += sin(translate) / 2.0;  
        colorVarying = color;  
    }else {  
        f_texCoord = v_texCoord;  
        gl_Position = position;  
    }  
}  

//frag shader
void main()  
{  
    if (tunnel==0) {  
        gl_FragColor = colorVarying;  
    }  else {  
        gl_FragColor = texture2D(s_texture, f_texCoord);    
    }   
}

Upvotes: 3

Views: 901

Answers (1)

Brad Larson
Brad Larson

Reputation: 170317

Without actual code, it will be difficult to pick out where the bottleneck is. However, you can get an idea of where the problem is by using Instruments to localize the causes.

Create a new Instruments document using both the OpenGL ES instrument and the new Time Profiler one. In the OpenGL ES instrument, hit the little inspector button on its right side, then click on the Configure button. Make sure pretty much every logging option is checked on the resulting page, particularly the Tiler Utilization % and Renderer Utilization %. Click Done and make sure that both of those statistics are checked in the Select statistics to list page.

Run this set of instruments against your application on the iPad for a little while during rendering. Stop it and look at the numbers. As explained in Pivot's answer to my question, if you are seeing the Tiler Utilization % in the OpenGL ES instrument hitting 100%, you are being limited by your geometry (unlikely here). Likewise, if the Renderer Utilization % is near 100%, you are fill-rate limited. You can also look to the other statistics you've logged to pull out what might be happening.

You can then turn to the Time Profiler results to see if you can narrow down the hotspots in your code where things might be getting slowed down. Find the items near the top of the list there. If they are in your code, double-click on them to see what's going on. If they are in system libraries, filter the results until you see something more relevant by right-clicking on the symbol name and choosing either Charge Library to Callers or Charge Symbol to Caller.

At some point, you'll start seeing OpenGL-related symbols up there, which should clue you in to what the GPU is doing. Also, you may be surprised to find some of your own code slowing things down.

There's another OpenGL ES instrument that you might try, but it's part of the Xcode 4 beta and is currently under NDA. Check out the WWDC 2010 session videos for more about that one.

Upvotes: 3

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