Re: [Openexr-devel] Finding out if a file contains all tiles

[Florian Kainz]

Once you have opened the file by constructing an input file
object, isComplete() only checks a flag that has been computed
by the constructor.  However...

The time it takes to construct the input file object depends
on whether any tiles or scan lines are missing, independent
of whether you will be calling isComplete() or not.  If the
file is complete, the constructor is fast.  If scan lines or
tiles are missing, then constructing the input file object
takes a little longer:

An OpenEXR file consists of a header, a tile offset table and
the tiles.  (For more details, see our new file layout document:
http://www.openexr.com/openexrfilelayout.pdf)

After opening a file for writing, the IlmImf library immediately
writes the file header, followed by an empty tile offset table.
The library knows the size of the table, but it does not yet
know the location of the tiles in the file.  Immediately before
before closing the file, the library replaces the empty table
with a table that contains the actual tile locations.

When you open a file for reading, the tile offset table may be
empty or incomplete.  In this case the input file constructor
scans through the rest of the file to determine where the tiles
are.  This process is significantly faster than reading the
entire file.  Only a 20-byte header at the start of every tile
is read; the rest of the file is skipped.

Reconstructing the file's tile offset table happens every
time you open an incomplete file for reading.  Once you have
successfully opened the file, the cost of calling isComplete()
is negligible.

Florian



David Larsson wrote:
> Interesting.
> What's the expected running time of this function? Is it a quick flag 
> check or more of a loop over all tiles to see if they are available.
>
> Cheers
> /David L
>
> On 4/25/06, *Florian Kainz* <address@hidden <mailto:address@hidden>> wrote:
>
>     Class InputFile has an isComplete() method that allows you to check
>     if any tiles or scan lines are missing.  ImfInputFile.h says:
>
>          isComplete() returns true if all pixels in the data window are
>          present in the input file, or false if any pixels are missing.
>          (Another program may still be busy writing the file, or file
>          writing may have been aborted prematurely.)
>
>     I guess the function should be mentioned in the documentation...
>
>     Florian
>
>
>
>     David Larsson wrote:
>      > Hi,
>      > I wonder if there is a fast way to figure out if an exr-file is
>     missing
>      > any tiles, preferably not involving reading all tiles.
>      >
>      > /David Larsson
>      >
>      >
>      >
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