Question

I am having problem with iOS 3.2, the same code works fine on iOS 4.2, but with the 3.2,
CGBitmapContextGetData return nil

my code:

CGContextRef context = CGBitmapContextCreate(
                                                 NULL,
                                                 image.size.width,
                                                 image.size.height,
                                                 8,
                                                 image.size.width * 4,
                                                 CGImageGetColorSpace(image.CGImage),
                                                 kCGImageAlphaPremultipliedLast
                                                 );

    if (context != NULL))   {

        CGContextDrawImage(context, CGRectMake(0.0f, 0.0f, image.size.width, image.size.height), image.CGImage);

        UInt32 *contextData = CGBitmapContextGetData( context );
}

contextData is nil when running on iOS 3.2, and returns the right value when running on iOS 4.2

Any one can explain why the result is different, and how to solve it?

Thanks.

Answer 1

Before iOS4 you had to explicity allocate memory yourself, from the documentation

data

pointer to the destination in
memory where the drawing is to be
rendered. The size of this memory
block should be at least
(bytesPerRow*height) bytes. In iOS 4.0
and later, and Mac OS X v10.6 and
later, you can pass NULL if you want
Quartz to allocate memory for the
bitmap. This frees you from managing
your own memory, which reduces memory
leak issues.

So you should do something like

unsigned char* dataPtr = (unsigned char*)malloc(image.size.height*image.size.width*4)

use that i.s.o. of NULL as your first parameter, and free() it at an appropriate time.