NOTE: this is a document salvaged from our old wiki for a tool which is now available in stable versions. It should eventually be transformed into a proper specification with final UX choices.

Idea and background⁠

A plugin named iWarp already exists in Gimp, but suffer from different troubles (small preview, uneasy manipulation). This year’s Google Summer of Code have a project to implement this behavior as a real live on-canvas tool in Gimp. This project is taken by Michael Muré (batolettre@gmail.com, irc: Bat`O on #gimp@irc.gnome.org).

Basically, this would allow users to deform an image by a series of cursor action on it. This can be seen as a sort of sculpt. See for example the case where user can move pixel with a stroke of mouse. Others behavior can also be implemented, see below.

A quick technical sneak peek⁠

This tool will be a complete rewrite, no parts will be taken from the plugin. The tool will works with Gegl. The plugin can be seen as a proof of concept.

Technically, this tool will work in two steps:

• a series of tools, one by behavior, act on a image-sized buffer that hold relative coordinate for each pixel. That mean that each pixel of the final image will have on this buffer a coordinate from where the final color come from.
• the final image is computed from the original image and this buffer.

The approach with a coordinate buffer instead of acting directly on the image allows to:

• Render the tool effect in a constant time, no matter the number of stroke.
• No information lost. See for example the case of a pinch done multiple times in the same area so the input pixel would collapse. If you un-pinch the area, the map-absolute operator will fetch the right pixels, with no data loss.

Potential behavior⁠

This technical approach gives a good liberty when implementing behavior. Following is a list a potential behavior, but other can be found and implemented.

• move: the stroke moves pixels under the cursor
• swirl (left and right) - rotary transform
• erase transform: cancel the transformation locally (ie set the relative coordinate to zero)
• pinch/concentrate
• expand
• smooth: tend to flatten the transformation

All operations are applied with a feathered “brush”, so effect is largest at center and smallest at edge.

Tool options⁠

This tool would have as options at least:

• size of the effect
• shape of the effect (round or square ?) -i dont think offering other shapes than round provides much… [[User:AlexiaDeath|AlexiaDeath]] 20:41, 2 June 2011 (UTC)
• Hardness of the effect (see hardness in round brushes)

This part, as the others, need to be thought in details, an probably expanded.

Undo⁠

Undo should be achievable with this tool. What is exactly doable (one step undo ? more ?) is currently unclear and need to be discussed.

Possible way to achieve that:

• keeping copies of the coordinate buffer
  • it will quickly eat a lot of memory
• applying the stroke in reverse,
  • it lead to approximation problem
  • not all behavior are easy to undo (see for instance the cancel or smooth behavior)
• recording all the stroke applied, and recomputing the coordinate buffer when undoing
  • can take a lot of time if there is many strokes
• mixed solution: keeping “keyframe” coordinate buffer, every n stroke + every strokes, and recompute from this keyframe when undoing
• keeping modified tiles for every stroke done, and recompose a buffer when undoing
• stack one gegl op for each stroke in a gegl graph to compute the final coordinate buffer, and unstack op for undo

Quality⁠

Since editing the coordinate buffer and rendering the transformation are two separated process, we can render the image with different quality without affecting the deformation.

Pressure sensitivity⁠

Options like the size of the effect or the sharpening of the shape could be pressure sensitive.