Reversing a halftone image back to a full-color image is a challenging task. Halftoning works by turning a smooth, continuous-tone image into a pattern of black and white dots, which means the original color details are essentially lost in the process. Because of that, it is not possible to recover the exact original colors.
To handle this, a common workaround is to break the problem into two steps. First, inverse halftoning is used to reconstruct a grayscale version of the original image from the black-and-white halftone pattern. This helps in getting back some of the lost visual smoothness. After that, a colorization technique is applied to bring color into the grayscale image. One simple way to do this is by using pseudocoloring, which maps different shades of gray to a predefined set of colors.
The following procedure outlines the steps:
- Inverse Halftoning: This step converts the halftone dot patterns back into continuous gray levels. A common method is to use a low-pass filter, such as a Gaussian filter. This filter blurs the image, averaging the dots to approximate the original grayscale intensity. The “imgaussfilt” function from the Image Processing Toolbox is suitable for this purpose.
- Pseudocolor Application: Once a grayscale image is obtained, color can be added. The simplest way is to map the grayscale intensities to a predefined color map. This process is called pseudocoloring. It assigns a specific color to each gray level. The functions “gray2ind” and “ind2rgb” can be utilized for this.
You can utilize the following function as a sample case:
rgb_original = imread('image.png');
gray_original = rgb2gray(rgb_original);
halftone_image = dither(gray_original);
reconstructed_gray = imgaussfilt(double(halftone_image), 2);
[indexed_image, map] = gray2ind(reconstructed_gray, 256);
pseudocolor_image = ind2rgb(indexed_image, parula(256));
figure;
subplot(1, 3, 1);
imshow(halftone_image);
title('Original Halftone Image');
subplot(1, 3, 2);
imshow(reconstructed_gray, []);
title('Reconstructed Grayscale');
subplot(1, 3, 3);
imshow(pseudocolor_image);
title('Final Pseudocolor Image');
This code first generates a halftone image for the demonstration. It then applies a Gaussian filter to reconstruct a grayscale image. Finally, it applies the 'parula' colormap to create a pseudocolor version.
For more information on the functions used, please refer to the following official MathWorks documentation:
