Here is the code you're looking for, which I found in this related question. You need to add this to your .m file and also add the method declaration to your .h file.
- (CGContextRef)createARGBBitmapContextFromImage:(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 = CGColorSpaceCreateDeviceRGB();
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;
}