how to use imresize function. (2024)

"Why the number of 3 have"

IMRESIZE does not convert an RGB image into a grayscale or select color channels or whatever unspecified operation you expect to occur. IMRESIZE uses a fairly universal definition of "resizing" an image: it changes the number of row/columns an image has, exactly as its documentation clearly states "If A has more than two dimensions, then imresize only resizes the first two dimensions". It is highly recommended to read the documentation for functions that you are using, to learn what those functions do and how to call them properly. Reading the documentation is faster and more reliable than guessing.

If you want one channel of your image then use indexing.

If you want to convert an RGB (e.g. truecolor) image to grayscale then use IM2GRAY:

Thats means the number of 3 showing the image is RGB.

"Thats means the number of 3 showing the image is RGB."

Probably, but not necessarily: it might be an Lab image, or use any of countless other colorspaces.

The number of channnels does not uniquely determine what colorspace an image is encoded with, that can only be determined with prior-knowledge (e.g. the color encoding stored as part of in an image file).

% Read in reference image and it's colormap

[indexedImage, cmap] = imread('image2988.png');

% Get it's size.

[rows, columns, numberOfColorChannels] = size(indexedImage)

if isempty(cmap)

% If colormap does not exist, use gray scale.

cmap = gray(256);

end

% Read in unknown RGB image

unknownImage = imread('unknown.png');

% Resize image. Note this may change colors obviously.

unknownImage = imresize(unknownImage, [rows, columns]); % It's still an RGB image after this.

% Turn into an indexed image using the same colormap as the reference image.

unknownImage = rgb2ind(unknownImage, cmap);

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