How to display grayscale image using imshow function
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Why the below code not showing grayscale image
f = imread('lena.bmp');
imshow(f, [0 80])
I'm reading a book Digital Image Processing using Matlab, they have discussed the below syntax to show grayscale image.
imshow(f, [low high])
displays as black all values less than or equal to low, and as white all values greater than or equal to high
I'm using Matlab R2015a
1 Kommentar
Antworten (2)
Guillaume
am 21 Okt. 2015
Bearbeitet: Guillaume
am 21 Okt. 2015
The problem is that your f (what a bad variable name for an image!) is not a greyscale image as greyscale is not supported by the 'bmp' format. It is an RGB image that may happen to have all three channels identical, so it appear grey. A true greyscale image has only one colour channel.
imshow(f(:, :, 1), [0 80]) %since all three channels should be identical
%or
imshow(rgb2gray(f), [0 80]) %works even if all 3 channels are not identical, but then the original image is not greyscale
imshow with a RGB image appears to ignore the 'DisplayRange' argument.
4 Kommentare
Guillaume
am 21 Okt. 2015
Bearbeitet: Guillaume
am 21 Okt. 2015
By the way, it's why I always recommend using PNG as an image format. The PNG format support all three types of images (indexed, greyscale, rgb), plus optional transparency, plus up to 16 bit per channel, plus storage of metadata. And it's got fast non-lossy compression to boot.
Atinesh S
am 24 Okt. 2015
4 Kommentare
Guillaume
am 24 Okt. 2015
You have to differentiate between images in memory and image file format. You can have a greyscale image in memory but if you save it to BMP it has to be converted either to indexed or rgb. In JPG, it's always RGB. Regardless, IF you know the image is grayscale, you can simply load it and convert the rgb/indexed image back to grayscale with no loss of data. With indexed, just discard the colour map, with rgb just discard two of the channels.
There are not many image formats that will store an image as greyscale. As mentioned in my answer, PNG is one of them, and the one I'd recommend. TIF is another (I think, that format is a mess).
Walter Roberson
am 25 Okt. 2015
With indexed, do not just discard the colormap: the index is not necessarily in order of grayscale level. You should instead use ind2gray()
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