TIP #97 Version 1.4: Moving Vertices of Canvas Items

This is not necessarily the current version of this TIP.

Abstract

This TIP proposes a canvas subcommand (or possibly two) that allows for replacing characters in text objects and to move individual vertices of line and polygon items.

Rationale

Interactive graphics programs often allow users to modify shapes of objects by selecting and dragging the vertices. Moving one vertex of a canvas item in the current version of Tk, (at least as far as I can find out from the documentation), can only be done by first removing the coordinate by dchars and then insert the new one by insert, or for geometric items like lines and polygons using the coords command to obtain and reset the coordinates, after having modified the coordinate list by lrepace.

The most important issue here, I think, is performance. I believe that the current way of moving a vertex can be slow in some scenarios.

The rchars command is proposed merely to conform with the dchars and insert commands, which both operate on lines, polygons and text items, hence rchars should do that as well.

Specification

Two canvas widget commands are proposed: imove and rchars. The following command is proposed to move a vertex of any canvas item:

canvas imove tagOrID index x y

This command will move the indexth point of the item identified by tagOrID to the new position given by x and y. The command should work for all canvas items.

The following command provides a similar functionality, but conforms to the current insert and dchars commands.

canvas rchars tagOrID first last string

This command will:

for text items

replace the characters in the range first and last (inclusive) with the characters in string.

for line and polygon items

replace the coordinates in the range first and last (inclusive) with the coordinates specified in string.

The names of the commands are negotiable. The imove command is not strictly necessary as the rchars command can be used to obtain the same result. However, I believe that a separate imove command can be implemented with a better performance with rchars.

Copyright

This document has been placed in the public domain.

This is not necessarily the current version of this TIP.