Convert Color-Indexed Image to Indexes - image

Question

I have a directory of images in png format, which use a fixed color index for their encoding. I'm interested in producing images whose grayscale values correspond to the index itself, not the color for that index.
For example, suppose the color index has the entry index 3 -> (255, 0, 0) (red). I want to replace every instance of (255, 0, 0) in the RGB image with the grayscale value of 3.
My first thought is to 1) hard code the reversed color index into a lookup table 2) load the image, 3) iterate over pixels, doing a replace based on the look up table.
The problem with this is hard coding the lookup table. I can get it (via imagemagick identify), but that's tedious. Are there any libraries that can do this for me? I'm looking for either 1) cmd line transformations or 2) code libraries that get the color index for a pixel.

Answer 1

If you open a palettised image in PHP using GD, but accidentally forget to tell GD that it is palettised, you will actually get the palette index in the blue pixel. So you could take advantage of that to create a grayscale image by creating a truecolour image the same size as your original and then writing the palette index three times, once each for the Red, Green and Blue channels. It's easier than it sounds, here is the code:
#!/usr/local/bin/php
<?php
// Read in a palettised image
$im = imagecreatefrompng("pal.png");
// Create output image same size
$out = imagecreatetruecolor(imagesx($im), imagesy($im));
for ($x = 0; $x < imagesx($im); $x++) {
for ($y = 0; $y < imagesy($im); $y++) {
$pixel = imagecolorat($im, $x, $y);
$index = $pixel & 0xFF;
$outCol = imagecolorallocate($out,$index,$index,$index);
imagesetpixel($out,$x,$y,$outCol);
// printf("[%d,%d] Palette index:%d\n",$x,$y,$index);
}
}
imagepng($out,"result.png");
?>
So, if I create a 3 pixel palettised image with ImageMagick like this:
convert xc:red xc:lime xc:blue +append pal.png
and check it has palette like this
identify -verbose pal.png | more
Image: pal.png
Format: PNG (Portable Network Graphics)
Mime type: image/png
Class: PseudoClass
Geometry: 3x1+0+0
Units: Undefined
Type: Palette <--- it has a palette
Endianess: Undefined
Colorspace: sRGB
Depth: 8/1-bit
Channel depth:
red: 1-bit
green: 1-bit
blue: 1-bit
Channel statistics:
Pixels: 3
Red:
min: 0 (0)
max: 255 (1)
mean: 85 (0.333333)
standard deviation: 120.208 (0.471405)
kurtosis: -1.5
skewness: 0.707107
entropy: 0.918296
Green:
min: 0 (0)
max: 255 (1)
mean: 85 (0.333333)
standard deviation: 120.208 (0.471405)
kurtosis: -1.5
skewness: 0.707107
entropy: 0.918296
Blue:
min: 0 (0)
max: 255 (1)
mean: 85 (0.333333)
standard deviation: 120.208 (0.471405)
kurtosis: -1.5
skewness: 0.707107
entropy: 0.918296
Image statistics:
Overall:
min: 0 (0)
max: 255 (1)
mean: 85 (0.333333)
standard deviation: 120.208 (0.471405)
kurtosis: -1.5
skewness: 0.707107
entropy: 0.918296
Colors: 3
Histogram:
1: ( 0, 0,255) #0000FF blue
1: ( 0,255, 0) #00FF00 lime
1: (255, 0, 0) #FF0000 red
Colormap entries: 4
Colormap: <--- here is the palette
0: (255, 0, 0) #FF0000 red
1: ( 0,255, 0) #00FF00 lime
2: ( 0, 0,255) #0000FF blue
3: (255,255,255) #FFFFFF white
Then check the result.png after running the PHP script, it now looks like this - i.e. greyscale and the colours match the former indices.
identify -verbose result.png
Image: result.png
Format: PNG (Portable Network Graphics)
Mime type: image/png
Class: DirectClass
Geometry: 3x1+0+0
Units: Undefined
Type: Grayscale <--- it is now greyscale, no palette
Endianess: Undefined
Colorspace: sRGB
Depth: 8-bit
Channel depth:
gray: 8-bit
Channel statistics:
Pixels: 3
Gray:
min: 0 (0)
max: 2 (0.00784314)
mean: 1 (0.00392157)
standard deviation: 0.816497 (0.00320195)
kurtosis: -1.5
skewness: 0
entropy: 1
Colors: 3
Histogram:
1: ( 0, 0, 0) #000000 gray(0)
1: ( 1, 1, 1) #010101 gray(1)
1: ( 2, 2, 2) #020202 gray(2)
Note that if your original image had very few colour palette entries, the output image will be dark so you will want to contrast-stretch or normalize it...