Save image from opengl (pyqt) in high resolution: - image

Question

I have a PyQt application in which an QtOpenGL.QGLWidget shows a wind turbine.
I would like to save the turbine as an image in high resulution, but so far I am only able to save the rendered screen image, i.e. in my case 1680x1050 minus borders,toolbars etc.
glPixelStorei(GL_PACK_ALIGNMENT, 1)
data = glReadPixels(0, 0, self.width, self.height, GL_RGBA, GL_UNSIGNED_BYTE)
How can I get around this limitation?
EDIT
I have tried using a framebuffer,
from __future__ import division
import OpenGL
from OpenGL.GL import *
from OpenGL.GLU import *
from OpenGL.GLUT import *
from PIL import Image
import time, sys
import numpy as np
WIDTH = 400
HEIGHT = 300
def InitGL():
glMatrixMode(GL_PROJECTION)
gluPerspective(45.0, float(WIDTH) / float(HEIGHT), 0.1, 100.0)
glMatrixMode(GL_MODELVIEW)
def DrawGLScene():
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT)
glLoadIdentity()
glTranslatef(0, 0., -3)
glutWireTeapot(1)
glFlush()
def capture_screen():
DrawGLScene()
glPixelStorei(GL_PACK_ALIGNMENT, 1)
data = glReadPixels(0, 0, WIDTH, HEIGHT, GL_RGBA, GL_UNSIGNED_BYTE)
image = Image.fromstring("RGBA", (WIDTH, HEIGHT), data)
image.transpose(Image.FLIP_TOP_BOTTOM).show()
def capture_fbo(width=800, height=600):
fbo = glGenFramebuffers(1)
render_buf = glGenRenderbuffers(1)
glBindRenderbuffer(GL_RENDERBUFFER, render_buf)
glRenderbufferStorage(GL_RENDERBUFFER, GL_RGBA, width, height);
glBindFramebuffer(GL_FRAMEBUFFER, fbo)
glFramebufferRenderbuffer(GL_DRAW_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, render_buf);
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, fbo);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT)
glMatrixMode(GL_MODELVIEW)
glLoadIdentity()
glTranslatef(1, 1., -3)
glScale(width / WIDTH, height / HEIGHT, 1)
glutWireTeapot(1.0)
glFlush()
glReadBuffer(GL_COLOR_ATTACHMENT0);
data = glReadPixels(0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE)
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0);
image = Image.fromstring("RGBA", (width, height), data)
image.transpose(Image.FLIP_TOP_BOTTOM).show()
glDeleteFramebuffers(1, [fbo]);
glDeleteRenderbuffers(1, [render_buf]);
glutInit(sys.argv)
glutInitDisplayMode(GLUT_RGBA | GLUT_DEPTH)
glutInitWindowSize(WIDTH, HEIGHT)
window = glutCreateWindow("")
glutDisplayFunc(DrawGLScene)
InitGL()
DrawGLScene()
capture_screen()
capture_fbo()
glutMainLoop()
but nothing is drawn at the areas outside the normal screen window area

Answer 1

The way glReadPixels works is it reads the pixels from the currently selected framebuffer.
Not sure what a framebuffer is? Here's the wiki page: http://www.opengl.org/wiki/Framebuffer_Object
A quick explanation is, you can think of the framebuffer as a "canvas" that your graphics card draws on. You can have as many framebuffers as you want (sorta... most drivers define a limit somewhere), but only one will be displayed. This is the selected framebuffer and can only be as large as the window that you are displaying on screen.
Here's a tutorial on making a framebuffer: http://www.lighthouse3d.com/tutorials/opengl-short-tutorials/opengl_framebuffer_objects/
Modify it so that the framebuffer you are creating is the resolution that you want to take a screenshot from.
Once you have the framebuffer set up, you can draw to it like so:
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, buffer);
/* drawing code */
glReadPixels();
If you want to be able to still see something on screen, you will need to draw to the default frame buffer after drawing to the new framebuffer:
glBindFramebuffer(GL_DRAW_FRAMEBUFFER, 0); //"0" means return to default framebuffer
/* same drawing code */
Note that this means you will be drawing your entire scene twice! Make sure your framerate can handle it.