[?] Re-Mapping an image

Do you have a quick way to re-map the colors in an image to a preset pallete?

Lets suppose I have loaded a typical 16 million color image and I want to change it's colors to a 4 color pallete of say:

0 - $000000 ' black
1 - $ffffff ' white
2 - $cc1211 ' red
3 - $32ef30 ' green

What routine/function would you use?
I have a modified GreyScale converter (Birdies) which is along the lines of what I'm trying to do:

Function ConvertToGreyScale(i:TPixmap)
  For Local y=0 Until i.height
    For x=0 Until i.width
      Local col = ReadPixel( i, x, y )
      Local a = ( col & $ff000000)
      Local r = ( col & $ff0000 ) Shr 16
      Local g = ( col & $ff00 ) Shr 8
      Local b = ( col & $ff )
      Local cc= (r+g+b)/3
      col = a | (cc Shl 16) | (cc Shl 8) | cc
      WritePixel i, x, y, col
    Next
  Next
EndFunction

The bit I'm stuck on is comparing the source pixel color with the available palette colors and choosing the nearest match.

Well, i put already an example in the Wiki, http://www.blitzwiki.org/index.php/ReadPixel

You can also try my GFX module:
http://blempereur.einden.com/work/bmaxmod/ag.mod.rar
The user guide online:
http://blempereur.einden.com/work/bmaxmod/commands.html

Are you planning on adding a 'RemapPixmap' function?
That is, a function that will remap a pixmap to a fixed set of palette colors?

BTW: Nice set of image manipulation functions.

I'm still in a muddle. Should I compare each R,G,B component of the source pixel against the list colors in the palette or use the argb result?

I need to find the closest matching color:

argb=ReadPixel(mypixmap,x,y)
For pal=EachIn PaletteList
 If pal.rgb {{is a close match to}} argb
   WritePixel mypixmap,x,y,pal.rgb
 EndIf
Next

Got it working by using:

For pal=EachIn palettelist
 r=Abs(sourceRed-palRed)
 g=Abs(sourceGreen-palGreen)
 b=Abs(sourceBlue-palBlue)
 pal.index=(r+g+b)/3
Next

After that, I sort the palettelist then pick off the first entry which is the closest match.

If you finish any pixmap function, i can add it to my module, send me, you can write yourself the doc and explaination.
Coz i'm not sure that i understand what you are trying to do.

I have the re-mapping almost perfectly working now. It's just a matter of choosing an easy way to implement it into your image processing functions. The awkward part is how to pass the palette set.

This is what my function does:

Takes a loaded pixmap, re-maps the colours in the image to a preset range of colours from a palette.

Example:



As you can see, the remapping code needs more work. It fails to pick CYAN as a close match for the background.

Currently, I have the palette stored in a TYPE with r,g,b fields. Basically, the routine just needs to run through a list of colours and choose the closest match.

Maybe you would get a better color match using Hue Saturation and Value, rather than RGB?

Also, how about dithering? Floyd Steinberg? That girl on the right looks ill ;-)

Or something from the C64 days. Not really surprising considering the chosen palette.

Even with a limited palette it would look a lot more like the original with dithering, but then you have to look into building histograms of color usage and selecting an optimum palette.

Well spotted that man. I'm doing a Retro screensaver which loads up an image from a given folder then converts it to a retro-style C64 screenie.
So far it pretty much looks the business.

I'm almost there with RGB but I'm sure there was a formula for matching 3 values to a list which contains sets of 3 values. Any ideas?

What I'm doing at the moment is:

argb=ReadPixel(i,x,y)
srcR = (argb & $ff0000) Shr 16
srcG = (argb & $ff00)  Shr 8
srcB = (argb & $ff)
For pal=EachIn palettelist
	r=Abs(srcR-pal.r)
	g=Abs(srcG-pal.g)
	b=Abs(srcB-pal.b)
	pal.index=(r+g+b)/3
Next
SortList palettelist ' lowest index value comes first
fp:palette=palette(palettelist.first())
argb = $ff000000 | fp.r Shl 16 | fp.g Shl 8 | fp.b
WritePixel i, x, y, argb

I don't think this is the best way though.

Example:

source pixel color:
-------------------------
006,254,254 ' cyan


palette choices:
-------------------------
092,250,253 ' cyan
158,251,171 ' light green

My routine seems to choose light green here.