Re: [Vxl-maintainers] unsigned int -> size_t for indexing

[Gehua Y. <yan...@gm...>]

On Tue, Dec 7, 2010 at 1:19 PM, Brad King <bra...@ki...> wrote:

>
> Actually there are more reasons to use uint64_t rather than size_t.
> Some APIs may support loading a small part of a gigantic image.
> In that case the underlying indexing and file offsets must be 64-bit
> even though pointers might be only 32-bit.  Types like size_t are
> good for dealing with in-memory buffers (and thus standard library
> APIs) but not for indexing something that might be mostly out-of-core.
>
> BTW, a patch like that attached will be helpful for dealing with 64-bit
> integer types.  This adds "long long" overloads for vnl_math functions
> that are supposed to be available for all fundamental integer types.
>
> -Brad
>
>
Brad,

Please forgive me for being the pain.  I am not against what you proposed.
Just want to raise a couple points for discussion:

First, I have not heard of any images as gigantic as having one of the
dimensional index goes beyond 2^32.  Hypothetically, if such an image exists
and it is close to a square, it takes roughly 2^64 bytes to store a
grayscale byte image, which seems unrealistically large and will break
64-bit pointers anyway.  Therefore, we do not gain any real benefits from
uint64_t index type unless  1) image is not square but a narrow strip or 2)
image is sparse, similar to the idea of a sparse matrix. This second idea is
beyond my knowledge.

Second, using uint64_t index type on 32-bit systems will likely result in
performance loss when applied to vil_image_view<T>.  To access a pixel,
which usually occurs in a double loop, arithmetic operations performed on
64-bit indices take more than twice the number of CPU instructions than on
32-bit indices.   At the end, the pointer still needs to be truncated to 32
bits in order to access the memory.   Maybe we shall apply uint64_t to only
a selected subset of APIs or employ overloading functions?

Please feel free to correct me if I am wrong or partial.

Best regards,
Gehua Yang