Perspective Transform + Crop in iOS with OpenCV

So after a few days of trying to solve it, I came up with a solution (Ignore the blue dots on the second image):

As promised, here's a complete copy of the code:

- (void)confirmedImage{    cv::Mat originalRot = [self cvMatFromUIImage:_sourceImage];    cv::Mat original;    cv::transpose(originalRot, original);    originalRot.release();    cv::flip(original, original, 1);    CGFloat scaleFactor =  [_sourceImageView contentScale];    CGPoint ptBottomLeft = [_adjustRect coordinatesForPoint:1 withScaleFactor:scaleFactor];    CGPoint ptBottomRight = [_adjustRect coordinatesForPoint:2 withScaleFactor:scaleFactor];    CGPoint ptTopRight = [_adjustRect coordinatesForPoint:3 withScaleFactor:scaleFactor];    CGPoint ptTopLeft = [_adjustRect coordinatesForPoint:4 withScaleFactor:scaleFactor];    CGFloat w1 = sqrt( pow(ptBottomRight.x - ptBottomLeft.x , 2) + pow(ptBottomRight.x - ptBottomLeft.x, 2));    CGFloat w2 = sqrt( pow(ptTopRight.x - ptTopLeft.x , 2) + pow(ptTopRight.x - ptTopLeft.x, 2));    CGFloat h1 = sqrt( pow(ptTopRight.y - ptBottomRight.y , 2) + pow(ptTopRight.y - ptBottomRight.y, 2));    CGFloat h2 = sqrt( pow(ptTopLeft.y - ptBottomLeft.y , 2) + pow(ptTopLeft.y - ptBottomLeft.y, 2));    CGFloat maxWidth = (w1 < w2) ? w1 : w2;    CGFloat maxHeight = (h1 < h2) ? h1 : h2;    cv::Point2f src[4], dst[4];    src[0].x = ptTopLeft.x;    src[0].y = ptTopLeft.y;    src[1].x = ptTopRight.x;    src[1].y = ptTopRight.y;    src[2].x = ptBottomRight.x;    src[2].y = ptBottomRight.y;    src[3].x = ptBottomLeft.x;    src[3].y = ptBottomLeft.y;    dst[0].x = 0;    dst[0].y = 0;    dst[1].x = maxWidth - 1;    dst[1].y = 0;    dst[2].x = maxWidth - 1;    dst[2].y = maxHeight - 1;    dst[3].x = 0;    dst[3].y = maxHeight - 1;    cv::Mat undistorted = cv::Mat( cvSize(maxWidth,maxHeight), CV_8UC1);    cv::warpPerspective(original, undistorted, cv::getPerspectiveTransform(src, dst), cvSize(maxWidth, maxHeight));    UIImage *newImage = [self UIImageFromCVMat:undistorted];    undistorted.release();    original.release();    [_sourceImageView setNeedsDisplay];    [_sourceImageView setImage:newImage];    [_sourceImageView setContentMode:UIViewContentModeScaleAspectFit];}- (UIImage *)UIImageFromCVMat:(cv::Mat)cvMat{    NSData *data = [NSData dataWithBytes:cvMat.data length:cvMat.elemSize() * cvMat.total()];    CGColorSpaceRef colorSpace;    if (cvMat.elemSize() == 1) {        colorSpace = CGColorSpaceCreateDeviceGray();    } else {        colorSpace = CGColorSpaceCreateDeviceRGB();    }    CGDataProviderRef provider = CGDataProviderCreateWithCFData((__bridge CFDataRef)data);    CGImageRef imageRef = CGImageCreate(cvMat.cols,                                     // Width                                        cvMat.rows,                                     // Height                                        8,                                              // Bits per component                                        8 * cvMat.elemSize(),                           // Bits per pixel                                        cvMat.step[0],                                  // Bytes per row                                        colorSpace,                                     // Colorspace                                        kCGImageAlphaNone | kCGBitmapByteOrderDefault,  // Bitmap info flags                                        provider,                                       // CGDataProviderRef                                        NULL,                                           // Decode                                        false,                                          // Should interpolate                                        kCGRenderingIntentDefault);                     // Intent    UIImage *image = [[UIImage alloc] initWithCGImage:imageRef];    CGImageRelease(imageRef);    CGDataProviderRelease(provider);    CGColorSpaceRelease(colorSpace);    return image;}- (cv::Mat)cvMatFromUIImage:(UIImage *)image{    CGColorSpaceRef colorSpace = CGImageGetColorSpace(image.CGImage);    CGFloat cols = image.size.height;    CGFloat rows = image.size.width;    cv::Mat cvMat(rows, cols, CV_8UC4); // 8 bits per component, 4 channels    CGContextRef contextRef = CGBitmapContextCreate(cvMat.data,                 // Pointer to backing data                                                    cols,                       // Width of bitmap                                                    rows,                       // Height of bitmap                                                    8,                          // Bits per component                                                    cvMat.step[0],              // Bytes per row                                                    colorSpace,                 // Colorspace                                                    kCGImageAlphaNoneSkipLast |                                                    kCGBitmapByteOrderDefault); // Bitmap info flags    CGContextDrawImage(contextRef, CGRectMake(0, 0, cols, rows), image.CGImage);    CGContextRelease(contextRef);    return cvMat;}

Hope it helps you + happy coding!

I think the point correspondence in getAffineTransform is incorrect.

Check the point coordinates output by box.points(pts);

Why not just use p1 p2 p3 p4 to calculate the transformation?