This is not necessarily the current version of this TIP.This is not necessarily the current version of this TIP.
| TIP: | 164 |
| Title: | Add Rotate Subcommand to the Canvas Widget |
| Version: | $Revision: 1.1 $ |
| Author: | Arjen Markus <arjen dot markus at wldelft dot nl> |
| State: | Draft |
| Type: | Project |
| Tcl-Version: | 8.5 |
| Vote: | Pending |
| Created: | Monday, 17 November 2003 |
| Keywords: | Tk, canvas |
This TIP proposes to add a rotate subcommand to the canvas widget to facilitate rotating items in very much the same way as is now possible with scaling and moving.
The canvas currently allows a programmer to scale and move items, but the third common affine transformation, rotation, is not supported. This can in itself be simulated by a script (simply get the coordinates of all items involved, transform them and set the new coordinates), but if you have several hundreds or even thousands of items this gets very slow.
Rotation is easy for polygons and lines: simply transform the coordinates. For circles and circular arcs new bounding boxes must be calculated and in addition for arcs the starting angle must be changed.
Implementing rotation should consider what to do with items that can not (easily) be rotated: text items, rectangles, non-circular ovals and arcs, widgets and images. Currently, text can not be drawn at an arbitrary angle, so I propose to only transform the coordinates at which the text is "anchored". A similar strategy can be used for widgets and images.
For rectangles and general ovals and arcs there are two choices:
Transforming the item into a polygon would solve the definition problem (these items are defined by a bounding box and the orientation is implicitly assumed), but might break scripts that look for a particular type of item.
Therefore we will simply calculate a new bounding box and ignore changes in orientation.
The new command would become:
$canvas rotate $itemOrTag $x $y $angle
where the arguments are very similar to those of the scale subcommand and angle is in degrees (a positive angle means rotate counter-clockwise).
A reference implementation does not yet exist.
Should this TIP also consider the possibility of reflections in a line?
Should this TIP also consider more general linear transformations (to be specified via a 2x2 matrix)? What about full affine transformations (i.e. with the translation components)?
If we implement these from the start, we need to do only a little more work.
This document is placed in the public domain.
This is not necessarily the current version of this TIP.