Re: Tk::Canvas createGroup() Undocumented

Hi Ken,

This probably can shed some light, from TK Canvas: Group vs. tags:

For instance, if you tried to rotate a bunch of "tags", they would all
+ rotate independently; but if they are grouped, you can specify the g
+roup center point, where they would rotate around.
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So, my guess would be that (0,0) is the group's "handle" point for geometric transformations.

bw, bliako

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Re^2: Tk::Canvas createGroup() Undocumented by kcott (Archbishop) on Mar 20, 2021 at 23:49 UTC
G'day bliako, ++ Thanks for taking the trouble to dig out that old (2004) post. The text regarding rotations from zentara (which you've quoted) actually refers to `Tk::Zinc`; rotations are not possible with `Tk::Canvas`. From "Tk::Canvas - TRANSFORMATIONS": ... Canvases do not support scaling or rotation of the canvas coordinate system relative to the window coordinate system. Individual items may be moved or scaled using methods described below, but they may not be rotated. The thinking behind your guess is valid; unfortunately, it cannot work. That's a pity because it would have been useful. For instance, rotating the bend through 90° clockwise, could have made the creek flow north, instead of south, without the need to code a separate group. Presumably, had that been possible, the `[0, 0]` would have needed to be `[50, 50]` to rotate about the centre. — Ken	[reply] [d/l] [select]

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Re^2: Tk::Canvas createGroup() Undocumented
by kcott (Archbishop) on Mar 20, 2021 at 23:49 UTC

G'day bliako,

++ Thanks for taking the trouble to dig out that old (2004) post.

The text regarding rotations from zentara (which you've quoted) actually refers to Tk::Zinc; rotations are not possible with Tk::Canvas. From "Tk::Canvas - TRANSFORMATIONS":

... Canvases do not support scaling or rotation of the canvas coordinate system relative to the window coordinate system.

Individual items may be moved or scaled using methods described below, but they may not be rotated.

The thinking behind your guess is valid; unfortunately, it cannot work.

That's a pity because it would have been useful. For instance, rotating the bend through 90° clockwise, could have made the creek flow north, instead of south, without the need to code a separate group. Presumably, had that been possible, the [0, 0] would have needed to be [50, 50] to rotate about the centre.

— Ken

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