GLKit, Opengl ES 3.0, get depth with glReadPixels

Question

Ho can I get distance point on screen? Here is code:

- (void)viewDidLoad {
    [super viewDidLoad];
    // Do any additional setup after loading the view.
    context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES3];

    if (!context || ![EAGLContext setCurrentContext:context]) {
        NSLog(@"Failed to create ES context");
    }

    GLKView *view = (GLKView *)self.view;
    view.context = context;
    view.drawableDepthFormat = GLKViewDrawableDepthFormat16;

    glEnable(GL_DEPTH_TEST);

    shader = [[BaseEffect alloc] initWithVertexShader:@"Shader.vsh"
                                       fragmentShader:@"Shader.fsh"];

    // setup projection and model matrix
}

- (void)glkView:(GLKView *)view drawInRect:(CGRect)rect
{
    glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
    glClearDepthf(1.0f);
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);


    [shader prepareToDraw];
    // draw point

    glBindVertexArray(vertexArray);
    glDrawArrays(GL_POINTS, 0, pointsCount);
}

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event
{
    UITouch *touch = [touches anyObject];
    CGPoint tapLoc = [touch locationInView:self.view];

    GLfloat depth = 0;
    glReadPixels(tapLoc.x, tapLoc.y, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &depth);
    NSLog(@"Depth %f", depth);
}

All draws perfectly, but I can't get distance to drew points.

Edit:

I log depth, also experimenting with color. Color is reading only whith this arguments:

glReadPixels(p.x, p.y, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, pixelBuffer);

When use GL_FLOAT, rgb can not be read. I also try get values after drawing:

- (void)glkView:(GLKView *)view drawInRect:(CGRect)rect
{
    // preparing and drawing

    if (testDepthPoint.x != 0) {
        [self testDepth:testDepthPoint];
        testDepthPoint = CGPointZero;
    }
}

- (void)testDepth:(CGPoint)p {
    GLfloat depth = -99.0;
    glReadPixels(p.x, p.y, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &depth);
    NSLog(@"Depth %f", depth); // always log -99.0
}

Not worked for me

Answer 1

OpenGL ES 2.0 not supported to read GL_DEPTH_COMPONENT from buffer. More info here:

https://stackoverflow.com/a/18805281/820795

It works for Opengl ES 3.0

In ES 3.0 and later,there is support for depth textures. Using this, you can get the depth using the following steps.

• Render the scene to an FBO, using a texture as the depth attachment.
• Render a screen sized quad with a shader that samples the depth texture generated in the previous step, and writes the value to the color buffer.
• Use glReadPixels() on the color buffer.