I have computed an image with values between 0 and 255. When I use imageview(), the image is correctly displayed, in grey levels, but when I want to save this image or display it with imshow, I have a white image, or sometimes some black pixels here and there:
Whereas with imageview():
Can some one help me?
I think that you should use imshow(uint8(image)); on the image before displaying it.
Matlab expects images of type double to be in the 0..1 range and images that are uint8 in the 0..255 range. You can convert the range yourself (but change values in the process), do an explicit cast (and potentially loose precision) or instruct Matlab to use the minimum and maximum value found in the image matrix as the white and black value to scale to when visualising.
See the following example with an uint8 image present in Matlab:
im = imread('moon.tif');
figure; imshow(im);
figure; imshow(double(im));
figure; imshow(double(im), []);
figure; imshow(im2double(im));
Related
clear all;
clc;
imag = imread('286502.png');
image_binary = im2bw(imag,0.85); %converte image to binary
image_binary = not(image_binary);
figure(1);clf
imagesc(image_binary);colormap(gray)
I am using this code to genarate a binary image of an ellipse having its inside as white while outside as black.But , the problem i am facing is that whenever the foreground and background of my input ellipse is either light or dark then the binary image becomes all black or all white.
if you want to keep the gray-scale nature of our image, you may remove the line im2bw command because that flattens the gray-scale values into binary (0-1 or white/black).
if you just want to show a binary image in gray color, you can try customizing the colormap
colormap([0 0 0;0.5 0.5 0.5]);
im2bw(imag,0.85) thresholds at a fixed value, which will work for some images but not for others. I recommend that you binarize with a method such as Otsu, which determines an optimal threshold for each image individually.
image_binary = imbinarize(imag);
I have an image with dark blue spots on a black background. I want to convert this to inverse gray scale. By inverse, I mean, I want the black ground to be white.
When I convert it to gray scale, it makes everything look black and it makes it very hard to differentiate.
Is there a way to do an inverse gray scale where the black background takes the lighter shades?
Or, another preferable option is to represent the blue as white and the black as black.
I am using img = rgb2gray(img); in MATLAB for now.
From mathworks site:
IM2 = imcomplement(IM)
Is there a way to do an inverse gray scale where the black
background takes the lighter shades?
Based on your image description I created an image sample.png:
img1 = imread('sample.png'); % Read rgb image from graphics file.
imshow(img1); % Display image.
Then, I used the imcomplement function to obtain the complement of the original image (as suggested in this answer).
img2 = imcomplement(img1); % Complement image.
imshow(img2); % Display image.
This is the result:
Or, another preferable option is to represent the blue as white and
the black as black.
In this case, the simplest option is to work with the blue channel. Now, depending on your needs, there are two approaches you can use:
Approach 1: Convert the blue channel to a binary image (B&W)
This comment suggests using the logical operation img(:,:,3) > 0, which will return a binary array of the blue channel, where every non-zero valued pixel will be mapped to 1 (white), and the rest of pixels will have a value of 0 (black).
While this approach is simple and valid, binary images have the big disadvantage of loosing intensity information. This can alter the perceptual properties of your image. Have a look at the code:
img3 = img1(:, :, 3) > 0; % Convert blue channel to binary image.
imshow(img3); % Display image.
This is the result:
Notice that the round shaped spots in the original image have become octagon shaped in the binary image, due to the loss of intensity information.
Approach 2: Convert the blue channel to grayscale image
A better approach is to use a grayscale image, because the intensity information is preserved.
The imshow function offers the imshow(I,[low high]) overload, which adjusts the color axis scaling of the grayscale image through the DisplayRange parameter.
One very cool feature of this overload, is that we can let imshow do the work for us.
From the documentation:
If you specify an empty matrix ([]), imshow uses [min(I(:)) max(I(:))]. In other words, use the minimum value in I as black, and the maximum value as white.
Have a look at the code:
img4 = img1(:, :, 3); % Extract blue channel.
imshow(img4, []); % Display image.
This is the result:
Notice that the round shape of the spots is preserved exactly as in the original image.
This question already has an answer here:
What does the index refer to when selecting a pixel on an image in Matlab?
(1 answer)
Closed 6 years ago.
I have gray scale image "lena.bmp". I want read this image in matlab using imread() function.
When i use code below to read and show image my image is dark (black).
img = imread('lena.bmp');
imshow(img);
But when i use code below, I have no problem to view.
[img map]= imread('lena.bmp');
imshow(img,map);
It seems that my first code doses not reading image in grayscale mode (like what rgb2gray function generate).
My image is as follows:
What can i do to solve this problem?
Your image is an "indexed" image. That means it contains integer values which act as "labels" more than anything, and each of those labels is mapped to a colour (i.e. an rgb triplet). Your map variable represents that mapping; at row 5 you have the rgb triplet that corresponds to 'label' "5", for instance.
