# TIP 164: Add Rotate Subcommand to the Canvas Widget

``````Author:         Arjen Markus <[email protected]>
Author:         Donal K. Fellows <[email protected]>
State:          Final
Type:           Project
Vote:           Done
Created:        17-Nov-2003
Post-History:
Keywords:       Tk,canvas
Tcl-Version:    8.7
Tk-Branch:      tip-164
Votes-For:      DKF, JN, FV, SL, AK
``````

# Abstract

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.

# Rationale

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.

Implementing rotation should consider what to do with items that can not (easily) be rotated: rectangles, non-circular ovals and arcs, widgets, bitmaps and images. For the items that only define a single coordinate, the anchor point, we just transform that anchor point. (It would require major work to Tk's rendering engine to allow rotated embedded images, and rotated windows would be even more complex than that.)

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. This is best left to user code.

• Therefore we will simply calculate a new bounding box and ignore changes in orientation. However, this cannot be done by just applying the general rotation transformation to the bounding box definition coordinates because this radically transforms the shape of the item, in some cases in ways that are highly surprising. Instead, we compute a virtual anchor point for the item (arbitrarily chosen to be its center), rotate that about the origin point, and then update the item bounding box to be the same shape as before in the same orientation as before but now centered on the new virtual anchor point.

Text items can be rotated, but they are often used with rectangles to make labelled boxes so the rotation of them with respect to their anchor is left for user code. Similarly, arc items have angles associated with them for where they start and end, but it isn't clear how those should be rotated when the item itself cannot be rotated (since we don't allow choosing angles for the major and minor axes of the underlying ellipse). Circular arcs could be handled simply, but we do not detect that case.

# Proposal

Canvases will have a new subcommand, rotate, which will rotate the coordinates of chosen items by a specified angle about a specified point.

canvas rotate itemOrTag x y angle

Note that most of the arguments are very similar to the canvas's scale subcommand. The angle is measured in degrees, with positive values indicating anti-clockwise rotation.

## Canvas Item C API Alterations

The `Tk_ItemType` structure gains a new field, ```Tk_ItemRotateProc *rotateProc```, which comes after the `nextPtr` field. This field allows the creator of the item type to define a callback that handles rotation; if defined as `NULL`, which it is by default in most existing code, then the default rotation algorithm is applied, which simply rotates the coordinate list using the `coordProc`.

The definition of `Tk_ItemRotateProc` is:

``````typedef void (Tk_ItemRotateProc)(
Tk_Canvas canvas,
Tk_Item *itemPtr,
double originX,
double originY,
``````

The `canvas` and `itemPtr` are the usual arguments for these item callbacks. The `originX` and `originY` describe the point about which rotation is to be done. The `angleRadians` is the amount to rotate, in radians (i.e., suitable for directly using with `sin()` and `cos()`).

# Reference Implementation

See the `tip-164` branch.

# Future Work

Should we also consider the possibility of reflections in a line?

Should we 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.

It might be useful to utilize the `-anchor` Tk option in all canvas items and add a new `-anchoroffset \$x,\$y` option, where `\$x` and `\$y` be canvas distances relevant to the anchor position. Once defined, or with their default values, the `-anchor` and `-anchoroffset` values combined will provide the item's center, for placement and rotation purposes. Item sets, constituting symbols, can then be manipulated easily.
It could probably be beneficial to add an `-angle` option (and an accompanying `-angleunit` one), to persist a rotation angle in the configuration database. The presence of an angle value other than zero would help negate rotation, thus re-instating the item to its initial orientation.