How can I use an CGAffineTransform scale and flip simultaneously

cgaffinetransform, cgaffinetransformscale, ios

Solution

I'm assuming `fromRect.origin` is `CGPointZero`.

You need a compound transform that moves the origin to the lower left, flips the Y axis, and applies the scaling you want.

You can make a compound transform using the functions that modify a transform, like this:

- (CGAffineTransform)transformFromValueToScreen {
    CGRect fromRect = self.valueSpace;
    CGRect viewRect = self.screenSpace;

    CGSize scales = CGSizeMake(viewRect.size.width/fromRect.size.width,
                               viewRect.size.height/fromRect.size.height);

    CGAffineTransform transform = CGAffineTransformIdentity;

    // Move origin from upper left to lower left.
    transform = CGAffineTransformTranslate(transform, 0, fromRect.size.height);

    // Flip the sign of the Y axis.
    transform = CGAffineTransformScale(transform, 1, -1);

    // Apply value-to-screen scaling.
    transform = CGAffineTransformScale(transform, scales.width, scales.height);

    return transform;
}

Problem

Original source