Question

Given an image G with dimensions w x h, how can I surround the image with white to produce a new image that is 320 x 480 where G is centered, surrounded by white. The width and height of the incoming image might be larger than 320 or 480, repsecitively, so first I would need to scale it to ensure that it is less than 320×480 (or equal to).

-(UIImage *)whiteVersion:(UIImage *)img {

}

I would use these steps:

Firstly, scale image if needed...then:
1. Create a UIImage of size 320x480
2. Paint the whole thing white
3. Paint G (the image) in the center by using some simple math to find the location

However, I can’t figure out step 1 or 2.

Answer 1

You don’t draw into images. You draw into graphics contexts. There are different kinds of graphics contexts. An image graphics context lets you take a snapshot of its contents as an image.

- (UIImage *)whiteVersion:(UIImage *)myImage {
    UIGraphicsBeginImageContextWithOptions(CGSizeMake(320, 480), YES, myImage.scale);
    [[UIColor whiteColor] setFill];
    UIRectFill(CGRectInfinite);
    [myImage drawAtPoint:CGPointMake((320 - myImage.size.width) / 2, (480 - myImage.size.height) / 2)];
    UIImage *myPaddedImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return myPaddedImage;
}

You can learn more about drawing on iOS by perusing the Drawing and Printing Guide for iOS and the Quartz 2D Programming Guide.