Re: [boost] Generic Image Library (GIL) accessing raw memory

From: Stefan Seefeld (seefeld_at_[hidden])
Date: 2006-10-18 19:38:25

Lubomir Bourdev wrote:
>  
> Tim Shead wrote:
>> Another question / observation on the GIL API.  When 
>> interfacing with external code, particularly "C" libraries, 
>> it's often necessary to pass a pointer to the memory managed 
>> by an image
>> (gdk_pixbuf_create_from_raw_data() in my case).  So far, I've 
>> been using
>>
>>   &image[0]
>>
>> to get a pointer to memory, but that's pretty ugly, and I'm 
>> thinking it might not work for all cases.  Is there a better 
>> way to do it?  If not, I'd suggest adding an "image::data()" 
>> method that would provide canonical access to memory.  The 
>> choice of name is for consistency with std::basic_string::data().
>>
>> Cheers,
>> Tim
>>
> 
> The problem of course is that the image view may not at all be
> memory-based, like the Mandelbrot set.
> Even for memory-based image views, and for images, they may be planar,
> so in this case you really need to get pointers to each color plane.
> 
> If the image is interleaved you can just do what you would do with
> std::vector:
> 
> rgba8_image_t img(100, 100);
> void* data=&img[0];
> 
> If the image is planar, you can get the pointers to each plane like
> this:
> 
> rgb8_planar_image_t planar_img(100, 100);
> void* plane0=&planar_img[0]->channel<0>();
> void* plane1=&planar_img[0]->channel<1>();
> ...
> 
> Useful MPL predicates:
> 
> // plain memory-based views/images with no dereference adaptors
> view_is_basic<View>
> image_is_basic<Image>
> 
> view_is_planar<View>
> image_is_planar<Image>
> 
> I am open to adding an interface like data() but I am not sure how best
> to do this to accommodate these variations.

We use something similar in VSIPL++ (http://www.codesourcery.com/vsiplplusplus),
where we have e.g. Matrix<std::complex<float> > with interleaved and split storage.
There we have some 'direct data access' metafunctions which provide a 'data()'
accessor returning

a) std::complex<float> *, for interleaved storage
b) std::pair<float *,float *>, for split storage
c) nothing, if there is no direct data access possible.

We have been thinking about extending it to Image Processing, where the element-type
is Pixel (such as defined by GIL), and storage is interleaved or planar.

Regards,
                Stefan

-- 
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