update pixels in random order?

Programming Questions

phone.bone

update pixels in random order?

in Programming Questions • 2 years ago

Hi there,

After some time searching the web and trying different things I can't figure this out. So I hope somebody on the forums can assist me with this.

I have a picture loaded in, and now I want all the pixels to turn black, in a random order, and every pixel just once, so:

loadPixels();
if (i < (width*height)) {
pixels[i] = color(0);
i++;
}
updatePixels();

loadPixels();
if (i < width*height){
pixels[int(random(width*height))] = color(0);
i++;
}
updatePixels();

won't work because it goes over pixels more than once. Also I would like it to stop after it has changed all pixels to black.

Can someone please help me?
Thanks in advance!

Replies(6)

kooogy

Re: update pixels in random order?

2 years ago

how many pixels?

if it's not too many then create an array (of ints?); set item[i] = i; knuth shuffle them, then iterate through them setting pixel[item[i]] to black.

there's a way to do it without the array, but it requires MATHS and i can't find the relevant bits. it involves co-primes and group theory and something like a^i mod p

phone.bone

Re: Re: update pixels in random order?

2 years ago

1026720 pixels (1240 x 828). So quite a bit..
I'm not very skilled, what is knuth shuffle? And co-primes and a^i mod p don't ring a bell either :S

What I have now in my draw is:

if(part > 0){
loadPixels();
for (int w=0; w<(width*height)/25; w++) {
i = int(random(pixels.length));
j = int(random(i, pixels.length));
if ( j<pixels.length && i<pixels.length && pixels[i] > pixels[j]/part ) {
c = pixels[i];
pixels[i] = pixels[j];
pixels[j] = c;
}
}
updatePixels();
}else {
loadPixels();
if (k < (width*height)) {
int p = int(random(2, width-2*height-2));
pixels[p] = color(0);
k++;
println("k: "+k);
}
updatePixels();
}
part = part-0.015;
updatePixels();

The first part destroys the image, and when part < 0 that stops and the screen should gradually be filled with black.. but it is way too slow this way (also because it goes over all the pixels again instead of just the pixels it has not gone over yet).

kooogy

Re: update pixels in random order?

2 years ago

the aforementioned MATHS

http://en.wikipedia.org/wiki/Linear_congruential_generator

choose m to be just slightly bigger than your width x height and just ignore the ones that are outside the set.

kooogy

Re: update pixels in random order?

2 years ago

knuth shuffle. it's a shuffle, invented by knuth! like you'd shuffle cards, changing their order. runs in linear time, very simple (but easy to get wrong):

http://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle

1M ints isn't a lot, these days, 4Mb. and you only have to do it once.

PhiLho

Re: update pixels in random order?

2 years ago

You can pre-fill an array with all the possible indices in the pixel array, randomized:

int idx = 0;
int[] indices;
void setup()
{
size(100, 100);
background(255);
indices = new int[width * height];
// Generate a list of random indices
// http://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle
// inside-out algorithm
for (int i = 1; i < indices.length; i++)
{
int j = int(random(0, i+1));
indices[i] = indices[j];
indices[j] = i;
}
}
void draw()
{
if (idx >= width * height)
{
println("Done!");
noLoop();
return;
}
loadPixels();
pixels[indices[idx++]] = 0xFF000000 + idx * 64; //#000000;
updatePixels();
}

[EDIT] Mmm, funny, I started that early but had to do something else and just finished now. I missed the conversation above. I see we had the same idea...

phone.bone

Re: Re: update pixels in random order?

2 years ago

Thank you both very much! I am still struggling to get it working but I haven't read everything thoroughly yet. I stop working on it for now but I will try to get it finished tomorrow morning. I hope I'll be able to finish it with the above suggestions.

Top Reply