Sub-pixel rendering in OpenGL - accuracy issue

Question

I need to generate a few series of test images for a photogrammetric application. These should contain simple objects (disks, rectangles, etc) with a very precisely known position.

Considering a 8bit grayscale image of a black rectangle on white background, a smallest observable displacement (after interpolation) should be 1/256 pixel, as there are 256 possible intensity levels of each pixel.

I decided to use OpenGL (with python + pyglet) to render such images, as later on I will have to render more complicated (3d scene, stereo image pairs)

Unfortunately, the best accuracy I have achieved is around pixel/10, the full intensity depth is not used.

Is it possible to do better, ideally - achieving full 1/256pixel accuracy? Any hints on how to do that, please?

sample code, generating images of a partial disk, moved 0.01 pixel more in each frame

#-*- coding: utf-8 -*-
import pyglet
from pyglet.gl import *
from PIL import Image, ImageGrab

config = pyglet.gl.Config(width = 800, height = 600, sample_buffers=1, samples=16)
window = pyglet.window.Window(config=config, resizable=True) 
window.set_size(800, 600)

printScreenNr = 0

@window.event
def on_draw():
     global printScreenNr
     window.clear()
     glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );

     glLoadIdentity()
     glEnable(GL_LINE_SMOOTH)
     glHint(GL_LINE_SMOOTH_HINT, GL_NICEST);

     glTranslated(200, 200,0)

     circleQuad = gluNewQuadric()
     glTranslated(200+(printScreenNr*0.01), 0, 0)
     gluPartialDisk(circleQuad, 0, 50, 500, 500, 0, 180)

@window.event
def on_resize(width, height):
    glViewport(0, 0, width, height)
    glMatrixMode(GL_PROJECTION)
    glLoadIdentity()
    glOrtho (0, width, height, 0, 0, 1);
    glMatrixMode(GL_MODELVIEW)
    return pyglet.event.EVENT_HANDLED

@window.event
def on_text(text):
    global printScreenNr
    if(text == "p"):
        pyglet.image.get_buffer_manager().get_color_buffer().save('photo'+str(printScreenNr)+'.tiff')
        printScreenNr += 1

pyglet.app.run()

(the code above is using gluPartialDisk, but I have tested the issue also using quadrangles, results' accuracy did not differ)

Sub-pixel rendering in OpenGL - accuracy issue

2 Answers