如何在单触后通过CGBitmapContext组合两个图像时避免质量损失

Question

I have two images. First ( avatar ) is 640x640, second ( like ) is 21x20. When I'm trying to draw second on first, I get it, but with very low quality. Here is the code:

var bounds = targeImageView.Bounds;
CGImageAlphaInfo alphaInfo = CGImageAlphaInfo.PremultipliedLast;
CGColorSpace colorSpaceInfo = CGColorSpace.CreateDeviceRGB ();
CGBitmapContext resultBitmap = new CGBitmapContext (IntPtr.Zero, (int)bounds.Width, (int)bounds.Height, 8, 4 * (int)bounds.Width, colorSpaceInfo, alphaInfo);
resultBitmap.DrawImage (bounds, avatar.CGImage);
var likeY = 0;
var likeX = 0;
var likeHeight = (float)Math.Floor (bounds.Height / 2);
var likeWidth = likeHeight;
resultBitmap.DrawImage (new RectangleF (likeX, likeY, likeWidth, likeHeight), like.CGImage);
targetImageView.Image = UIImage.FromImage (resultBitmap.ToImage ());

Here is the avatar picture on the same image view but without upper code, just imageView.Image = avatar;

Here is the same but after drawing.

Answer 1

You got better eyesight than me - at least at that small size. If it's a retina issue (I think so) then the following code should help you as it creates a context that will adjust itself to a retina (or not) display.

UIImage avatar = ...;
UIImage like = ...;
UIGraphics.BeginImageContextWithOptions (avatar.Size, false, 0);
using (CGContext ctx = UIGraphics.GetCurrentContext ()) {
    var rect = new RectangleF (Point.Empty, avatar.Size);
    avatar.Draw (rect);
    // coordinates are likely reversed (UIKit wrt CoreGraphics) you might have to fix them (untried)
    rect = new RectangleF (0, avatar.Height / 2, avatar.Width / 2, avatar.Height / 2);
    like.Draw (rect);

    targetImageView.Image = UIGraphics.GetImageFromCurrentImageContext ();
    UIGraphics.EndImageContext ();
    // dispose of avatar and like UIImage if not used anymore
}

Answer 2

1. Use PNGs they are lossless, and better quality than JPG or Bitmap.

   NSData *imageData = UIImagePNGRepresentation(image);

   UIImage *image=[UIImage imageWithData:data];

2. Allocate UIImageView with frame for placement using CGRectMake (). You will be able to float the image anywhere you want.

3. Set the ImageView's Image with [ImageView setImage:...]

4. The Image will be rendered much cleaner and you will have full control over quality and placement.

I have used a 256 image compressed down to a 15px x 15px and it still held a very good shape. In contrast I have also stretched images the other way around.