Using glReadPixels to read value at Tap Point ? OpenGL 2.0 iOS

Question

Say I have a square which is defined as follows :

typedef struct {
float Position[3];
float Color[4];
} Vertex;

const Vertex Vertices[] = {
{{2, 0, 0}, {1, 0, 0, 1}},
{{4, 0, 0}, {1, 0, 0, 1}},
{{4, 2, 0}, {1, 0, 0, 1}},
{{2, 2, 0}, {1, 0, 0, 1}}
};

const GLubyte Indices[] = {
0, 1, 2,
2, 3, 0
};

And I am applying the following projection and modelview matrices :

- (void)update {

//Projection matrix.
float aspect = fabsf(self.view.bounds.size.width / self.view.bounds.size.height);
projectionMatrix = GLKMatrix4MakePerspective(GLKMathDegreesToRadians(_projectionAngle), aspect, 4.0f, 10.f);
self.effect.transform.projectionMatrix = projectionMatrix;

//Modelview matrix.
modelViewMatrix = GLKMatrix4MakeTranslation(_xTranslation, _yTranslation, -7.0);
self.effect.transform.modelviewMatrix = modelViewMatrix;

}

I now want to read the pixel colour of the object where a user taps on the screen. I am trying glkMathUnproject in combination with glReadPixels as follows, however glReadPixels is returning incorrect colour values for the tap point :

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

bool testResult;

GLint viewport[4];
glGetIntegerv(GL_VIEWPORT, viewport);

GLKVector3 nearPt = GLKMathUnproject(GLKVector3Make(tapLoc.x, (tapLoc.y-1024)*-1, 0.0), modelViewMatrix, projectionMatrix, &viewport[0] , &testResult);

GLKVector3 farPt = GLKMathUnproject(GLKVector3Make(tapLoc.x, (tapLoc.y-1024)*-1, 1.0), modelViewMatrix, projectionMatrix, &viewport[0] , &testResult);

farPt = GLKVector3Subtract(farPt, nearPt);

GLubyte pixelColor[4];
glReadPixels(farPt.v[0], farPt.v[1], 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, &pixelColor[0]);
NSLog(@"pixelColor %u %u %u %u", pixelColor[0],pixelColor[1],pixelColor[2], pixelColor[3]);

}

Can anyone advise how I can get the pixel colour accurately ?

Answer 1

Guy Cogus' code looks like the right approach, but you should know why what you posted isn't working. The point of unprojecting the vector using GLKUnproject is to get it out of screen space and into object space (the coordinate system of your geometry before it is transformed by the projection or modelview matrices), not into screen space, which is what glReadPixels is in (by definition, since you are reading pixels).