Question

I have a custom UIView that contains an interactive drawing that is drawn in the drawRect function of the view. On the iPad the drawing size is 1024 x 768. For the iPhone I shrink the drawing for the iPhone using CGContextScaleCTM. To keep the proper aspect ratio on my drawings I shrink the view to 480 x 360 and set the y value of the view to -20 (effectively cropping 20 pixels off the top and bottom of the View, which is fine). Everything looks correct now, but I need to convert the touch coordinates from iPhone to iPad coordinates for the interactive portions of my view to work. If I make the uiview 320 high and use

point.y *=768/320 

for converting the y value the locations are correct (but my drawing is distorted) I’ve done some tests hard coding point in so I know this should work but I’m having a hard time getting the math to work with the crop. Here is what I have so far:

CGPoint point = [[touches anyObject] locationInView:self.view];
[self endTouch:&point];
NSLog(@"true point: %@",NSStringFromCGPoint(point));
if (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPhone) {
point.x *= 1024/480;
point.y *= 768/360;
NSLog(@"corrected point: %@",NSStringFromCGPoint(point));
}

Can anybody help me with doing this conversion? Thanks!

Answer 1

1024/480 and 768/360 perform integer division. The result of both operations is 2, but you really want 2.133333. Replace the relevant lines in your code with the following:

point.x *= 1024.0/480.0;
point.y *= 768.0/360.0;

This will return the value you’re looking for and scale your point’s x and y values accordingly.

You might be better served (in terms of readability) by replacing these literals with a #define macro. At the top of your code, put the following:

#define SCALING_FACTOR_X (1024.0/480.0)
#define SCALING_FACTOR_Y (768.0/360.0)

and then modify your code to use that:

point.x *= SCALING_FACTOR_X;
point.y *= SCALING_FACTOR_Y;

That way, it will be more clear as to what you’re actually doing.