OpenGL ES 2.0 Object Picking on iOS

Question

What is the best method to select objects that have been drawn in OpenGL ES 2.0 (iOS)?

I am drawing points.

Answer 1

Here is how i do, based on the above solution, with depth buffer for 3D picking and GLKVector3 signature to retrieve the seal :

• all my objects got a seal
• i use a GLKVector3 named color to retrieved the seal from the read pixel. With this solution you can store 255 *255 * 255 = 16,581,375 objects.

In the Shader (vertex or fragment as you want)

add a picking boolean which tell you if you do the pick pass or not

uniform bool picking;

and the GLKVector3

uniform vec3 color;

if the boolean picking is activated so the color will be the GLKVector3

if(picking)
{
    colorVarying = vec4(color, 1.0);
}

In the ViewController

the method which create the GLKVector3 from a seal (when i create a new 3D object) :

-(GLKVector3)pack:(uint)seal
{
    GLKVector3 hash;

    float r = seal % 255;
    float g = (seal / 255) % 255;
    float b = (seal / (255 * 255)) % 255;
    hash = GLKVector3Make(r/255, g/255, b/255);

    return hash;
}

And the view controller code where you get the pixel from the touch position and get the selected seal from the selected color :

-(uint)getSealByColor:(GLKVector3)color
{
    color = GLKVector3DivideScalar(color, 255);
    for (MyObject *o in _objects) {
        if(GLKVector3AllEqualToVector3(o.color, color))
        {
            return o.seal;
        }
    }
    return 0;
}

-(void)tap:(UITapGestureRecognizer*)recognizer
{
    CGPoint p = [recognizer locationInView:self.view];
    GLKVector3 i = [self pickingAt:p];
    _sealSelected = [self getSealByColor:i];
}

-(GLKVector3)pickingAt:(CGPoint)position
{
    CGFloat scale = [UIScreen mainScreen].scale;

    GLsizei w = self.view.bounds.size.width * scale;
    GLsizei h = self.view.bounds.size.height * scale;

    GLuint fb;
    GLuint rb;
    GLuint db;

    Byte pixelColor[4] = {0,};

    glGenFramebuffers(1, &fb);
    glBindFramebuffer(GL_FRAMEBUFFER, fb);
    glGenRenderbuffers(1, &rb);
    glBindRenderbuffer(GL_RENDERBUFFER, rb);

    glRenderbufferStorage(GL_RENDERBUFFER, GL_RGBA8_OES, w, h);
    glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, rb);

    //here we also create a depth buffer for 3D objects picking
    glGenRenderbuffers(1, &db);
    glBindRenderbuffer(GL_RENDERBUFFER, db);

    glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT16, w, h);
    glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, db);

    GLenum status = glCheckFramebufferStatus(GL_FRAMEBUFFER);
    if (status != GL_FRAMEBUFFER_COMPLETE) {
        NSLog(@"Framebuffer status: %x", (int)status);
        return GLKVector3Make(0.0, 0.0, 0.0);
    }

    //we render the scene with our picking boolean activated
    [self render:YES];

    glReadPixels(position.x * scale, (h - (position.y * scale)), 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, pixelColor);

    glDeleteRenderbuffers(1, &db);
    glDeleteRenderbuffers(1, &rb);
    glDeleteFramebuffers(1, &fb);

    return GLKVector3Make(pixelColor[0], pixelColor[1], pixelColor[2]);
}

Hope this help someone, this is how i do to allow color picking for more than 255 objects.