How to use UIGraphicsBeginImageContextWithOptions?

Question

I'm trying to adapt THIS question to work with a retina display. Here's how I figured out how to work with retina graphics for UIGraphicsBeginImageContext.

if (UIGraphicsBeginImageContextWithOptions != NULL)
{
    UIGraphicsBeginImageContextWithOptions(self.view.bounds.size, NO, 0.0f);
}
else
{
    UIGraphicsBeginImageContext(self.view.bounds.size);
}

However, when I use it the image looks really huge and doesn't scale down to fit the display. Any ideas how I can tell it to resize the captured image to fit the boxes? (Please refer to the other question to understand what I mean).

score 12 · Accepted Answer · answered Feb 28 '11 at 21:56

I've written this code to perform exactly the same thing described in the other post. It works on any iOS device with at least iOS 3.1.

_launcherView is the view that need to be photographed

CGFloat scale = 1.0;
if([[UIScreen mainScreen]respondsToSelector:@selector(scale)]) {        
    CGFloat tmp = [[UIScreen mainScreen]scale];
    if (tmp > 1.5) {
        scale = 2.0;    
    }
} 

if(scale > 1.5) {
    UIGraphicsBeginImageContextWithOptions(_launcherView.frame.size, NO, scale);
} else {
    UIGraphicsBeginImageContext(_launcherView.frame.size);
}

[_launcherView.layer renderInContext:UIGraphicsGetCurrentContext()];

UIImage *screenshot = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

CGRect upRect = CGRectMake(0, 0, _launcherView.frame.size.width*scale, (diff - offset)*scale);
CGImageRef imageRefUp = CGImageCreateWithImageInRect([screenshot CGImage], upRect);
[self.screenshot1 setFrame:CGRectMake(0, 0, screenshot1.frame.size.width, diff - offset)];
[screenshot1 setContentMode:UIViewContentModeTop];
UIImage * img1 = [UIImage imageWithCGImage:imageRefUp];
[self.screenshot1 setBackgroundImage:img1 forState:UIControlStateNormal];
CGImageRelease(imageRefUp);

CGRect downRect = CGRectMake(0, (diff - offset)*scale, _launcherView.frame.size.width*scale, (screenshot.size.height - diff + offset)*scale);
CGImageRef imageRefDown = CGImageCreateWithImageInRect([screenshot CGImage], downRect);
[self.screenshot2 setFrame:CGRectMake(0, screenshot1.frame.size.height ,  screenshot2.frame.size.width, _launcherView.frame.size.height - screenshot1.frame.size.height)];
[screenshot2 setContentMode:UIViewContentModeTop];
UIImage * img2 = [UIImage imageWithCGImage:imageRefDown];
[self.screenshot2 setBackgroundImage:img2 forState:UIControlStateNormal];
CGImageRelease(imageRefDown);

Thanks! I don't have access to my machine ATM, but I'll try it as soon as I can and will accept it as answer. — sudo rm -rf, Feb 28 '11 at 22:00
If you have any problem please tell me, but i'm sure it will be easily adapted to fit your needs. — MacTeo, Feb 28 '11 at 22:03
Works quite nicely, thanks! I'm just wondering if this could be done with `UIImage`s instead of `UIButton`s. I dislike the "click" effect that the buttons force. — sudo rm -rf, Mar 01 '11 at 04:06
Oh never mind, I can just use `screenshot1.adjustsImageWhenHighlighted = NO;` :) — sudo rm -rf, Mar 01 '11 at 04:14
What the @#$@# - this answer is not only incomplete but if you use this code it has nothing but compile problems - variables are not even defined! — Sam B, Apr 19 '13 at 16:24
If you look at the code is was written for iOS 3.1: four years ago... It is obvious that now has problems. — MacTeo, Apr 21 '13 at 10:01

RY_ Zheng · Answer 2 · 2020-05-03T03:29:16.940

According to this session, iOS Memory Deep Dive, we had better use ImageIO to downscale images.

Memory use is related to the dimensions of the images, not the file size.
UIGraphicsBeginImageContextWithOptions always uses SRGB rendering-format, which use 4 bytes per pixel.
A image have load -> decode -> render 3 phases.

For the following image, if you use UIGraphicsBeginImageContextWithOptions we only need 590KB to load a image, while we need 2048 pixels x 1536 pixels x 4 bytes per pixel = 10MB when decoding

while UIGraphicsImageRenderer, introduced in iOS 10, will automatically pick the best graphic format in iOS12. It means, you will save 75% of memory by replacing UIGraphicsBeginImageContextWithOptions with UIGraphicsImageRenderer if you don't need SRGB.

let url = NSURL(fileURLWithPath: filePath)

func resize() =

let imgSource = CGImageSourceCreateWithURL(url, nil)
if let imageSource = imgSource {
    let options: [NSString: Any] = [
        kCGImageSourceThumbnailMaxPixelSize: 100,
        kCGImageSourceCreateThumbnailFromImageAlways: true
    ]
    let scaledImage = CGImageSourceCreateThumbnailAtIndex(imageSource, 0, options as CFDictionary)
}

I took some notes here for this session.

score -2 · Answer 3 · edited Nov 24 '17 at 04:32

-2

- (UIImage *)getNewimage{
    UIGraphicsBeginImageContextWithOptions(newSize, NO, 0.0);
    [image drawInRect:CGRectMake(0, 0, newSize.width, newSize.height)];
    UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    return newImage;
}

edited Nov 24 '17 at 04:32

Bhumit Mehta

16,278
11
50
64

answered Feb 28 '17 at 04:16

Oshitha Wimalasuriya

431
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11

2

Please add more description and/or information about your answer and how it solves the asked problem so others can easily understand it without asking for clarification – koceeng Feb 28 '17 at 04:47
it corrupts the image. – Ananta Prasad May 23 '18 at 08:35

How to use UIGraphicsBeginImageContextWithOptions?

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