In binary image, how to convert large black pixels area into white ?

I am working on skew detection and correction. I had corrected the skewness of the image, but I found that there is large size of Black areas on all sides of the corrected image. I tried to convert the black pixels (large area only) to white using the following codes. But it is not working properly. Sometimes its also remove black pixel from the pictures itself and sometimes it does not remove the larger area of the black pixels.

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    measurements = regionprops(SkewCorrectedImage == 0, 'Area','PixelIdxList');
    Regions = find([measurements.Area] >=200000);%%
    theIndexes = vertcat(measurements(Regions).PixelIdxList);
    c=SkewCorrectedImage;
    (theIndexes)=1

[Jan]

You should be able to set the added pixels to white directly in the code for rotating the image. This would be much smarter that using an auto-detection of the black triangles afterwards. So please post the code to obtain the shown image.

[Y.L.K KHUMAN]

 Here is the rotation part code:

     props = regionprops(joinedBlobs, 'BoundingBox','Orientation'); % Find bounding box.
     boundingBox = props.BoundingBox;
     angle = props.Orientation
     angleToRotate = -angle(1);
     figure, imshow(IPImg), rectangle('Position', boundingBox, 'EdgeColor', 'red');
     croppedImage = imcrop(IPImg, props.BoundingBox);
     figure, imshow(croppedImage), title('cropped Image');
     SkewCorrectedImage = imrotate(croppedImage, angleToRotate);
     figure, imshow(SkewCorrectedImage), title('corected image');
     imsave();

Answer 1

[Akira Agata]

You can easily do that by using imclearborder function, like:

BWout = ~yourBinaryImage;
BWout = imclearborder(BWout);
BWout = ~BWout;

[Y.L.K KHUMAN]

Thank for your help. When I try this code, it removes some boundary part, which is also a part of the image. Rotated Image:

After Using your code: