hi, i don't really feel comfortable with it yet myself, but i'd say take a look at loadPixels(), updatePixels() and the pixels[] array, as well as bitwise operations (bit-shifting).

heh yeah, couple of weeks ago i was trying to send images over wifi between two computers and used the OSC protocol for this, i.e. i sent the image pixel by pixel and stitched it back together at the other side... it was fast, but some amount of pixels just got lost in the air. Unintended, glitchy effect.
i guess glitch art has to be hacky...
anyway, i've been doing
this using pixel array and some experimenting with simple +- operations...

thanks  

it's a prototype for a glitched-out 2D shooter:
http://dl.dropbox.com/u/1358257/shooter/3d_prototype/applet/index.html
(browser needs to be in 32bit mode, and you have to trust it as it will load enemy sprites from flickr. there's not much gameplay in there.. but you can control avatar 1 with cursor keys, shoot with alt).

for the glitches:
Code:

void pixelShifter()
{
	loadPixels();
	sincounter1 +=0.01;
	sincounter2 +=0.005;
	int bla = (int)(sin(sincounter1)*1000);
	int bla2 = (int)(sin(sincounter1)*6);
	
	counter = counter +counter;
	if(counter > pixels.length) counter = 1;
	
	for(int i = 0; i < pixels.length; i++){
		if(i < pixels.length-bla2-width*2)
		{
			pixels[i] = pixels[i] | lastPixels[i+bla2+width*2]+bla;	
		}
		lastPixels[i] =  pixels[i];
	}
	updatePixels();
}
 

there's basically the pixel array which is there from beginning, and an array i called lastPixels as a temporary buffer. Each frame these arrays are mixed into each other, not directly on top but offset in a certain direction, determined by "bla2".
Code:

pixels[i] = pixels[i] | lastPixels[i+bla2+width*2]+bla; 

also there is some arbitrary number added to each pixel ("bla") and i don't know what exactly it does, but it totally messes up color   . bla and bla2 are controlled by sine waves, so the glitches are slowly changing direction...

it's fun to experiment with values in there but i have to say... i've seen the OS X curtain of Death quite a lot when running a sketch, probably doing something the gfx card does not want me to do.

the difficult thing imo is to gain actual control over glitches, i.e. changing intensity or location on screen...