WebGL/OpenGL: comparing the performance

For educational purposes I need to compare the performance of WebGL with OpenGL. I have two equivalent programs written in WebGL and OpenGL, now I need to take the frame rate of them and compare them.

In Javascript I use requestAnimationFrame to animate, and I noticed that it causes the frame rate to be always at 60 FPS, and it goes down only if I switch tab or window. On the other hand if I always call the render function recursively, the window freezes for obvious reasons.

This is how I am taking the FPS:

var stats = new Stats();
stats.domElement.style.position = 'absolute';
stats.domElement.style.left = '450px';
stats.domElement.style.top = '750px';

document.body.appendChild( stats.domElement );

setInterval( function () {

    stats.begin();
    stats.end();
}, 1000 / 60 );


var render= function() {
    requestAnimationFrame(render);
    renderer.render(scene,camera);
}

render();

Now the problem if having always the scene at 60 FPS is that I cannot actually compare it with the frame rate of OpenGL, since OpenGL redraws the scene only when it is somehow modified (for example if I rotate the object) and glutPostRedisplay() gets called.

So I guess if there is a way in WebGL to redraw the scene only when it is necessary, for example when the object is rotated or if some attributes in the shaders are changed.

Answers:

Answer

You can't compare framerates directly across GPUs in WebGL by pushing frames. Rather you need to figure out how much work you can get done within a single frame.

So, basically pick some target framerate and then keep doing more and more work until you go over your target. When you've hit your target that's how much work you can do. You can compare that to some other machine or GPU using the same technique.

Some people will suggest using glFinish to check timing. Unfortunately that doesn't actually work because it stalls the graphics pipeline and that stalling itself is not something that normally happens in a real app. It would be like timing how fast a car can go from point A to point B but instead of starting long before A and ending long after B you slam on the brakes before you get to B and measure the time when you get to B. That time includes all the time it took to slow down which is different on every GPU and different between WebGL and OpenGL and even different for each browser. You have no way of knowing how much of the time spent is time spent slowing down and how much of it was spent doing the thing you actually wanted to measure.

So instead, you need to go full speed the entire time. Just like a car you'd accelerate to top speed before you got to point A and keep going top speed until after you pass B. The same way they time cars on qualifying laps.

You don't normally stall a GPU by slamming on the breaks (glFinish) so adding the stopping time to your timing measurements is irrelevant and doesn't give you useful info. Using glFinish you'd be timing drawing + stopping. If one GPU draws in 1 second and stops in 2 and another GPU draws in 2 seconds and stops in 1, your timing will say 3 seconds for both GPUs. But if you ran them without stopping one GPU would draw 3 things a second, the other GPU would only draw 1.5 things a second. One GPU is clearly faster but using glFinish you'd never know that.

Instead you run full speed by drawing as much as possible and then measure how much you were able to get done and maintain full speed.

Here's one example: http://webglsamples.org/lots-o-objects/lots-o-objects-draw-elements.html

It basically draws each frame. If the frame rate was 60fps it draws 10 more objects the next frame. If the frame rate was less than 60fps it draws less.

Because browser timing is not perfect you might be to choose a slightly lower target like 57fps to find how fast it can go.

On top of that, WebGL and OpenGL really just talk to the GPU and the GPU does the real work. The work done by the GPU will take the exact same amount of time regardless of if WebGL asks the GPU to do it or OpenGL. The only difference is in the overhead of setting up the GPU. That means you really don't want to draw anything heavy. Ideally you'd draw almost nothing. Make your canvas 1x1 pixel, draw a single triangle, and check the timing (as in how many single triangles can you draw one triangle at a time in WebGL vs OpenGL at 60fps).

It gets even worse though. A real app will switch shaders, switch buffers, switch textures, update attributes and uniforms often. So, what are you timing? How many times you can call gl.drawBuffers at 60fps? How many times you can call gl.enable or gl.vertexAttribPointer or gl.uniform4fv at 60fps? Some combination? What's a reasonable combination? 10% calls to gl.verterAttribPointer + 5% calls to gl.bindBuffer + 10% calls to gl.uniform. The timing of those calls are the only things different between WebGL and OpenGL since ultimately they're talking to the same GPU and that GPU will run the same speed regardless.

Answer

You actually do not want to use framerate to compare these things because as you just mentioned you are artificially capped to 60 FPS due to VSYNC.

The number of frames presented will be capped by the swap buffer operation when VSYNC is employed and you want to factor that mess out of your performance measurement. What you should do is start a timer at the beginning of your frame, then at the end of the frame (just prior to your buffer swap) issue glFinish (...) and end the timer. Compare the number of milliseconds to draw (or whatever resolution your timer measures) instead of the number of frames drawn.

Answer

The correct solution is to use the ANGLE_timer_query extension when available.

Quoting from the specification:

OpenGL implementations have historically provided little to no useful timing information. Applications can get some idea of timing by reading timers on the CPU, but these timers are not synchronized with the graphics rendering pipeline. Reading a CPU timer does not guarantee the completion of a potentially large amount of graphics work accumulated before the timer is read, and will thus produce wildly inaccurate results. glFinish() can be used to determine when previous rendering commands have been completed, but will idle the graphics pipeline and adversely affect application performance.

This extension provides a query mechanism that can be used to determine the amount of time it takes to fully complete a set of GL commands, and without stalling the rendering pipeline. It uses the query object mechanisms first introduced in the occlusion query extension, which allow time intervals to be polled asynchronously by the application.

(emphasis mine)

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