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: Christian Henning (chhenning_at_[hidden])
Date: 2010-12-06 11:06:16

Hi Phil,

>
> The string ctor copies all the data, and then the istringstream ctor may or
> may not copy it all again (does anyone know?).

Did you see Kenny's email regarding boost::iostreams?

>> About libjpeg, thanks for pointing out one potential inefficiency. As
>> far as I can tell rec_outbuf_height is set to 1 be default. A user can
>> change that number to 2 or 4 but the lib would need to be recompiled.
>> The user also need to set the UPSAMPLE_MERGING_SUPPORTED compiler
>> flag.
>
> Well, here's what it says in jpeglib.h:
>
>  /* Description of actual output image that will be returned to application.
>   * These fields are computed by jpeg_start_decompress().
>   * You can also use jpeg_calc_output_dimensions() to determine these values
>   * in advance of calling jpeg_start_decompress().
>   */
>
>  JDIMENSION output_width;      /* scaled image width */
>  JDIMENSION output_height;     /* scaled image height */
>  int out_color_components;     /* # of color components in out_color_space
> */
>  int output_components;        /* # of color components returned */
>  /* output_components is 1 (a colormap index) when quantizing colors;
>   * otherwise it equals out_color_components.
>   */
>  int rec_outbuf_height;        /* min recommended height of scanline buffer
> */
>  /* If the buffer passed to jpeg_read_scanlines() is less than this many
> rows
>   * high, space and time will be wasted due to unnecessary data copying.
>   * Usually rec_outbuf_height will be 1 or 2, at most 4.
>   */
>
> To me, that looks like the library sets the value not the user.
>

There is a function called use_merged_upsample which returns only true
when UPSAMPLE_MERGING_SUPPORTED is set during compilation. Otherwise

cinfo->rec_outbuf_height = 1;

That's the only assignment I can find in the whole libjpeg source
code. Maybe I'm missing something?

Regards,
Christian