[PyOpenGL-Users] Rolling spectrogram with pyopengl

[Timothée L. <tim...@lp...>]

Dear pyopengl users,

I am writing (as a hobby) an application that does real-time 
visualization of audio data. The main widget is a rolling spectrogram, 
that is a colored image where the horizontal axis is the time, the 
vertical axis is the frequency, and the color of each pixel represents 
the intensity of the corresponding spectrum component (see 
http://www.flickr.com/photos/41584197@N03/3832486029/in/set-72157622072708326/ 
for example). Each column of the image is computed with a FFT of the 
audio data every 20 ms or so, and the whole image is displayed on screen 
and "rolls" and time goes by.

I am trying to use OpenGL to improve the performance of the part of the 
application that displays the image on the screen, but I can't manage to 
get it really faster than a simple 2D blitting, so I am asking for your 
help !

Currently, I first set up a texture with the following :

        # Create Texture
        GL.glGenTextures(1, self.texture) # generate one texture name
        GL.glBindTexture(GL.GL_TEXTURE_2D, self.texture) # bind a 2d 
texture to the generated name
        GL.glPixelStorei(GL.GL_UNPACK_ALIGNMENT, 1)
        GL.glTexImage2D(GL.GL_TEXTURE_2D, 0, GL.GL_RGBA, 
2*self.canvas_width, height, 0, GL.GL_RGBA, GL.GL_UNSIGNED_BYTE, None)
        GL.glTexParameterf(GL.GL_TEXTURE_2D, GL.GL_TEXTURE_WRAP_S, 
GL.GL_CLAMP)
        GL.glTexParameterf(GL.GL_TEXTURE_2D, GL.GL_TEXTURE_WRAP_T, 
GL.GL_CLAMP)
        GL.glTexParameterf(GL.GL_TEXTURE_2D, GL.GL_TEXTURE_WRAP_S, 
GL.GL_REPEAT)
        GL.glTexParameterf(GL.GL_TEXTURE_2D, GL.GL_TEXTURE_WRAP_T, 
GL.GL_REPEAT)
        GL.glTexParameterf(GL.GL_TEXTURE_2D, GL.GL_TEXTURE_MAG_FILTER, 
GL.GL_NEAREST)
        GL.glTexParameterf(GL.GL_TEXTURE_2D, GL.GL_TEXTURE_MIN_FILTER, 
GL.GL_NEAREST)
        GL.glTexEnvf(GL.GL_TEXTURE_ENV, GL.GL_TEXTURE_ENV_MODE, GL.GL_DECAL)

Then, every 20 ms or so, I modify two columns of that texture with :
        GL.glTexSubImage2D(GL.GL_TEXTURE_2D, 0, self.offset, 0, 1, 
self.height, GL.GL_BGRA, GL.GL_UNSIGNED_BYTE, byteString)
        GL.glTexSubImage2D(GL.GL_TEXTURE_2D, 0, self.offset + 
self.canvas_width, 0, 1, self.height, GL.GL_BGRA, GL.GL_UNSIGNED_BYTE, 
byteString)

And I draw part of that texture to my widget with :
        GL.glLoadIdentity()
        GL.glBegin(GL.GL_QUADS)
        xoff = float(self.offset)/(2*self.canvas_width)
        GL.glTexCoord2f(xoff, 0.)
        GL.glVertex2f(0, 0)
        GL.glTexCoord2f(1.+xoff , 0.)
        GL.glVertex2f(2*self.canvas_width, 0)
        GL.glTexCoord2f(1.+xoff, 1.)
        GL.glVertex2f(2*self.canvas_width, self.height)
        GL.glTexCoord2f(xoff, 1.)
        GL.glVertex2f(0,  self.height)
        GL.glEnd()

Profiling shows that the two GL.glTexSubImage2D take a lot of time (more 
than twice as much as the whole drawing part) whereas it's "just" two 
single columns of the texture being updated... What can I do to optimize 
this ? Is there a smarter way to achieve the same result ?

Thanks for your help !

Timothée Lecomte