Counting colours

Kris_a · Post by **Kris_a** » Fri Mar 26, 2004 3:20 pm

Hi,

Anyone know a really fast way to count the colours in an image? The only way I know is to check each pixel, then look for a matching pixel in the palette, adding the colour to the palette if it isn't already there. This method is really slow, though, especially with pictures over 100x100 pixels.

Paint Shop Pro (7) can count the colours in just about any size of picture in under 0.3 sec. How on Earth does it do it?

Thanks,
Kris

LarsG · Post by **LarsG** » Fri Mar 26, 2004 3:41 pm

It probably loads the picture into an array upon loading...
It would then be just a matter of sorting the numbers of this array to get the colors..

(atleast that's what I would guess...

)

MLK · Post by **MLK** » Thu Apr 01, 2004 2:12 am

just read today about:

Code: Select all

GetDeviceCaps_(HDC, nIndex)

so, maybe

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GetDeviceCaps_(HDC,#NUMCOLORS)

could be interesting

just take a look at the api help

Kris_a · Post by **Kris_a** » Thu Apr 01, 2004 4:24 pm

Thanks, I have actually figured a way out

Glad to share it if anyone needs it. It's really fast (well, about the same speed as PSP's one)

PS. GetDeviceCaps only works with 8-bit images

LarsG · Post by **LarsG** » Thu Apr 01, 2004 5:30 pm

well then... just post yer code.. or tell us how you did it?!?

Kris_a · Post by **Kris_a** » Thu Apr 01, 2004 5:50 pm

Think it'd be easier to just tell you, especially since it's in C++.

All you have to do is copy the image to a buffer then use a sorting algorithm (I suggest radix, it's the fastest one for me) on it.
Then do something like this:

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last_colour = buffer(0);
num_colours = 1

for i = 0 to width*height-1
  if buffer(i) <> last_colour
    num_colours = num_colours + 1
    last_colour = buffer(i)
  endif
next

I was talking with a friend about counting colours, and this just popped into my mind. I didn't expect it to be so fast! I'd been looking for a method like this for a while, so I'm really happy about that.