How to Deskew Points Using Perspective Transformation and Affine Transforms?

Perspective Transformation for Deskewing

To achieve a deskewing effect on a set of points using perspective transformation, it's essential to understand the following:

Point Ordering:

The order of points matters in perspective transformation. To ensure accuracy, the order must be consistent in both the source and destination vectors.

Image Size:

If you want the resulting image to only contain the object of interest, set its width and height to match the resulting rectangle's dimensions.

Performance Considerations:

For affine transformations like rotate, resize, and deskew, it's more efficient to use the affine counterparts:

• getAffineTransform()
• warpAffine()

Affine Transform:

getAffineTransform() only requires three points and provides a 2x3 matrix, while warpAffine() performs the warping.

Resizing the Resulting Image:

To resize the resulting image to a different size than the input, use:

cv::Size size(box.boundingRect().width, box.boundingRect().height);
cv::warpPerspective(src, dst, size, ... );

Example:

vector<Point> not_a_rect_shape;
not_a_rect_shape.push_back(Point(408, 69));
not_a_rect_shape.push_back(Point(72, 2186));
not_a_rect_shape.push_back(Point(1584, 2426));
not_a_rect_shape.push_back(Point(1912, 291));

RotatedRect box = minAreaRect(cv::Mat(not_a_rect_shape));

Point2f pts[4];
box.points(pts);

Point2f src_vertices[3];
src_vertices[0] = pts[0];
src_vertices[1] = pts[1];
src_vertices[2] = pts[3];

Point2f dst_vertices[3];
dst_vertices[0] = Point(0, 0);
dst_vertices[1] = Point(box.boundingRect().width-1, 0);
dst_vertices[2] = Point(0, box.boundingRect().height-1);

Mat warpAffineMatrix = getAffineTransform(src_vertices, dst_vertices);

cv::Size size(box.boundingRect().width, box.boundingRect().height);
warpAffine(src, rotated, warpAffineMatrix, size, INTER_LINEAR, BORDER_CONSTANT);

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