[Vm-dev] JPEGReadWriter2, BitBlt and reusing previously allocated bitmaps

[David T. Lewis]

Done.

Thanks Laura!

Dave


>  Hi David, Hi All,
>
>    I'd like Laura to be added as a committer to source.squeak.org/VMMaker.
>
> On Wed, Jun 15, 2016 at 7:28 AM, Laura Perez Cerrato <
> lauraperezcerrato at gmail.com> wrote:
>
>>
>> Hello everyone,
>>
>> Working on JPEGReadWriter2 I noticed that both reading and writing
>> primitives include a sanity check that ensures that the
>> source/destination
>> Smalltalk bitmap has the exact size in bytes needed, instead of checking
>> that its size is at least that needed. Some BitBlt primitives perform
>> the
>> same check, thus not allowing operations with forms with associated
>> bitmaps
>> with a size greater than needed.
>>
>> When performing operations with images, and specially when such images
>> are
>> large in size, actually processing the images takes a small fraction of
>> the
>> time it takes to perform the whole operation, while allocating and
>> deallocating correctly sized bitmaps takes much longer. If one would
>> wish
>> to process a series of similarly sized images (with a definite maximum
>> size), it would be desirable to allocate a bitmap big enough to hold any
>> of
>> them only once and then reuse it, thus avoiding the aforementioned cost.
>> Checking that source and destination bitmaps are big enough instead of
>> checking that their size is exactly that which is expected would allow
>> that
>> optimization.
>>
>> A brief exploration of BitBlt and JPEGReadWriter2's code, accompanied
>> with
>> some experimenting of what would happen if such sanity checks were
>> modified
>> as proposed, has lead me to thinking that these changes would be
>> benefitial. I haven't observed any undesirable side effects (meaning,
>> nothing seems to have blown up :)). However, I'm inexperienced in
>> working
>> with the VM, so it's rather expectable that I miss something :) Your
>> input
>> on the topic would be extremely appreciated.
>>
>> The attached changesets contain the proposed set of changes.
>>
>> Cheers!
>>
>> -Laura Perez Cerrato
>>
>>
>
>
> --
> _,,,^..^,,,_
> best, Eliot
>