How can I extract the data from an image and process it pixel by pixel?

Question

For example, to change the color of each pixel to the mean color of the three channels, I tried

i = ExampleData[{"TestImage", "Lena"}];
Mean[i]

but it just remains unevaluated:

How can I read the colors of an image into a list or matrix and change the color codes and save it back to an image?

Answer 1

Certainly. For instance, here's how to reduce the number of colours to 10 (randomly chosen in RGB space):

i = Import["ExampleData/lena.tif"]

You can try ImageData[i] to see the actual RGB values for each pixel. Now produce ten random triplets of reals between 0. and 1., and construct a function to quickly pick the one closest to some given number:

colours = RandomReal[{0, 1}, {10, 3}];
nf = Nearest[colours];

Then map the thing over the RGB values of the image and look at it:

Map[First[nf[#]] &, ImageData[i], {-2}] // Image

Try increasing the number of randomly selected colours to see what happens:

Manipulate[
 Module[{colours = RandomReal[{0, 1}, {num, 3}], nf},
  nf = Nearest[colours];
  Map[First[nf[#]] &, ImageData[i], {-2}] // Image
  ],
 {{num, 10}, 1, 1000, 1}
 ]

Answer 2

The ImageApply function applies any suitable Mathematica function to every pixel in an image. You just have to specify the transformation you want to make. I recently asked this: Image levels: how to alter 'exposure' of dark and light areas? question - and got many good answers...

Answer 3

Have you looked through the included documentation?

guide/ImageProcessing

tutorial/ImageProcessing

There are also a number of entries on the Wolfram Blog that relate to image processing:

the incredible convenience of mathematica image processing

aMAZEing image processing in mathematica

the battle of the marlborough maze at blenheim palace continues

fun with line-art

fixing bad astrophotography

To learn more on the subject ask Heike. ;-)

Answer 4

There are also several built-in effects available through ImageEffect. For example:

ImageEffect[ExampleData[{"Image","Lena"}], #]& /@ 
    {"Charcoal", {"OilPainting", 10}, {"Posterization", 5}}