如何通过CGImage得到bitmap

在目前(iOS4.3版本已经出来了)cocoa没有提供从CGimage到bitmap的接口，想得到bitmap还得费一番周折。以下是我从苹果开发者网站上找到的一段，可以实现。大体思路为：先创建满足自己要求的Context，再将原图片画到Context上，然后再从Context上得到图片的bitmap数据。

 

原文：http://developer.apple.com/library/ios/#qa/qa1509/_index.html

 

creates a bitmap context with a 8-bits per component ARGB color space, draws the source image to this context, then retrieves the image bits in this color space from the context. Regardless of what the source image format is (CMYK, 24-bit RGB, Grayscale, and so on) it will be converted over to this color space.

 

For more information about creating bitmap contexts for other pixel formats, see the Quartz 2D Programming Guide.

Listing 2  Accessing the pixel data of a CGImage.

void ManipulateImagePixelData(CGImageRef inImage){    // Create the bitmap context    CGContextRef cgctx = CreateARGBBitmapContext(inImage);    if (cgctx == NULL)     {         // error creating context        return;    }     // Get image width, height. We'll use the entire image.    size_t w = CGImageGetWidth(inImage);    size_t h = CGImageGetHeight(inImage);    CGRect rect = {{0,0},{w,h}};     // Draw the image to the bitmap context. Once we draw, the memory     // allocated for the context for rendering will then contain the     // raw image data in the specified color space.    CGContextDrawImage(cgctx, rect, inImage);     // Now we can get a pointer to the image data associated with the bitmap    // context.    void *data = CGBitmapContextGetData (cgctx);    if (data != NULL)    {        // **** You have a pointer to the image data ****        // **** Do stuff with the data here ****    }    // When finished, release the context    CGContextRelease(cgctx);     // Free image data memory for the context    if (data)    {        free(data);    }}CGContextRef CreateARGBBitmapContext (CGImageRef inImage){    CGContextRef    context = NULL;    CGColorSpaceRef colorSpace;    void *          bitmapData;    int             bitmapByteCount;    int             bitmapBytesPerRow;     // Get image width, height. We'll use the entire image.    size_t pixelsWide = CGImageGetWidth(inImage);    size_t pixelsHigh = CGImageGetHeight(inImage);    // Declare the number of bytes per row. Each pixel in the bitmap in this    // example is represented by 4 bytes; 8 bits each of red, green, blue, and    // alpha.    bitmapBytesPerRow   = (pixelsWide * 4);    bitmapByteCount     = (bitmapBytesPerRow * pixelsHigh);    // Use the generic RGB color space.    colorSpace = CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB);    if (colorSpace == NULL)    {        fprintf(stderr, "Error allocating color space/n");        return NULL;    }    // Allocate memory for image data. This is the destination in memory    // where any drawing to the bitmap context will be rendered.    bitmapData = malloc( bitmapByteCount );    if (bitmapData == NULL)     {        fprintf (stderr, "Memory not allocated!");        CGColorSpaceRelease( colorSpace );        return NULL;    }    // Create the bitmap context. We want pre-multiplied ARGB, 8-bits     // per component. Regardless of what the source image format is     // (CMYK, Grayscale, and so on) it will be converted over to the format    // specified here by CGBitmapContextCreate.    context = CGBitmapContextCreate (bitmapData,                                    pixelsWide,                                    pixelsHigh,                                    8,      // bits per component                                    bitmapBytesPerRow,                                    colorSpace,                                    kCGImageAlphaPremultipliedFirst);    if (context == NULL)    {        free (bitmapData);        fprintf (stderr, "Context not created!");    }    // Make sure and release colorspace before returning    CGColorSpaceRelease( colorSpace );    return context;}

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