Possible Retina issue with OpenGL ES on iPhone?

问题:

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];

回答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.

回答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.