Re: [Openexr-devel] Read OpenEXR from memory data source ?

[Paul Schneider]

Hi, Gernot,

it sounds like the buffer you're passing doesn't contain valid EXR data  
- that error means that it tried to read the "magic number" from the  
beginning of the stream, and it wasn't the correct magic number.  See  
ImfHeader.cpp line 804 for where this error occurs.

Where are you getting the stream from?  The bytes in your memory buffer  
should be identical to the bytes of an EXR file on disk.  Also, I think  
that the EXR library expects the input stream to be little-endian - you  
don't happen to be running this on a big-endian machine, do you?

This would be a good way of testing your class:

char*  buf;
FILE* file;
int size;

// open a known-good EXR file on disk
file = fopen (path_to_exr_file, "rb");

// see how big it is
fseek (file, 0, SEEK_END);
size = ftell (file);
fseek (file, 0, SEEK_START);

// read the entire thing into a buffer
buf = new char[size];
fread (buf, size, 1, file);
fclose (file);

// read the EXR image data out of the buffer
Mem_IStream stream (buf, size);
Imf::InputFile input (stream);

// allocate framebuffer, call readPixels(), etc.
// ...



On Mar 6, 2005, at 1:19 PM, Gernot Ziegler wrote:

> Hej !
>
> I am currently implementing an OpenEXR input plugin and would need to  
> read
> from a memory source that is defined in the call:
>
> load_openexr(unsigned char *mem, int size)
>
> So far, I have implemented a Mem_IStream (see code below), but the  
> problem
> is that I can't get past the filename check -
> > OpenEXR-load: testing input
> > Cannot read image file "dummy". File is not an image file.
>
> Did I miss something here ?
>
> IStream should by design be able to read from memory, right ?
>
> Hints are welcome :-)
>
> ----
> class Mem_IStream: public IStream
> {
>  public:
>
>   Mem_IStream (unsigned char *exrbuf, int exrsize):
>     IStream("dummy"), _exrpos (0), _exrsize(exrsize)  { _exrbuf =  
> exrbuf;
> }
>
>   virtual bool  read (char c[], int n);
>   virtual Int64 tellg ();
>   virtual void  seekg (Int64 pos);
>   virtual void  clear ();
>
>  private:
>
>   Int64 _exrpos;
>   Int64 _exrsize;
>   unsigned char *_exrbuf;
> };
>
> bool Mem_IStream::read (char c[], int n)
> {
>   if (n + _exrpos < _exrsize)
>     {
>       memcpy(c, (void *)(_exrbuf[_exrpos]), n);
>       _exrpos += n;
>       return true;
>     }
>   else
>     return false;
> }
>
> Int64 Mem_IStream::tellg ()
> {
>   return _exrpos;
> }
>
> void Mem_IStream::seekg (Int64 pos)
> {
>   _exrpos = pos;
> }
>
> void Mem_IStream::clear ()
> {
> }
>
>
> Servus,
>   Gernot
>
> --  
> T----------------------------W-E-L-C-O-M- 
> E------------------------------T
> |  The Austria <=> Sweden <=> Germany <=> Netherlands connection.....   
>  H
> O  http://www.mpi-sb.mpg.de/~gziegler | http://lysator.liu.se/~gz       
>  E
> \------------------------------F-U-T-U-R- 
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