Re: [Openexr-devel] previews...

[Florian Kainz]

Hi Wayney,

just a quick update: I implemented a preview image attribute
that can be updated at any time while writing an OpenEXR file.  
We intend to release an updated version of the OpenEXR libraries
in the next couple of weeks; the release will include the preview 
image.

Regarding the EXR acronym - it stands for extended range.

Florian


address@hidden wrote:
> 
> Hi Florian,
> 
> > I would prefer to use a single attribute with three fields:
> 
> Yes, that's a neater way. I get worried when I see fixed structures because
> of versioning problems. But if the preview format ever changed (probably
> unlikely, but I've assumed that before... :)), it could just use a different
> attribute name.
> 
> > attributes are specified when an image file is opened, but if a program
> > generates an image incrementally, one scan line at a time, and each scan
> > line is written as soon as it is available, then the preview image is not
> > known when the file is opened for writing.  We will have to come up a way
> > to store the preview image pixels in the file later, after the header, and
> > possibly some or all of the pixel data have been written.
> 
> Ack, that's a good point, I'm currently generating my images in memory and
> didn't think of that :-/
> 
> Perhaps the application can 'reserve' the preview space (ie. Create a
> preview attribute with a specified width\height but with black pixels) when
> opening the file and fill in the required data later.
> 
> ImfHeader
> {
>         bool reservePreview( int width, int height );
>         bool writePreviewImage( byte *pPixels, int length );
> };
> 
> If 'reservePreview' is not called, no preview is written. If it is called
> and a preview image is not written using 'writePreviewImage' the file fails.
> If the length parameter passed to writePreviewImage does not match the image
> size, the file fails. If 'reservePreview' is not called but
> 'writePreviewImage' is, then 'writePreviewImage' returns false but the file
> is still written.
> 
> 'reservePreview' could store the position within the stream for the preview
> as well as the image size allowing a direct seek and write when
> 'writePreviewImage' is called.
> 
> Seems a bit hacky to me though, sorry, just putting it out there :)
> 
> As an aside, could anyone tell me what 'exr' is short for? I've looked at
> the documentation and not found anything, taking a stab at it I'd guess
> 'EXtended Resolution' or something, but probably miles away?
> 
> Thanks,
> Wayney
> 
> -Virus scanned and cleared ok