changing the rgb values of the pixels in a jpg image

Question

roshana 2011-4-6

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https://ww2.mathworks.cn/matlabcentral/answers/4936-changing-the-rgb-values-of-the-pixels-in-a-jpg-image

编辑： Walter Roberson 2017-7-16

J=imread('A.jpg');
for i=1:r
for j=1:c
J(i,j,1)=newrgb(1,1);
J(i,j,2)=newrgb(1,2);
J(i,j,3)=newrgb(1,3);
end
end

%%newrgb is a matrix with 3 rgb values.

when I check the newrgb matrix, it has double values larger than one,i guess 1 to 255.

I know imread gives me values of uint8, but i tried converting the newrgb values to uint8, or even converting the rgb of my image to double, but the result is a white picture, what should i do?

thank you

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Poornakant 2017-7-15

Hello Roshana, I'm working on something similar to this question of yours. Changing the RGB values of a pixel to get a specific curve of an image. I'm new to MATLAB coding could you please help me with the method you've used to sort your problem. Thank you.

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Answer 1

Walter Roberson 2011-4-6

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https://ww2.mathworks.cn/matlabcentral/answers/4936-changing-the-rgb-values-of-the-pixels-in-a-jpg-image#answer_6857

在 MATLAB Online 中打开

Replace your 3 existing assignments with

J(i,j,:) = newrgb .* 255;

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roshana 2011-4-6

still not working. you see the rgb values of J are in uint8.when I divide the newrgb values by 255, instead of multiplying, it gives me a maroon picture. in case of multiplying it is only a white page

Walter Roberson 2011-4-6

What value range is newrgb ? And what data class is the variable?

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Answer 2

roshana 2011-4-7

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newrgb values are between 1 to 100, and of type double.

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Walter Roberson 2011-4-7

If J is uint8 and your newrgb values are in the range you indicate, then your original code will set the entire block J(1:r,1:c,:) to be the color indicated by newrgb. If r and c are the size of J then the result would be a monochrome picture. If the values in newrgb are all the same, then the monochrome picture would be a shade of gray.

After you assign the values, look at J(1,1,:) to see whether the values match the (rounded) values of newrgb .

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Answer 3

roshana 2011-4-7

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https://ww2.mathworks.cn/matlabcentral/answers/4936-changing-the-rgb-values-of-the-pixels-in-a-jpg-image#answer_6927

the problem is solved. it seems matlab kept the previous wrong answer in the memory in a way. I came to know this when I changed my image, the result was still the previous image, I totally changed the variable, and it works. Maybe because I used J as image variable, and j as the loop variable ...

Thank you