MATLAB: Change grayscale pixel color by xy coordinates

colorcoordinatepixel

I have a piece of code that returns the x and y coordinates of a rectangle that I draw on a grayscale image.
frameA=read('abc.jpg'); 
imshow(frameA);
H=imrect;
Position = wait(H);
Position contains [x y h w], where x,y = xy coordinates of top left corner of rectangle, h=rectangle height, w =rectangle width. So I have all the coordinates I need.
How can I use these coordinates to modify the color of the pixels in this rectangle?
If I use frameA(x,y) = 1 for instance, the x,y here corresponds to the indices of the pixels and not their spatial coordinates.

Best Answer

With frameA() you need to pass in the row and column, in that order, not x and y in that order. So you'd need frameA(y, x) but even then you'd need to convert y and x to integers because you can't have fractional values: frameA(round(y), round(x)).
To set the values in the rectangle to a particular gray scale, you do something like this
[rows, columns, numberOfColorChannels] = size(frameA);
if numberOfColorChannels > 1
    % Convert to grayscale if it's color.
    frameA = rgb2gray(frameA);
end
col1 = round(Position(1));
col2 = round(col1 + Position(3));
row1 = round(Position(2));
row2 = round(row1 + Position(4));
frameA(row1:row2, col1:col2) = desiredGrayLevel;
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