绘制屏幕外的彩色正方形将返回灰度图像 [英] Drawing an offscreen coloured square returns a greyscale image

问题描述

我早些时候问过这个问题，现在正在尝试探索制作屏幕外图像的想法。

出问题了-我想也许是色彩空间？我已经反复地编写代码，直到最终我只用几行就可以证明问题。

我有一个带imageview的视图（称为iv ）和一个按钮，按下该按钮时会调用 push

  
-（UIImage *）块：（CGSize ）size {
 UIGraphicsBeginImageContext（size）; 
 CGContextRef context = UIGraphicsGetCurrentContext（）; 
 CGContextSetFillColor（context，CGColorGetComponents（[UIColor redColor] .CGColor））; 
 CGContextFillRect（上下文，CGRectMake（0，0，size.width，size.height））; 
 UIImage * result = UIGraphicsGetImageFromCurrentImageContext（）; 
 UIGraphicsEndImageContext（）; 
的返回结果； 
} 
 
 
-（IBAction）推送{
 self.iv.image = [self block：CGSizeMake（50.0,50.0）]; 
} 
  

这里的问题是，与其绘制指定颜色的50x50块，

我使用位图上下文尝试了相同的操作

  
-（CGContextRef）createBitmapContextOfSize：（CGSize）size {
 CGContextRef context = NULL; 
 CGColorSpaceRef colorSpace; 
 void * bitmapData; 
 int bitmapByteCount; 
 int bitmapBytesPerRow; 
 
 bitmapBytesPerRow =（size.width * 4）; 
 bitmapByteCount =（bitmapBytesPerRow * size.height）; 
 
 colorSpace = CGColorSpaceCreateDeviceRGB（）; 
 bitmapData = malloc（bitmapByteCount）; 
 if（bitmapData == NULL）{
 fprintf（stderr，未分配内存！）; 
返回NULL; 
} 
 context = CGBitmapContextCreate（bitmapData，
 size.width，
 size.height，
 8，//每个组件的位
 bitmapBytesPerRow，
 colorSpace，
 kCGImageAlphaPremultipliedLast）; 
 // CGContextSetAllowsAntialiasing（context，NO）; 
 if（context == NULL）{
 free（bitmapData）; 
 fprintf（stderr，未创建上下文！）； 
返回NULL; 
} 
 CGColorSpaceRelease（colorSpace）; 
返回上下文； 
} 
 
-（UIImage *）块：（CGSize）size {
 UIGraphicsBeginImageContext（size）; 
 CGContextRef context = [self createBitmapContextOfSize：size]; 
 CGContextSetFillColor（context，CGColorGetComponents（[UIColor blueColor] .CGColor））; 
 CGContextFillRect（上下文，CGRectMake（0，0，size.width，size.height））; 
 UIImage * result = [UIImage imageWithCGImage：CGBitmapContextCreateImage（context）]; 
 CGContextRelease（context）; 
 UIGraphicsEndImageContext（）; 
的返回结果； 
} 
  

具有相同的结果。灰色框（在这种情况下不是蓝色框）。

我确定是否可以使此框正常工作。

解决方案

如果您有CGColor对象，只需使用 CGContextSetFillColorWithColor 函数。。 p>

I asked this question earlier and am now trying to explore the idea of making an offscreen image.

Something is going wrong - I think maybe with colour space? I've boiled the code down and down and down, until I eventually can demonstrate the problem with just a few lines.

I have a view with an imageview (called iv) and a button, which when pushed, calls "push"

- (UIImage *) block:(CGSize)size {
    UIGraphicsBeginImageContext(size);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetFillColor(context, CGColorGetComponents([UIColor redColor].CGColor));
    CGContextFillRect (context, CGRectMake(0, 0, size.width, size.height));
    UIImage *result =  UIGraphicsGetImageFromCurrentImageContext(); 
    UIGraphicsEndImageContext();
    return result;  
}


- (IBAction) push {
    self.iv.image = [self block:CGSizeMake(50.0,50.0)];
}

The issue here is that instead of drawing a 50x50 block in the specified colour, it draws it in a shade of grey, which leads me to think its a colour space error.

I tried the same thing with a Bitmap Context using

- (CGContextRef) createBitmapContextOfSize:(CGSize) size {
    CGContextRef    context = NULL;
    CGColorSpaceRef colorSpace;
    void *          bitmapData;
    int             bitmapByteCount;
    int             bitmapBytesPerRow;

    bitmapBytesPerRow   = (size.width * 4);
    bitmapByteCount     = (bitmapBytesPerRow * size.height);

    colorSpace = CGColorSpaceCreateDeviceRGB();
    bitmapData = malloc( bitmapByteCount );
    if (bitmapData == NULL) {
        fprintf (stderr, "Memory not allocated!");
        return NULL;
    }
    context = CGBitmapContextCreate (bitmapData,
                                     size.width,
                                     size.height,
                                     8,      // bits per component
                                     bitmapBytesPerRow,
                                     colorSpace,
                                     kCGImageAlphaPremultipliedLast);
    //CGContextSetAllowsAntialiasing (context,NO);
    if (context== NULL) {
        free (bitmapData);
        fprintf (stderr, "Context not created!");
        return NULL;
    }
    CGColorSpaceRelease( colorSpace );
    return context;
}

- (UIImage *) block:(CGSize)size {  
    UIGraphicsBeginImageContext(size);
    CGContextRef context = [self createBitmapContextOfSize:size];
    CGContextSetFillColor(context, CGColorGetComponents([UIColor blueColor].CGColor));
    CGContextFillRect (context, CGRectMake(0, 0, size.width, size.height));
    UIImage *result = [UIImage imageWithCGImage: CGBitmapContextCreateImage (context)];
    CGContextRelease(context);
    UIGraphicsEndImageContext();
    return result;
}

with identical results. Grey boxes not (in this case) a blue box.

I'm sure if I can get this to work everything else will follow.

解决方案

If you have a CGColor object, just use the CGContextSetFillColorWithColor function.

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