Digikam/GSoC2010/NonDestructiveEditing

"Never touch the original"

Parts needed in the foundation:

1. The format to store changes
2. Adapting digiKam's plugins and filters
   
3. Storing change information in image metadata and internal database
4. Naming scheme for versions of new (modified) images

Parts needed in the UI:

1. No more Save/Save As in the image editor, automatically save as new version
2. Some sort of menu to choose the image version - original, modified, modified_2 etc.
3. Menu/button for exporting the modified image
4. Stacks of images (original/current version) in the album icon view --needs some thinking
5. Widget to display version history - a tree view with buttons to show information would be nice

Details of foundation parts

1. The format to store changes

• Basically XML file in image metadata (using XMP)
• Use OpenRaster specification as a basepoint for changes description - http://create.freedesktop.org/wiki/OpenRaster
• see below for some thoughts on how to refer to existing files in this format

2. Adapting digiKam's plugins and filters

• We need to have an interface in EditorTool to get the serialized operation that has been applied
• From an image plugin, we need a list of filter names that are supported, and a way to execute a described operation on a given image
• Plugins will have to have unique name composed of filter name and some unique part (as there can be two plugins with same name) and also plugin version will be stored (as newer versions can have different parameters)
• Naming like < ... name="digikam:charcoal-hbr9Gh" version="2" > with version stored as another parameter for better parsing

3. Storing change information in image metadata and internal database

• Use XMP - defining a custom namespace and writing our changes
• Use OpenRaster specification as a basepoint for changes description - http://create.freedesktop.org/wiki/OpenRaster
• Always store the latest version (the current) in a normal file - good for thumbnailing and editing with external apps like Gimp
• If user edits some exotic format or unsupported RAW, or if he edits a read-only file on a read-only device, ask him, where to store the changed file
• Create some small wrapper classes to read/write that format
• mck182 - I would store changes like cropping immediately into new file as recropping image everytime it's displayed could be quite expensive

4. Naming scheme for versions of new (modified) images

• New files will be put next to the original with just modified name
• Names will be without spaces and not a translatable string if possible (because of possible non-ascii chars, which may cause problem on some other systems)
• For example image01.jpg (original), image01_v1.jpg (first version), image01_v2.jpg (second version) and so on
• the user can choose a default image format to store his edits. By recommendation lossless (PNG, TIFF, PGF), if the user really wants, lossy (JPEG)
• mck182 - When exactly will be the new files created? With every new modification to the original image (new modification of the original=new version)? I would let the first modification stay just in metadata, another new modification of the original would create _v1 file for the first modifications and the new modifications would go into original image's metadata. With another modification to the original image, dump the current modifications in metadata to _v2 file and write the new ones in metadata of the original etc etc. But that way we would also need something like _curr (current) version of the file for thumbnailing and working with other apps.
  • My suggestion: For every version (independent line of development starting from an original image) there is a current version. In this file's metadata all editing steps from the original to the resulting file are described. Intermediate versions need not be stored, unless an editing step cannot be replayed automatically. Forking a new version can be done from any point in this line of development.
  • this requires a two-dimensional numbering - lines of development, and step in this line of development

How to refer to a file?

• In a format describing changes from an original file over intermediate results to a current version, the previous files need to be uniquely referred to
• for intermediate versions we can expect to be able to write to the metadata. For original images, we cannot guarantee this.
• full-file hashes as used by git are very elegant for version management, but fail in our case: We modify the metadata all the time, changing the hash. For the same reason, the smaller uniqueHash currently in use by digiKam is not suited for this purpose. A hash on only the image pixel data is currently technically not available (and would require support for each image format separately)
• there can be metadata fields with unique ids (Exif.Photo.ImageUniqueId). These can have two different meanings:
  • 1) showing that a group of different pictures result from the same original (simplest case, groups a RAW with the corresponding JPEG)
  • 2) uniquely identifying a single picture version
• Suggesting a combined approach:
  • Use a unique id (type 2), added to the metadata on modification, or, if allowed, on import of original. Caveat: Will not be changed when editing externally.
  • Use filename, creationDate or unique id (type 1) for identifying original pictures

TODO

• specify details of the format for storing changes
• think more about the UI changes (would be great to involve usability team)
• email Krita mailing-list for their opinion?