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PORelationship Between Skew and Angle - CGAffineTransform
 

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  1. PORelationship Between Skew and Angle - CGAffineTransform
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    <p>I am working on an iPad application that allows the user to draw a line with their finger, then using the beginning and end points of that line I calculate the angle of the line in relation to the x/y plane of the iPad. </p> <p>My problem is that I want to use the angle of the drawn line to set the skew of an image that I am drawing into the current context. </p> <p>This is the relavent code from my drawRect method:</p> <pre><code> CGSize size = CGSizeMake(1024, 768); UIGraphicsBeginImageContext(size); CGContextRef ctx = UIGraphicsGetCurrentContext(); CGAffineTransform skewIt = CGAffineTransformMake(1, 0, skewValue, 1, 0, 0); CGContextConcatCTM(ctx, skewIt); CGContextDrawImage(UIGraphicsGetCurrentContext(), CGRectMake(0,0,size.width, size.height), theImage.CGImage); UIImage *image = UIGraphicsGetImageFromCurrentImageContext(); UIGraphicsEndImageContext(); </code></pre> <p>I calculate the angle of the line in my touchEnded method, the point values of the drawn line are stored in an array named skewArray:</p> <pre><code>- (void) touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event{ if (stepThree){ CGContextClearRect(skewContext, CGRectMake(0, 0, 1024, 668)); CGPoint pt1 = CGPointMake([[skewArray objectAtIndex:1]point3].x, [[skewArray objectAtIndex:1]point3].y); CGPoint pt2 = CGPointMake([[skewArray objectAtIndex:[skewArray count]-1]point3].x, [[skewArray objectAtIndex:[skewArray count]-1]point3].y); CGPoint pt3 = CGPointMake([[skewArray objectAtIndex:[skewArray count]-1]point3].x, [[skewArray objectAtIndex:1]point3].y); CGFloat dis1 = sqrt((pow((pt2.x - pt1.x), 2) + pow((pt2.y - pt1.y), 2))); CGFloat dis2 = sqrt((pow((pt3.x - pt2.x), 2) + pow((pt3.y - pt2.y), 2))); CGFloat dis3 = sqrt((pow((pt1.x - pt3.x), 2) + pow((pt1.y - pt3.y), 2))); angle = acos(((-1*pow(dis2, 2))+pow(dis1, 2)+pow(dis3, 2))/(2*dis1*dis3)) * 180/3.14; //Do something with the angle to produce the appropriate skew value [self setNeedsDisplay]; } } </code></pre> <p>Thank you in advance for your help!</p>
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    1. COWhat is your question? What's not working? What results are you getting that are different from what you expect? Your description mentions 2 points, but your code manipulates 3 points. What do the points represent?
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    2. COThe three points are the the first point in the line, the last point in the line and a third point derived from the first two points to create a right triangle. From the three points I calculate the lengths of each side. I then use the lengths of the sides to calculate the angle of the drawn line. My question is how do I translate the angle in degrees (or radians) to a value that can be used to skew a context via a CGAffineTransform, so that the angle of the skew matches the angle of the line drawn by the user.
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    3. COCheck out this [Wikipedia article](http://en.wikipedia.org/wiki/Shear_mapping) on shear mapping. It looks like you'd be better off getting the slope and using 1.0 / m as you skew value.
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