Re: [boost] [gil io_new review] Reading images from in-memory sources

Subject: Re: [boost] [gil io_new review] Reading images from in-memory sources
From: Kenny Riddile (kfriddile_at_[hidden])
Date: 2010-12-06 10:03:28

On 12/5/2010 3:46 PM, Phil Endecott wrote:
>> review of Christian Henning's extensions to the Boost Generic Image
>> Library
>> starts on December 1st and lasts until December 10th, 2010.
>
> I've been looking at how to read images from in-memory data. For
> example, I have various places where I mmap() something that contains
> encoded images (e.g. a large map made up from many tiles in a single
> file). I can also imagine retrieving an encoded image from a database,
> for example.
>
> According to io.qbk:
>
> [h2 Reading And Writing In-Memory Buffers]
> Reading and writing in-mempory buffers are supported as well. See as
> follows:
>
> ``
> // 1. Read an image.
> ifstream in( "test.tif", ios::binary );
>
> rgb8_image_t img;
> read_image( in, img, tiff_tag() );
>
> // 2. Write image to in-memory buffer.
> stringstream out_buffer( ios_base::out | ios_base::binary );
>
> rgb8_image_t src;
> write_view( out_buffer, view( src ), tiff_tag() );
>
> // 3. Copy in-memory buffer to another.
> stringstream in_buffer( ios_base::in | ios_base::binary );
> in_buffer << out_buffer.rdbuf();
>
> // 4. Read in-memory buffer to gil image
> rgb8_image_t dst;
> read_image( in_buffer, dst, tag_t() );
>
>
> In the cases that I described above, using a stringstream would require
> that all of the data is copied at least once. I guess that if I knew
> more about how streambufs work I would know how to avoid that. Can
> anyone help me to implement this:
>
> void read_mem_jpeg( some_gil_image_tpye& dest, const char*
> jpeg_mem_begin, const char* jpeg_mem_end) { ... }
>
> in a way that doesn't copy all of the encoded data?
>
> Looking at formats/jpeg/read.hpp, it seems that irrespective of that the
> data will be copied again (into 'buffer') before being read by libjpeg.
>
> On the subject of copying, while reading PNGs it looks like the data is
> always copied into a temporary buffer in read_rows<>() even when no
> format conversion is needed (or is there some specialisation that I have
> missed?). And when reading JPEGs, it seems to always read single lines,
> yet libjeg advises that you should get at least rec_outbuf_height lines
> on each call to avoid extra copying within the library.
>
>
> Regards, Phil.
>
>
>
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I accomplish this using Boost.IOStreams...something like this works for me:

using namespace boost::iostreams;
stream< array_source > bufferStream( buffer, size );
read_image( bufferStream, dest, jpeg_tag() );