Writing an image with floating point values

Question

My code reads an RGB image, processing it to produce floating-point results. How can I write these values as an image, maintaining these floating-point values, in Matlab 7.6.0 (R2008a)?

Answer 1

It is actually possible to fit 64 bits of pixel data (i.e. the number of bits needed to represent a double-precision floating-point value) into certain image formats, specifically a PNG. And you can even recover the data exactly. The key is to encode everything as uint16 values and use a Truecolor RGB image (3 color planes of 16 bit data) along with an alpha transparency map (another 16 bits). Here's the encoding step:

data = rand(4);              % Just some sample data, a small 4-by-4 matrix
hexData = num2hex(data(:));  % Get the 16 digit hex codes for each pixel
C = mat2cell(hexData, numel(data), [4 4 4 4]);  % Split the hex codes into 4 groups of 4
C = cellfun(@(c) {uint16(hex2dec(c))}, C);  % Convert each hex value into a 16 bit integer
colorData = reshape([C{1:3}], [size(data) 3]);  % 4-by-4-by-3 uint16 color data
alphaData = reshape(C{4}, size(data));          % 4-by-4 uint16 alpha data
imwrite(colorData, 'double_data.png', 'Alpha', alphaData);  % Save image

You now have a valid image file, although it'll probably looks like random garbage if you view it. Now, you can decode it like so:

[imColor, ~, imAlpha] = imread('double_data.png');  % Load image
imSize = size(imColor);  % Get image size
imHex = [dec2hex(imColor(:, :, 1)) ...  % Convert each uint16 to hex and concatenate
         dec2hex(imColor(:, :, 2)) ...
         dec2hex(imColor(:, :, 3)) ...
         dec2hex(imAlpha)];
imdata = reshape(hex2num(imHex), imSize(1:2));  % Reproduce data

And an equality check:

>> isequal(imdata, data)

ans =

  logical

   1           % it's the same!