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How to access elements of single channel IplImage in Opencv

How can I access the IplImage Elements (one channel and IPL_DEPTH_8U depth).

I want to change the pixel value at a specific (x, y) position of an image.

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opencv provides CV_IMAGE_ELEM method to access IplImage elements, this is a macro,

define CV_IMAGE_ELEM( image, elemtype, row, col )       \
    (((elemtype*)((image)->imageData + (image)->widthStep*(row)))[(col)])


Second parameter - type

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Pixels are stored inside the imageData array. So, since your image is the only channel, you just need to do the following:

myimage.imageData[y*myimage.width+x] = 100;

This provides the correct offset from the beginning of the buffer in imageData and is more readable than any other pointer algebra operation.

In the images of N-channels, it is enough to multiply the array offset by N and add the number of channels for reading:



i.e. for RGB image

myimage.imageData[3*(y*myimage.width+x)+0] = 100; //Red
myimage.imageData[3*(y*myimage.width+x)+1] = 100; //Green
myimage.imageData[3*(y*myimage.width+x)+2] = 100; //Blue

Any optimization to avoid multiplying the data to get the index can be done according to the goal you need to achieve.

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A quick way to get the pixel value is to use a macro.

CV_IMAGE_ELEM( image_header, elemtype, y, x_Nc )

And in your case, the image is the only channel. So you can get the value of pixel i, j by

CV_IMAGE_ELEM(image, unsigned char, i, j)
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