如何在Core Graphics中生成RGBA图像？ [英] How do you generate an RGBA image in Core Graphics?

问题描述

我正在尝试从文本生成RGBA8图像以用作OpenGL ES 2.0纹理。

I'm trying to generate an RGBA8 image from text to use as an OpenGL ES 2.0 texture.

+(UIImage *)imageFromText:(NSString *)text
{
  UIFont *font = [UIFont systemFontOfSize:20.0];  
  CGSize size  = [text sizeWithFont:font];

  CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
  CGContextRef contextRef =  CGBitmapContextCreate (NULL,
                                                    size.width, size.height,
                                                    8, 4*size.width,
                                                    colorSpace,
                                                    kCGImageAlphaLast
                                                    );
  CGColorSpaceRelease(colorSpace);
  UIGraphicsPushContext(contextRef);

  [text drawAtPoint:CGPointMake(0.0, 0.0) withFont:font];
  UIImage *image = UIGraphicsGetImageFromCurrentImageContext();

  UIGraphicsPopContext();

  return image;
}

不幸的是，没有 CGColorSpaceCreateDeviceRGBA 和 CGColorSpaceCreateDeviceRGB 导致以下错误：

Unfortunately, there's no CGColorSpaceCreateDeviceRGBA, and CGColorSpaceCreateDeviceRGB results in the following error:

CGBitmapContextCreate: unsupported parameter combination: 8 integer bits/component; 32 bits/pixel; 3-component color space; kCGImageAlphaLast; 448 bytes/row.

创建OpenGL想要的正确RGBA8格式我缺少什么？

What am I missing to create the proper RGBA8 format that OpenGL wants here?

更新：我从 kCGImageAlphaNone CGBitmapContextCreate 的最后一个参数$ c>（当我复制粘贴代码时）是 kCGImageAlphaLast ，这是我错误尝试过的几种变体之一。

Update: I changed the last parameter of CGBitmapContextCreate from kCGImageAlphaNone (which it was when I copy pasted the code) to kCGImageAlphaLast, which is one of several variations I've tried in error.

Update 2 ： UIGraphicsGetImageFromCurrentImageContext（）如果未使用 UIGraphicsBeginImageContext创建上下文，则返回nil （），因此有必要以不同方式提取图像： [UIImage imageWithCGImage：CGBitmapContextCreateImage（contextRef）] 。

Update 2: UIGraphicsGetImageFromCurrentImageContext() returns nil if the context was not created with UIGraphicsBeginImageContext(), so it is necessary to extract the image differently: [UIImage imageWithCGImage:CGBitmapContextCreateImage(contextRef)].

推荐答案

您在创建过程中指定的颜色空间不会导致类似的错误。

The color space you specify during creation wouldn't cause an error like that.

你得到那个错误的原因是你为每个组件指定了8位，可能是 4 * size.width 为 bytesPerRow 传入的值，但 kCGImageAlphaNone的 bitmapInfo 参数。 kCGImageAlphaNone 仅表示 RGB ，而不是 RGBA 。如果你想要 RGBA ，你应该很可能指定 kCGImageAlphaLast kCGImageAlphaPremultipliedLast 。

The reason you're getting that error is that you've specified 8 bits per component, presumably 4 color components in the 4*size.width value you passed in for bytesPerRow, yet a bitmapInfo parameter of kCGImageAlphaNone. kCGImageAlphaNone means only RGB, not RGBA. If you want RGBA, you should most likely specify kCGImageAlphaLast kCGImageAlphaPremultipliedLast.

抱歉。我应该说 kCGImageAlphaPremultipliedLast ，而不是 kCGImageAlphaLast 。

sorry. I should have said kCGImageAlphaPremultipliedLast, not kCGImageAlphaLast.

所以，像这样：

CGColorSpaceRef colorSpace = CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB);
CGContextRef contextRef =  CGBitmapContextCreate(NULL,
                                                 size.width,
                                                 size.height,
                                                 8,
                                                 4 * size.width,
                                                 colorSpace,
                                                 kCGImageAlphaPremultipliedLast);

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