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Using for loop to loop over pixels in RGB image and brighten the image.

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nopah hardih
nopah hardih 2021년 9월 21일 2:34
댓글: Image Analyst 2021년 9월 21일 17:58
Hello I am new to matlab and looking for some guidance! I am trying to brighten an image by specifically using for loops to loop over each pixel and brighten them one by one. Im aware there are faster methods but this is where im starting. When I run my code, I dont get any errors but my output image looks exactly the same as the before. I was wondering if someone could point me in the right direction and tell me what im doing wrong. Thanks!
b = imread('cell.tif');
imwrite(b,'cell.jpg','jpg')
c = imread('cell.tif');
for x = 1:150 % incriment x from 1 to 150
x = x+1;
for y = 1:150 % incriment y from 1 to 150
y = y+1;
t = b(x,y, :);% print pixel value at location
if t<255
t=t+50;
else if t>=255
t=255;
end
end
b(x,y, :) = t;
end
end
subplot(2,1,1); imagesc(c); axis image; axis off; title('before');
subplot(2,1,2); imagesc(a); axis image; axis off; title('after');

채택된 답변

Image Analyst
Image Analyst 2021년 9월 21일 17:41
Not sure what your edit to your original question was, but apparently my first answer was not enough for you to figure it out, so here is a complete solution:
% Demo by Image Analyst
clc; % Clear the command window.
close all; % Close all figures (except those of imtool.)
clear; % Erase all existing variables. Or clearvars if you want.
workspace; % Make sure the workspace panel is showing.
format short g;
format compact;
originalImage = imread('cell.tif');
% Initialize an output image.
finalImage = originalImage;
[rows, columns, numberOfColorChannels] = size(originalImage)
brightnessIncrement = 50;
for x = 1:150 % Increment x from 1 to 150 (or columns)
for y = 1:150 % Increment y from 1 to 150 (or rows)
t = originalImage(y, x, :);
fprintf('For row %d, column %d, original brightness = %d\n', y, x, t); % Print pixel value at location
finalImage(y, x, :) = t + brightnessIncrement;
end
end
cmap = parula(256);
subplot(2,1,1); imshow(originalImage, [0, 255], 'Colormap', cmap); axis('on', 'image'); title('Before'); colorbar;
subplot(2,1,2); imshow(finalImage, [0, 255], 'Colormap', cmap); axis('on', 'image'); title('After'); colorbar;

추가 답변(1개)

Image Analyst
Image Analyst 2021년 9월 21일 4:30
What are you doing with c? Nothing. Anyway, c will be the same as the original b. Where are you actually displaying the images and assigning the titles? That code is not shown.
Do not assign x and y (or any loop iterator variable) inside the loop. The for line takes care of incrementing them so there is no need for you to do it manually.
In your b(x, y, :) you reversed x and y. It should be b(y, x, :) because y is the row and x is the column and arrays are indexed b(row, column, colorChannel)
t is uint8 so it will never get more than 255 so there is no need to check for that. It will just automatically clip it to 255.
When you say
t = b(x,y, :);% print pixel value at location
you're assuming b is an RGB image, though it might not be and the code will work even if it's color. However when you say
if t<255
if t came from a RGB image then t will be a 3 element vector and the if doesn't really make sense
  댓글 수: 2
Image Analyst
Image Analyst 2021년 9월 21일 17:58
b is from a tiff image and c is from a JPG image. Since c will have compression losses, it will not be the same as b for every pixel, either before or after b is changed. And neither will equal the badly-named a because there is no a defined in your code.
Yes, doing for x=1:150 makes x take on the values 1,2,3,4,5.....150. No need to assign x inside the loop.
What the instructions told you was unnecessary. It will automatically clip the values so there is no need to do it yourself. If you wanted to, you'd first have to case b to double and then values could exceed 255 and then you could clip, but this is totally unnecessary.
Your image is gray scale not color so t has only one value.
See my other answer for the solution.

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