Re: Memory issues to load a png image - any ideas?

On Wed, 3 Jul 2019, Ulisses wrote:
>
> Once it's the first time I'm working with compressed images, I'm a little
> lost:
>
> - Is there a way to display this image, even if I have no enough memory to
> allocate it "at once"? (I mean, any buffering tricks or something like that
> to work around this?)

I am by no means an expert with the libpng API but I think that you 
will want to use "progressive" APIs which provide decoded strips of 
pixels to your own code as soon as they are decoded.  Avoid simplified 
APIs which provide a whole decoded image to you in a simplified 
representation such as a large buffer.  If your display hardware has 
its own internal storage then you may be able to avoid 
double-buffering and deliver the (subset of) pixels directly to the 
display hardware.

PNG images are provided as a sequence of "chunks".  Even larger images 
are split into multiple chunks, which helps decrease memory 
requirements.  Each chunk is decoded/decompressed as it appears and if 
your code can accept input of the "chunk" size, then you can save lots 
of memory.  If you have control of the PNG images provided, then you 
can assure that they are optimized for your use.

If the height of the input image always matches the height of your 
display hardware, then this should help simplify the implementation 
since you can sub-sample the original image pixels into the horizontal 
resolution of your display hardware.

Bob
-- 
Bob Friesenhahn
[email protected], http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/
Public Key,     http://www.simplesystems.org/users/bfriesen/public-key.txt