[Digikam-devel] Experimental : image preview on main interface

Andreas Huggel ahuggel at gmx.net
Thu Jun 15 19:34:02 BST 2006

On Thursday 15 June 2006 22:48, Gilles Caulier wrote:
> On Thursday 15 June 2006 16:37, Laurent Rathle wrote:
> > Le Jeudi 15 Juin 2006 00:19, Caulier Gilles a écrit :
> > > Please, give me a feedback. Thanks in advance
> >
> > Previewing raw is effectively much faster with this than with the editor
> > on my Athlon XP 3200 / 1 Gb Ram. In the editor, it stops at 10 % and wait
> > for at least 30 s to load the rest. New preview is less than one second
> > :-)
> Yes, but take a care that preview isn't the RAW image data. It's a JPEG or
> TIFF or PPM file embedded into Camera Settings - Exif Makernotes metadata
> area. It's used by camera to display image on TV screen (it's more faster
> to render a small JPEG than the RAW file).
> Note this preview image is also available into JPEG, TIFF and PNG files if
> Makernote is available. Preview image extraction will be possible using
> Exiv2 when :
> - Exif data size limitation will be removed into Exiv2 about this metadata.
> - MakerNotes will be updated, especially CANON, NIKON, OLYMPUS, etc.
> Actually, only my Minolta makernote implementation include preview tags.
> This is not very complicated to do using Exiftools tags reference.
> Andreas, we have already talking about Exiv2 preview extraction in the
> past. Can you give me your viewpoint in this room ?
> Gilles

The Exif data of JPEGs usually contains a small thumbnail (typically only 
120x160 px), in JPEG or TIFF format. I guess that's pretty useless. Besides, 
some Makernotes point to a larger preview image somewhere in the JPEG (but 
possibly outside of the Exif data). In RAW files I have seen up to 4 images, 
including the RAW image. There is no standard for these preview images, they 
come in different sizes and image formats (but mostly JPEG).

I was first of all wondering if it is desireable/a good idea if these could be 
accessed through Exiv2. 

If so, two options come to mind: 
1) extract the relevant metadata tags into ExifData and let the application do 
the work.
+ Exiv2 interface remains the same
- apps need to know for each format which tags to look at
- these tags may need to be treated specially by Exiv2

2) extend the Exiv2::Image interface to access preview images
+ apps just need to decide which preview image to use if there are several
- a bit weird: the library would then give access to metadata and preview 
images, but not to the actual image

For the moment I like option 2 better, although I don't know how the new 
interface should look like, ideas welcome.


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