When reading an image, why is the map value blank?

Hi,

I am currently doing a project relating to image compression. And I'm trying to figure out a slight error, my code reads like this to read the image

[uploadedImage, map] = imread('Test.png');

But when i look at the variable uploadedImage it shows a 3-dimensional array. Which is fine, however the map variable has a value of [ ]. Can anyone tell me why this is happening?

Answer 1

[Walter Roberson]

This is common.

RGB images are r x c x 3 with an empty map

CMYK images (tiff only) are r x c x 4 with an empty map

RGBA images are sometimes r x c x 4 with an empty map, but are sometimes r x c x 3 with a third output that can indicate alpha

Grayscale images are r x c with an empty map. These have an implicit grayscale map.

Gray+alpha images are uncommon but are r x c x 2 with an empty map

Pseudocolor images are r x c with a nonempty map and can be converted to rgb with ind2rgb()

[Kishan Narotam]

Ah, that makes sense. Thank you so much.

[Carla Mirabella]

Could you explain how you can concatenate the two arrays image and map if they have different dimensions?

[Walter Roberson]

combined = ind2rgb(FirstImage, FirstMap);

combined_vertical(end+1:end+size(SecondImage,1), 1:size(SecondImage,2), :) = ind2rgb(SecondImage, SecondMap);

combined_horizontal(1:size(SecondImage,1), end+1:end+size(SecondImage,2), :) = ind2rgb(SecondImage, SecondMap);

If you want to get out a pseudocolor image then

combined_map = unique( [FirstMap; SecondMap; 0 0 0], 'rows');

pseudo_vertical = rgb2ind(combined_vertical, combined_map);

pseud_horizontal = rgb2ind(combined_horizontal, combined_map);

The 0 0 0 added before the unique is in case the original color maps did not have a pure black: when the images do not have the dimensions then padding was automatically added that is all 0 0 0 so we have to accomodate that in the color map.