包含UIView的UIImage / UIImageView重绘是否已缩放

Question

My iPad app has a navigation where I show screenshots of the different pages and because I want to show more than one screenshot at once I scale the container to around 24% of the original screenshots (1024x768).

- (void) loadView
{
    // get landscape screen frame
    CGRect screenFrame = [UIScreen mainScreen].bounds;
    CGRect landscapeFrame = CGRectMake(0, 0, screenFrame.size.height, screenFrame.size.width);

    UIView *view = [[UIView alloc] initWithFrame:landscapeFrame];
    view.backgroundColor = [UIColor grayColor];

    self.view = view;

    // add container view for 2 images
    CGRect startFrame = CGRectMake(-landscapeFrame.size.width/2, 0, landscapeFrame.size.width*2, landscapeFrame.size.height);
    container = [[UIView alloc] initWithFrame:startFrame];
    container.backgroundColor = [UIColor whiteColor];

    // add image 1 (1024x768)
    UIImage *img1 = [UIImage imageNamed:@"01.jpeg"];
    UIImageView *img1View = [[UIImageView alloc] initWithImage:img1];
    [container addSubview:img1View];

    // add image 2 (1024x768)
    UIImage *img2 = [UIImage imageNamed:@"02.jpeg"];
    UIImageView *img2View = [[UIImageView alloc] initWithImage:img2];

    // move img2 to the right of img1
    CGRect newFrame = img2View.frame;
    newFrame.origin.x = 1024.0;
    img2View.frame = newFrame;

    [container addSubview:img2View];

    // scale to 24%
    container.transform = CGAffineTransformMakeScale(0.24, 0.24);

    [self.view addSubview:container];
}

but when I scale images with "small" text it looks sth like this:

I have to use the big screenshots because if a user taps the image it should scale to 100% and be crispy clear.

is there a way how I can scale the images "smoothly" (on the fly) without ruining performance? it would be enough to have two versions: the full-px one and another for the 24% version.

Answer 1

The reason the scaled-down image looks crappy is it's being scaled in OpenGL, which is using fast-but-low-quality linear interpolation. As you probably know, UIView is built on top of CALayer, which is in turn a sort of wrapper for OpenGL textures. Because the contents of the layer reside in the video card, CALayer can do all of its magic on the GPU, independent of whether the CPU is busy loading a web site, blocked on disk access, or whatever. I mention this only because it's useful to pay attention to what's actually in the textures inside your layers. In your case, the UIImageView's layer has the full 1024x768 bitmap image on its texture, and that isn't affected by the container's transform: The CALayer inside the UIImageView doesn't see that it's going to be (let's see..) 246x185 on-screen and re-scale its bitmap, it just lets OpenGL do its thing and scale down the bitmap every time it updates the display.

To get better scaling, we'll need to do it in CoreGraphics instead of OpenGL. Here's one way to do it:

- (UIImage*)scaleImage:(UIImage*)image by:(float)scale
{
    CGSize size = CGSizeMake(image.size.width * scale, image.size.height * scale);
    UIGraphicsBeginImageContextWithOptions(size, YES, 0.0);
    [image drawInRect:CGRectMake(0, 0, size.width, size.height)];
    UIImage *imageCopy = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return imageCopy;
}

- (void)loadView
{
    // get landscape screen frame
    CGRect screenFrame = [UIScreen mainScreen].bounds;
    CGRect landscapeFrame = CGRectMake(0, 0, screenFrame.size.height, screenFrame.size.width);

    UIView *view = [[UIView alloc] initWithFrame:landscapeFrame];
    view.backgroundColor = [UIColor grayColor];

    self.view = view;

    // add container view for 2 images
    CGRect startFrame = CGRectMake(-landscapeFrame.size.width/2, 0, landscapeFrame.size.width*2, landscapeFrame.size.height);
    container = [[UIView alloc] initWithFrame:startFrame];
    container.backgroundColor = [UIColor whiteColor];

    // add image 1 (1024x768)
    UIImage *img1 = [UIImage imageNamed:@"01.png"];
    img1View = [[TapImageView alloc] initWithFrame:CGRectMake(0, 0, 1024, 768)];
    img1View.userInteractionEnabled = YES; // important!
    img1View.image = [self scaleImage:img1 by:0.24];
    [container addSubview:img1View];

    // add image 2 (1024x768)
    UIImage *img2 = [UIImage imageNamed:@"02.png"];
    img2View = [[TapImageView alloc] initWithFrame:CGRectMake(1024, 0, 1024, 768)];
    img2View.userInteractionEnabled = YES;
    img2View.image = [self scaleImage:img2 by:0.24];
    [container addSubview:img2View];

    // scale to 24% and layout subviews
    zoomed = YES;
    container.transform = CGAffineTransformMakeScale(0.24, 0.24);

    [self.view addSubview:container];
}

- (void)viewTapped:(id)sender
{
    zoomed = !zoomed;

    [UIView animateWithDuration:0.5 animations:^
    {
        if ( zoomed )
        {
            container.transform = CGAffineTransformMakeScale(0.24, 0.24);
        }
        else
        {
            img1View.image = [UIImage imageNamed:@"01.png"];
            img2View.image = [UIImage imageNamed:@"02.png"];
            container.transform = CGAffineTransformMakeScale(1.0, 1.0);
        }
    }
    completion:^(BOOL finished)
    {
        if ( zoomed )
        {
            UIImage *img1 = [UIImage imageNamed:@"01.png"];
            img1View.image = [self scaleImage:img1 by:0.24];
            UIImage *img2 = [UIImage imageNamed:@"02.png"];
            img2View.image = [self scaleImage:img2 by:0.24];
        }
    }];
}

And here's TapImageView, a UIImageView subclass that tells us when it's been tapped by sending an action up the responder chain:

@interface TapImageView : UIImageView
@end

@implementation TapImageView

- (void)touchesEnded:(NSSet*)touches withEvent:(UIEvent*)event
{
    [[UIApplication sharedApplication] sendAction:@selector(viewTapped:) to:nil from:self forEvent:event];
}

@end

Answer 2

Instead of scaling the container and all of its subviews. Create a UIImageView from the contents of the container and adjust its frame size to 24% of the original.

UIGraphicsBeginImageContext(container.bounds.size);
[container renderInContext:UIGraphicsGetCurrentContext()];
UIImage *containerImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

UIImageView *containerImageView = [[UIImageView alloc] initWithImage:containerImage];
CGRectFrame containerFrame = startFrame;
containerFrame.size.with *= 0.24;
containerFrame.size.height *= 0.24;
containerImageView.frame = containerFrame;

[self.view addSubView:containerImageView];