使用Core Graphics发生意外的图像颜色变化

Question

Following a general image creation and manipulation code suggestion in Add transparent space around a UIImage I ran into trouble from colors changing away from expected values.

(Please note that while I have simplified the code included herein such that the button is now the same size as the image, I was using the image manipulation code for a situation where the button is wider than the image, and placing that smaller image to the right end of the button's region. I had no trouble in that placement, but noted the color problem, so in creating this example, I reduced the button to the same size as the image, but retained the code that is evidently responsible for the undesirable color change.)

To reproduce the problem, start with a standard Xcode Game template project, in ObjC, using Metal. All of the following changes are made in GameViewController.m, mostly to the method viewDidLoad.

Start by commenting out the last 3 lines in viewDidLoad, which deal with the Renderer. This removes the rotating cube from the display, making the situation I'm detailing easier to see.

Then add the following code in its place...

UIButton *button = [UIButton buttonWithType:UIButtonTypeSystem];
button.frame = CGRectMake(350.0, 100.0, 90.0, 50.0);
[_view addSubview:button];

UIGraphicsBeginImageContextWithOptions(CGSizeMake(90.0, 50.0), NO, 1.0);
CGContextRef context = UIGraphicsGetCurrentContext();
UIGraphicsPushContext(context);

UIImage *oldImage = [UIImage imageNamed:@"SquiggleBox_90x50_01.png"];
//UIImage *oldImage = [UIImage imageNamed:@"SquiggleBox_90x50_01.jpg"];
[oldImage drawAtPoint:CGPointMake(0.0, 0.0)];

UIGraphicsPopContext();
UIImage *newImage = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

[button setImage:newImage forState:UIControlStateNormal];

(I have provided the two image files below. The first is a .PNG, the second is a .JPG, both 90x50 pixels. Both show a gold squiggle (a thin line that wanders the interior portion of the image) in a gold outline (a thin gold line on the rectangular border of the image. The PNG file has transparency where there is no gold color, the JPG uses a white fill. The PNG was created with Adobe Illustrator, and the JPG was converted via Photoshop. Save them with the names given below, and you can proceed with the code.)

Finally, add the following two image files to the project, so that the code can reference either of them.

SquiggleBox_90x50.png
SquiggleBox_90x50.jpg

When the PNG is used, the gold squiggle and outline turn blue. When the JPEG is used, the entirety of the button is solid blue.

Answer 1

Kevin Enax had the important clue... tinting. The solution is to add a single line to the code, immediately after the line defining newImage, before ending the context, put

newImage = [newImage imageWithRenderingMode:UIImageRenderingModeAlwaysOriginal];

This prevents tinting, and the original colors come through as desired.