To see what I mean, do unique(img) and you'll see that the values of your img array are in fact quite regular. The command rgbplot can demonstrate the actual colourmap graphically. Run rgbplot(map) on your map variable to see the mapping for each of the red green and blue colours.
Now, save and read the image below on your computer as img2 and compare the array values.
This image was generated by converting from the "indexed" image you linked to, to a "grayscale" one using photoediting software (the GIMP). The difference is that
in a grayscale image, the pixel values represent actual intensities, rather than integer 'labels'. Imread reads grayscale images as uint8 images by default, meaning it assigns intensity values to pixels ranging from 0 (black) to 255 (white). Since these values happen to be integers you could still cheat and treat them as 'labels' and force a colour-mapping on them. But if you assign a 'linear map' (i.e. value 1 = intensity 1, value 2 = intensity 2, etc) then your image will look as you would expect.
You'll see that the values from unique(img2) are quite different. If you imshow(img2) you'll see this displays as you'd expect. If you don't specify a colormap for imshow, it will assume that the map is a linear mapping from the lowest to the highest value in the image array, which explains why your indexed image looked weird, since its values were never supposed to correspond to intensities.
Also try imagesc(img2) which will show this but using the "current" colormap. imagesc causes the colormap to be "scaled", so that the lowest colour goes to the lowest value in the image, and similarly for the highest.
The default colormap is jet so you should see a psychedelic looking image but you should be able to make out lena clearly. If you try colormap gray you should see the gray version again. Also try colormap hot. Now to make sense of the colormaps, try the rgbplot command on them (e.g. rgbplot(gray), rgbplot(hot) etc).
So, going back to imshow, imshow basically allows you to display an indexed image, and specify what colormap you want to use to display it. If you don't specify the colormap, it will just use a linear interpolation from the lowest value to the highest as your map. Therefore imshow(img) will show the image pretty much in the same way as imagesc(img) with a gray colormap. And since the values in your first img represent evenly spaced 'labels' rather than actual intensities, you'll get a rubbish picture out.
EDIT: If you want to convert your indexed image to a grayscale image, matlab provides the ind2gray function, e.g.:
[img, map] = imread('lena.bmp');
img_gray = ind2gray(img, map);
This is probably what you need if you mean to process pixel values as intensities.
i have an image let say a=imread('example.bmp'i got all three channel from it :
R=a(:,:,1);
G=a(:,:,2);
B=a(:,:,3);
and i have the gray image of it:
igray=rgb2gray(a);
Can I get the red component from the gray image ?
No, you can't, since igray will be a two dimensional image (a is three dimensional, with the third dimension being colors planes), containing only intensity values for each pixel.
To convert an RGB image to grayscale, rbg2gray uses a formula you can find here
As you can see, it's a 3 variables equation, therefore you can't find them using intensity value alone.
The rgb2gray function effectively does this to every RGB pixel (type edit rgb2gray):
Gray = 0.298936021293776*Red+0.587043074451121*Green+0.114020904255103*Blue;
If you only have Gray in the equation above then you have one equation with three unknowns. More information is needed to solve for Red.
If you just want an RGB image where every channel has the same components, i.e., those created by rgb2gray, then use
igray(:,:,3) = rgb2gray(a); % Set last component first to fully allocate array
igray(:,:,1) = igray(:,:,3);
igray(:,:,2) = igray(:,:,3);
Or an RGB image where all the channels are equivalent to the red channel:
igray(:,:,3) = a(:,:,1);
igray(:,:,1) = a(:,:,1);
igray(:,:,2) = a(:,:,1);
The repmat function can be used as well if you prefer.
While nothing in horchler's very long answer is incorrect, I think you just want to get the red channel from the rgb image which is very easy.
A=imread('colorImg.jpg')
redChannel=A(:,:,1)
That's it!
That will return a matrix of type uint8, to convert to double, just do double(redChannel) and you can multipy/divide it by 255 as necessary.
When i wrote these commands
out = ones(size(ben))
imshow(out)
the output is a white picture but i expect almost dark picture because the rgb values are 1,1,1. when i give 255,255,255 it also gives a white picture. Isn't this a dilemma ?
Try out = ones(size(ben), 'uint8');
ones() by default creates an array of doubles. When imshow() gets an array of doubles it assumes that the pixel values range between 0 and 1, and assigns the white color to anything greater than 1. However, if you pass an array of uint8 to imshow() it will assume the range to be between 0 and 255.
You can also try using imagesc(); instead of imshow(), but you may need to do colormap gray after wards to get a grayscale image.
Another alternative is to rescale the image before display:
imshow(out / max(out(:)));