MonsieurDart MonsieurDart - 6 months ago 86
iOS Question

Transforming a rectangle image into a quadrilateral using a CATransform3D

I have an image and a set of four points (describing a quadrilateral Q). I want to transform this image so that it is fits the quadrilateral Q. Photoshop calls this transformation "Distort." But according to the source of this quadrilateral (the perspective of the image moving in space), it is in fact the combination of a scale, a rotation and a perspective matrix.

I am wondering if this is possible using a CATransform3D 4x4 matrix. Do you have any hints on how to do that? I've tried to take the four points and build 16 equations (out of A' = A x u) but it did not work: I'm not sure of what I should use as z, z', w and w' coefficients…

The following picture shows what I want to do:

Transforming a rectangle image into a quadrilateral using a CATransform3D

Here are some examples of points:

276.523, 236.438, 517.656, 208.945, 275.984, 331.285, 502.23, 292.344
261.441, 235.059, 515.09, 211.5, 263.555, 327.066, 500.734, 295
229.031, 161.277, 427.125, 192.562, 229.16, 226, 416.48, 256

Answer

I've created a kit for doing this on iOS: https://github.com/hfossli/AGGeometryKit/


Make sure your anchor point is top left (CGPointZero).

+ (CATransform3D)rectToQuad:(CGRect)rect
                     quadTL:(CGPoint)topLeft
                     quadTR:(CGPoint)topRight
                     quadBL:(CGPoint)bottomLeft
                     quadBR:(CGPoint)bottomRight
{
    return [self rectToQuad:rect quadTLX:topLeft.x quadTLY:topLeft.y quadTRX:topRight.x quadTRY:topRight.y quadBLX:bottomLeft.x quadBLY:bottomLeft.y quadBRX:bottomRight.x quadBRY:bottomRight.y];
}

+ (CATransform3D)rectToQuad:(CGRect)rect
                    quadTLX:(CGFloat)x1a
                    quadTLY:(CGFloat)y1a
                    quadTRX:(CGFloat)x2a
                    quadTRY:(CGFloat)y2a
                    quadBLX:(CGFloat)x3a
                    quadBLY:(CGFloat)y3a
                    quadBRX:(CGFloat)x4a
                    quadBRY:(CGFloat)y4a
{
    CGFloat X = rect.origin.x;
    CGFloat Y = rect.origin.y;
    CGFloat W = rect.size.width;
    CGFloat H = rect.size.height;

    CGFloat y21 = y2a - y1a;
    CGFloat y32 = y3a - y2a;
    CGFloat y43 = y4a - y3a;
    CGFloat y14 = y1a - y4a;
    CGFloat y31 = y3a - y1a;
    CGFloat y42 = y4a - y2a;

    CGFloat a = -H*(x2a*x3a*y14 + x2a*x4a*y31 - x1a*x4a*y32 + x1a*x3a*y42);
    CGFloat b = W*(x2a*x3a*y14 + x3a*x4a*y21 + x1a*x4a*y32 + x1a*x2a*y43);
    CGFloat c = H*X*(x2a*x3a*y14 + x2a*x4a*y31 - x1a*x4a*y32 + x1a*x3a*y42) - H*W*x1a*(x4a*y32 - x3a*y42 + x2a*y43) - W*Y*(x2a*x3a*y14 + x3a*x4a*y21 + x1a*x4a*y32 + x1a*x2a*y43);

    CGFloat d = H*(-x4a*y21*y3a + x2a*y1a*y43 - x1a*y2a*y43 - x3a*y1a*y4a + x3a*y2a*y4a);
    CGFloat e = W*(x4a*y2a*y31 - x3a*y1a*y42 - x2a*y31*y4a + x1a*y3a*y42);
    CGFloat f = -(W*(x4a*(Y*y2a*y31 + H*y1a*y32) - x3a*(H + Y)*y1a*y42 + H*x2a*y1a*y43 + x2a*Y*(y1a - y3a)*y4a + x1a*Y*y3a*(-y2a + y4a)) - H*X*(x4a*y21*y3a - x2a*y1a*y43 + x3a*(y1a - y2a)*y4a + x1a*y2a*(-y3a + y4a)));

    CGFloat g = H*(x3a*y21 - x4a*y21 + (-x1a + x2a)*y43);
    CGFloat h = W*(-x2a*y31 + x4a*y31 + (x1a - x3a)*y42);
    CGFloat i = W*Y*(x2a*y31 - x4a*y31 - x1a*y42 + x3a*y42) + H*(X*(-(x3a*y21) + x4a*y21 + x1a*y43 - x2a*y43) + W*(-(x3a*y2a) + x4a*y2a + x2a*y3a - x4a*y3a - x2a*y4a + x3a*y4a));

    const double kEpsilon = 0.0001;

    if(fabs(i) < kEpsilon)
    {
        i = kEpsilon* (i > 0 ? 1.0 : -1.0);
    }

    CATransform3D transform = {a/i, d/i, 0, g/i, b/i, e/i, 0, h/i, 0, 0, 1, 0, c/i, f/i, 0, 1.0};

    return transform;
}

I take no credit for this code. All I did was scouring the internet and put together various incomplete answers.