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  Trying to pixelate a section of an image  (Read 5006 times)
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Offline Porlus

Junior Devvie

« Posted 2013-04-18 18:38:12 »

Hi all, I've been looking around for a premade algorithm to pixelate an image or something similar that I could expand on although I've only been able to find descriptions of the algorithm so I've attempted to make it. What it does is loops through every pixel in a BufferedImage, loaded from a 16-bit PNG image file (byte for each component) and it divides the current X and Y components by the square size . From here it performs a further nested for loop to total the pixel colours in the block that this pixel is in.

It then divides by the block size, which is represented by the "pp" field, except it's squared because it samples pixels in a square along the X and Y axes. According to a few explanations I've read this should give me the average pixel colour for this block and every pixel in this block should be coloured in it. The algorithm I've written works somewhat in that the general shape of the image is preserved after the pixelation with far less detail, as is expected. The problem is that the colours are completely wrong and I'll show you how in the image below:

Here there's just a picture of a hamster I found. The algorithm magnifies the area by a sort of scale factor.

Also here's the algorithm:

Can anyone spot why the colours aren't averaging out right?


Offline SimonH
« Reply #1 - Posted 2013-04-18 19:14:15 »

You can't average colours like that, you have to break them into RGB;
int red;
int green;
int blue;

// loop over pp field of (numDots pixels)


// end loop

int averageColour=((red/numDots)<<16)|((green/numDots)<<8)|(blue/numDots);

// loop over pp field


// end loop

Edit: On reflection, it might be better just to use the first pixel of the pp field to colour the whole field - faster too!

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Offline RobinB

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« Reply #2 - Posted 2013-04-18 19:14:50 »

You cant just add ints and then devide, it works for numbers, but it really messes up colors.
this way 0x000100 / 2 becomes  0x000080, so very black-green becomes blue.
i guess you should separate the colors, and count these up or something.
Im sure there is some kind of blending algorithm.

*dahm, ninjad Smiley
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Offline Porlus

Junior Devvie

« Reply #3 - Posted 2013-04-18 19:24:31 »

Thanks for both your replies. Smiley I've substituted the single colour divides and used the components and it works great. I was just also wondering whether the magnification of the image is expected or whether I've glossed over another detail. :p Normally when I see pixelation the area of the image that's been pixelated is to scale with the rest of the image.
Offline Porlus

Junior Devvie

« Reply #4 - Posted 2013-04-18 21:04:04 »

I've sorted it now. Previously I'd made a magnification method and I think I borrowed too much logic from it. The working solution just loops through the outer for loops in steps the size of the block size. Then an inner set of nested for loops samples each of the pixels in that block, finds the average colour and then loops through them setting each to the colour of the average.
It's not the best solution but I'll post it here in case anyone needs a starting point.
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