UIImage.draw(in:) has a aliasing problem

Question

Created Mar ’21

Replies 2

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Views 1.8k

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I have a image with pixel size: 3024 * 3024
I have a custom UIView with logical size: 207 * 207, and layer.contentsScale is 3, so pixel size is 621 * 621.

I want to draw this UIImage in my custom UIView, the code in draw(rect:) like this;

Code Block var image: UIImage?  {
    didSet {
        setNeedDisplay()
    }
}
override func draw(_ rect: CGRect) {
    guard let ctx = UIGraphicsGetCurrentContext() else { return }
    ctx.addRect(bounds)
    ctx.setFillColor(UIColor.black.cgColor)
    ctx.fillPath()
     
    if var im = image {
        let size = Math.getMaxSizeWithAspect(size: CGSize(width: bounds.width, height: bounds.height), radioWidthToHeight: im.size.width / im.size.height)
      
        im.draw(in: CGRect(x: (bounds.width - size.width) / 2, y: (bounds.height - size.height) / 2, width: size.width, height: size.height))
}

and the result is very bad, picture is aliasing, I tried much solution, but not work well.

Code Block // not work list...
ctx.setShouldAntialias(true)
ctx.setAllowsAntialiasing(true)
ctx.interpolationQuality = .high
layer.allowsEdgeAntialiasing = true
layer.minificationFilter = .trilinear

Boost

Answer 1

Claude31 OP

Mar ’21

Is Math.getMaxSizeWithAspect a function of yours ?

If so, thanks to post the code.

Also try to change

.trilinear with .linear

or

207 in 252 (if that fits in your app)

0

Answer 2

PhonJac OP

Mar ’21

@Claude31, thank you for your reply, but is not work.

I found the solution

in UIView.draw function

create a CGImageContext with final image pixel size
draw UIImage into CGImageContext
get CGImage from CGImageContext
draw this CGImage into CGContext of UIView.draw

it works well.

there is my code:

Code Block override func draw(_ rect: CGRect) {
  guard let ctx = UIGraphicsGetCurrentContext() else { return }
  
  // fill background with black color
  ctx.addRect(bounds)
  ctx.setFillColor(UIColor.black.cgColor)
  ctx.fillPath()
   
  if var im = image {
    // image pixel size is 4032 * 4032, so aspect is 1:1=1,
    // bounds size is logical size (384 * 512), so I need found a max size in bounds and keep my image aspect
    // so 'size' is (384 * 384) 
    let size = Math.getMaxSizeWithAspect(size: CGSize(width: bounds.width, height: bounds.height), radioWidthToHeight: im.size.width / im.size.height)
    // pixelSize = (384 * 3, 384 * 3) = (1152, 1152)
    let pixelSize = CGSize(width: size.width * layer.contentsScale, height: size.height * layer.contentsScale)
    // I create a pixelSize ImageContext
    UIGraphicsBeginImageContextWithOptions(pixelSize, true, 1)
    guard let imgCtx = UIGraphicsGetCurrentContext() else { return }
    // draw UIImage into ImageContext
    im.draw(in: CGRect(x: 0, y: 0, width: pixelSize.width, height: pixelSize.height))
    // get cgImage
    guard let cgImg = imgCtx.makeImage() else { return }
    // there is another strange thing, CGContext draw cgImg will show a vertical flip result
    // so I have to vertical flip first
    ctx.scaleBy(x: 1, y: -1)
    ctx.translateBy(x: 0, y: -bounds.height)
    // draw cgImage into UIView's context
    ctx.draw(cgImg, in: CGRect(x: (bounds.width - size.width) / 2, y: (bounds.height - size.height) / 2, width: size.width, height: size.height))
     // it works like charm :D
  }
}

0