[Tutor] PIL's palette

Peter Szinek peter at rt.sk
Fri Jul 15 07:58:10 CEST 2005

Can you pls send me the image you ar working with?

D. Hartley wrote:
>>Your palette is a LookUp Table (LUT) - it is also named like this in
>>your prog ('lut') - which is a 768-long list (or as in this case, a list
>>of 256 lists, every inner list has 3 elements - so again you have 768
>>elements). So 3x256 or 1x768 is just an implementation detail. Let's see
>>the 3x256 version:
> Actually, it creates one list, of 768 values (numbers).  Not a list of
> lists, which could be indexed into.  Just one lonnnnng list.  So I
> dont know how it indexes into it or does anything with it! But I
> printed out "lut" and that's what it is, one long list. Anybody know
> how that works, then?
>>Now your picture looks like this:
> Even if it *did* have indices - why would my picture look like this,
> now? I dont think I understand what you're saying. Is this after I do
> the putpalette? All I know is, I start with one image, I do
> putpalette() and all of a sudden everything is a different color. And
> I dont know why it changes which colors to which. The greyscale thing
> makes a little more sense - it turns my color image into a black and
> white one, but I dont know why it chooses which grey color for which
> original color.
>>And when it is rendered, PIL looks up the color (hence lookup table) at
>>the given index. So the first pixel of the image will be the color held
>>at index 8 in the lookup table (or palette).
> Why would the first pixel be at index 8? What color is it looking up?
> I'm starting with an image (say it's a red square with a blue square
> inside it, that's more simple). What is it looking up? Pixel 1 already
> *has* a color - (255,0,0).  Putpalette() obviously ... picks some
> other value. I don't know why, or how. All the pixels in the original
> image start with a color value, and after putpalette() they end up
> with a different color value. I just dont know how it decides that, or
> why (and thus I dont know how I can control it to use it the way I
> want - for instance if I wanted to make the square yellow on the
> inside instead of blue).
>>So basically the palette is the comupter abstraction of a real painter's
>>palette - the real painter has x colors on his palette, and says (by
>>looking at the palette): ah, i need this intelligent shade of cobalt
>>blue. You do the same here - you have a palette (the look up table) and
>>say: ah, i need this color - so you put the INDEX of that color into the
>>image, and when the computer is 'painting' he looks up the color at that
> Again - I dont know how I'm telling the palette what color I want. I
> don't feel like I am telling it anything, at this point, ah ha. I just
> have a list of 768 numbers, which python somehow uses to change all
> the colors in my previous image.
>>Yep, because the first produces a palette like this:
>>index |
>>0     |  [255,127,0]    orange
>>1     |  [254,127,1]
>>2     |  [253,126,2]
>>...   |                 (some kind of purple, magenta, red ...)
>>...   |
>>255   |  [0,0,0]        black
>>So i think the indices in your original blue image point to colors in
>>the new palette which are 'red-like', hence the red-red combo.
> This seems to make a little more sense, but... why are there indices
> in my original image? What indices do you mean? And why do they point
> to colors in the new palette which are red-like?
> Also, if there is a different way to do the color-changing above, a
> simpler way that makes more sense, feel free to point it out. 
> Originally, I was changing all (0,255,0) to (255,0,0) (for example)
> with getpixel/putpixel.  But that only works if it's a blocky shape,
> no shading or anything like that.  colorize() does what I want but you
> have to start with black and white images.  I was just wondering if I
> could do something like colorize() but from red to blue for instance
> instead of from black or white to blue.  I *thought* putpalette would
> do it - and it seems to, I just have no control (at this point) over
> what colors it decides to change things to (because I dont know how
> this long 768-item list is working).
> Thanks again,
> Denise
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