zoukankan      html  css  js  c++  java
  • CALayer alpha mask not working

    I'm trying to create a simple example of drawing an image to a layer, and then setting that layer's alpha mask. I added the code below to my sample app's viewDidLoad. without setting the mask, i can see the image of the leaf. after i set the mask, i see nothing where my sublayer was. what am i doing wrong?

    Here are the two images i'm using (just samples i found on the net) http://sorenworlds.netfirms.com/Alpha/leaf.jpg

    http://sorenworlds.netfirms.com/Alpha/leaf-alfa.jpg

    self.view.layer.backgroundColor = [UIColor orangeColor].CGColor;
    self.view.layer.cornerRadius = 20.0;
    self.view.layer.frame = CGRectInset(self.view.layer.frame, 20, 20);
    
    
    CALayer *sublayer = [CALayer layer];
    sublayer.backgroundColor = [UIColor blueColor].CGColor;
    sublayer.shadowOffset = CGSizeMake(0, 3);
    sublayer.shadowRadius = 5.0;
    sublayer.shadowColor = [UIColor blackColor].CGColor;
    sublayer.shadowOpacity = 0.8;
    sublayer.frame = CGRectMake(30, 30, 128, 192);
    [self.view.layer addSublayer:sublayer];
    
    CGImageRef img = [UIImage imageNamed:@"leaf.jpg"].CGImage;
    sublayer.borderColor = [UIColor blackColor].CGColor;
    sublayer.borderWidth = 2.0;
    sublayer.contents = img;
    
    CGImageRef imgAlpha = [UIImage imageNamed:@"leaf_alpha.jpg"].CGImage;   
    CALayer *alphaLayer = [CALayer layer];
    alphaLayer.contents = (id)imgAlpha;
    sublayer.mask = alphaLayer;
    

    Any help is greatly appreciated.

    ---------------------------------------------------

    There are several problems with your code:

    1. The CALayer class reference says that the mask layer's alpha is used to mask the parent layer. Your mask layer (alphaLayer) is created with a JPG, which doesn't contain alpha values. Quartz doesn't understand that you want to use the grayscale RGB pixel values in the mask image as alpha values, and there's no code that you can call to do this (as far as I know).

      Quartz probably interprets the lack of alpha in the mask image as 0 alpha for every pixel which is why nothing is displayed.

      I suggest getting a paint program, like Pixelmator, and making a PNG file (which has alpha values built in) out of the original leaf image. Doing this removes the need for adding a mask layer in code.

    2. Your file name is "leaf-alfa.jpg", but it's "leaf_alpha.jpg" in your code.

    3. Sublayers must be laid-out in their parent layer's coordinates. You did not do this, nor did you set a boundary for the mask layer. A layer's default boundary is the empty rectangle, so even if you used an image with an appropriate alpha mask, you would still see nothing (or maybe just the original leaf image with no masking). You would need to do something like this:

      alphaLayer.bounds = CGRectMake(0, 0, 128, 192);
      alphaLayer.position = CGPointMake(sublayer.bounds.size.width/2.0,
                                        sublayer.bounds.size.height/2.0);
      
  • 相关阅读:
    lpc2103 rtc寄存器说明
    LPC21O3第一课:第一个实验,LED灯闪烁及ADS1.2的初步使用
    把FlvJoiner更新了一下
    Boost智能指针——weak_ptr
    买了一个USB无线网卡
    Boost的转换函数(二)
    把FlvDownloader更新了一下
    在C#中快速实现拖放操作
    Flv视频分割软件FlvSplitter发布
    用Apatch给Messenger去广告
  • 原文地址:https://www.cnblogs.com/allanliu/p/4207738.html
Copyright © 2011-2022 走看看