Re: [Openexr-devel] Implementing low level file IO

[Paul Schneider]

Hi, Brad,

I believe random access is required for EXR reading.  The library  
makes an effort to read as sequentially as possible, but in order to  
support tiled files, mipmaps, and arbitrary scanline access, you need  
to be able to seek.

The use of the filename[] argument depends on the implementation.   
Some implementations (like the std::ifstream and std::ofstream  
classes included with OpenEXR) use the filename[] to locate the file  
to read or write.  Others ignore it - I believe the Photoshop plugin  
just passes the string "EXR file".  If an exception occurs, the  
filename[] will be presented to the user as part of the error message  
by default ("the file "filename[]" could not be written, because you  
are out of disk space"), so it's probably a good idea to pass  
something meaningful.

- Paul



On May 23, 2005, at 1:23 AM, Brad Hards wrote:

> I'm looking at the manual section 7.2 (Custom Low-Level File I/O),  
> and having
> a few concerns. I think I need to do this to get good integration into
> Qt4/KDE4 - so I can use a QIODevice, which is the base class for  
> all kinds of
> IO concepts (network sockets, files, generic buffers and process  
> output).
>
> The problem is that sometimes a QIODevice doesn't support random  
> access - you
> cannot seek. So the question is "what is OpenEXR going to do if  
> tellg just
> returns zero"? If it is going to break, I'll just disable reading  
> files from
> non-seekable devices, but some indication of the dragons lurking  
> might be
> very useful before I get too much into this.
>
> Brad
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