Scaling UIImage for use with with CGContextDrawImage

Question

I have an image which measures 480px x 480px and I'd like to display it in a view which measures 375px x 375px using CGContextDrawImage as below. At the moment the image doesn't scale to fit the view - it is drawn at full size. How can I adjust the code below to scale the image to fit the view please?

self.image = [UIImage imageNamed:@"image2.png"];
CGContextRef layerContext = CGLayerGetContext(drawingLayer);
CGContextSaveGState(layerContext);
UIGraphicsBeginImageContext (self.viewRect.size);
CGContextTranslateCTM(layerContext, 0, self.image.size.width);
CGContextScaleCTM(layerContext, 1.0, -1.0);
CGContextDrawImage(layerContext, self.viewRect, self.image.CGImage);
UIGraphicsEndImageContext();
CGContextRestoreGState(layerContext);

Rob Rob · Accepted Answer · 2015-07-02T21:04:56

Nowadays you might use UIGraphicsImageRenderer, which gets you out of the weeds of all those CoreGraphics calls:

CGRect rect = CGRectMake(0, 0, 375, 375);
UIImage *smallImage = [[[UIGraphicsImageRenderer alloc] initWithBounds:rect] imageWithActions:^(UIGraphicsImageRendererContext * _Nonnull rendererContext) {
    [self.image drawInRect:rect];
}];

But my original answer is below.

The CGContextDrawImage function will scale the image drawing to fit the view. As the documentation for this function says:

Draws an image into a graphics context.

Quartz scales the image—disproportionately, if necessary—to fit the bounds specified by the rect parameter.

The only thing that looks highly suspect is the line that says:

CGContextTranslateCTM(layerContext, 0, self.image.size.width);

First, you want to translate vertically by the height, not the width. Second, you want to translate by the height of the viewRect, not of the image. Thus:

CGContextTranslateCTM(layerContext, 0, self.viewRect.size.height);

If the image still isn't getting scaled correctly, I'd suggest you double check the viewRect. But CGContextDrawImage definitely draws the image scaled within the specified CGRect.

Scaling UIImage for use with with CGContextDrawImage

1 Answers