iphone - 在 monotuch 中通过 CGBitmapContext 组合两个图像时如何避免质量损失

Question

我有两张图片。第一个 (avatar) 是 640x640，第二个 (like) 是 21x20。当我试图先画第二张时，我明白了，但质量很差。这是代码:

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 ());

这里是avatar图片在同一个image view但是没有上层代码，只是imageView.Image = avatar;

这里是相同的，但在绘制之后。

Answer 1

你的视力比我好 - 至少在这么小的尺寸下。如果它是视网膜问题(我认为是)，那么以下代码应该可以帮助您，因为它创建了一个上下文，可以将自身调整为(或不)视网膜显示。

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
}