PyopenGL glReadPixels

Question

I tried to use the glReadPixels method to color code a simple triangle in the screen, just without any secondary render functions, etc. but it didn't give the good result. Code:

import pygame as pg
from OpenGL.GL import *


pg.display.set_mode((500,500),pg.OPENGL)
glClearColor(0.5,0.0,0.5,0.0)
done=0

def first():
    glColor3f(0.5,0.6,0.7)
    glBegin(GL_TRIANGLES)
    glVertex(0.0,0.0,0.0)
    glVertex(1.0,0.0,0.0)
    glVertex(0.0,1.0,0.0)
    glEnd()
cl=0
clock=pg.time.Clock()
while not done:
    for event in pg.event.get():
        if event.type==pg.QUIT: done=1
        elif event.type==pg.MOUSEBUTTONDOWN:
            pos=pg.mouse.get_pos()
            color=glReadPixels(pos[0],pos[1],1,1,GL_RGB,GL_FLOAT)
            print color, pos[0], pos[1])



    glClear(GL_COLOR_BUFFER_BIT)
    first()
    pg.display.flip()
    clock.tick(20)
pg.quit()

But it always gives the SAME color output: [[[ 0.50196081 0. 0.50196081]]] 288 217 How can I fix it?

As far as I can see from the code, you are reading back the contents of the back buffer which are _undefined_ at this point in time. I don't know how pygame's `mouse.get_pos()` is supposed to work, but I suspect that it is not returning the correct OpenGL windows coordinates with the origin at bottom loft corner. From the numbers given, on a 500x500 window, and assuming identity transforms and full viewport, (288,217) is just the background, not in the triangle at all. — derhass, Apr 26 '15 at 00:01

Cosine · Accepted Answer · 2015-04-26T00:23:57.833

1

Your main problem is that glReadPixels accepts bottom-left origin and pygame gives top-left.

First, you should always save your pygame Surface (for the reason I will soon demonstrate). So:

window = pg.display.set_mode((500,500),pg.OPENGL)

Now you can access window.width and window.height. So now your glReadPixels will get the proper place:

color=glReadPixels(x,window.height-y,1,1,GL_RGB,GL_FLOAT)

PS: If you do want the most recent color, you should just put your colour detection after first() but before pg.display.flip() (OpenGL is always bound to the back buffer (unless you were to specify otherwise (but you wouldn't want to)))

EDIT: pg, not py

EDIT: I was wrong, it's in pixels. My bad.

edited Apr 26 '15 at 00:23

answered Apr 26 '15 at 00:17

Cosine

572
4
18

Thanks! But tell me, why theese returned colors isn't the same with the color values that I described the `glClearColor` & the `glColor`? – Uncle Dino Apr 28 '15 at 19:13
That is a good question... does pygame automatically clear the back buffer? It's possible that because you're reading from your back buffer you're getting empty memory. (See my "PS") – Cosine Apr 29 '15 at 22:41

PyopenGL glReadPixels

1 Answers1