MATLAB: Color-problem with imread and image

imageImage Processing Toolboximread

I'm working on a mirror anamorphosis by a cilindrical mirror. I'm having a problem with using colors. The problem can be reduced to this simple script:

picture = imread('pict.jpg')
image(picture)

This is the picture that I want to get:

This is the picture that I do get:

Does anyone know what went wrong? Many thanks.

Best Answer

It's an indexed image. Get the second output of imread(), which you are not asking for now. It will be the color map. Then apply it with the colornap() function.

Related Solutions

MATLAB: Resize and save transparent PNG file in matlab

For a start you never retrieve the transparency information so it's no wonder it's not preserved.

PNG images have two ways to specify transparency. For grayscale and true colour images, the transparency can be specified as an additional alpha channel the same size as the image. You get that in matlab from the 3rd output of imread (not the 2nd). Indexed images in PNG cannot have an alpha channel

For all types of images (so grayscale, true colour and indexed), PNG also allows you to specify a specific colour as transparent. To get that transparent colour in matlab you have to use at the SimpleTransparencyData field of imfinfo.

Assuming your image is actually true colour and that your code is a mistake, then:

[im, ~, alpha] = imread('image.png');  %transparency not valid for index images, so no point in getting map
im2 = imresize(im, [200, 200]);
alpha2 = imresize(alpha, [200 200]);
imwrite(im2, 'image2.png');

If the transparency is actually the second type ( imfinfo will have 'simple' for the Transparency field, then:

fileinfo = imfinfo('image.png');
assert(strcmp(fileinfo.Transparency, 'simple'), 'This file does not have simple transparency')
im = imread('image.png');  %if image is actually indexed, you also need to retrieve the colour map
im2 = imresize(im, [200 200]);
imwrite(im2, 'image2.png', 'Transparency', fileinfo.SimpleTransparencyData);

However, in that second case, note that because of the default interpolation, some transparent pixel may get assigned a colour that is no longer transparent. Using the non-default 'nearest' interpolation option in imresize would solve that problem.

MATLAB: Convert it into index type

          [image_mat, cmap] = imread('flage.png');
          if isempty(cmap)
              if ndims(image_mat) > 2
                  image_rgb = image_mat;
              else
                  image_rgb = repmat(image_mat, [1 1 3]);
              end
          else
              image_rgb = ind2rgb(image_mat, cmap);
          end
          
          imshow(image_rgb)

Best Answer

Related Solutions

MATLAB: Resize and save transparent PNG file in matlab

MATLAB: Convert it into index type

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