Possible Retina issue with OpenGL ES on iPhone?

Question

This is probably linked to another unsolved mystery of mine.

I'm drawing Orthographic 2d on iPhone, using real device and simulator. I'm trying to color my pixels a given color depending on how far they are from arbitrary point in pixelspace of 'A', which I pass in (hard code). I'm doing everything in Retina 960x640 resolution. I calculate distance from A to gl_FragCoord, and I color based on leaping between 2 colors with the 'max' being 300px distance.

When on simulator (with retina display) I need to give a center point of "460" pixels for screen midpoint X.. Y I give 160px, and I look for distance of '300'px.. to get the same effect on device I need center of 960X and distance of 150 to get the same results (interestingly, px 80 doesn't give the same results I want but 160 could be an overshoot on the original...)

Obviously a retina issue is at play. But where, and how, and how do I find and fix it?

I'm using:

glViewport(0, 0, 960.0f, 640.0f);

and:

glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_WIDTH, &framebufferWidth); glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_HEIGHT, &framebufferHeight);

And:

[self setView:[[EAGLView alloc] initWithFrame:[UIScreen mainScreen].bounds]];
[(EAGLView *)[self view] setContentScaleFactor:2.0f];

Answer 1

You shouldn't hardcode "glViewport(0, 0, 960.0f, 640.0f);", setup the viewport this way:

glViewport(0, 0, framebufferWidth, framebufferHeight);

Also don't hardcode the content scale, you can findout the content scale with:

float contentScale = 1.0f;
if ([[UIScreen mainScreen] respondsToSelector:@selector(displayLinkWithTarget:selector:)]) {
    contentScale = [[UIScreen mainScreen] scale];
}

About the pixel distance, since you want the distance to be the double with retina display, you can add an uniform to your shader with the content scale.

Answer 2

Now iOS devices can have multiple different screens, and a UIWindow can be placed in a non-main screen. In that case, you can use self.view.window.screen.scale to get current screen's scale dynamically.