As Christopher correctly points out, repeating-pattern "textures" are laid out starting at the top-left corner of a playfield.
Thus, if you use a graph-paper-like texture in a playfield, and you want a horizontal line in that texture to line up perfectly with the *bottom* of the playfield, it may be necessary for you to resize the playfield manually to make things line up along the bottom edge the way you want.
Most of the computer-graphics world operates with coordinate systems in which the origin is at the top-*left* corner and in which y-values get larger as you go *downward*. And indeed all of Squeak *outside the etoy system* operates this way.
However, most of the *mathematics* world operates with the conventional "Cartesian" coordinate system, in which y-values get larger as you *upward* in the 2-d plane. The Squeak etoy systm uses these "Cartesian coordinates" for "x" and "y" and "mouseX" and "mouseY" in Viewers and Scriptors.
Inevitably, situations arise in which there is a culture clash between the Cartesian view and the computer-graphics view. The situation that Christopher mentions is one. I wouldn't call it a "bug", because if repeating patterns were *not* laid out from the top-left-corner, the result would seem like *more* of a bug, to more people.
Cheers,
-- Scott
From: Christopher Sawtell csawtell@paradise.net.nz To: squeakland@squeakland.org Subject: Re: Seeking compelling high school examples Date: Fri, 2 May 2003 22:54:38 +1200
This project merely illustrates a very minor bug in the playfield.
It seems that the y origin for the graph paper is the top of the playfield, whereas the y origin for the graph is at the bottom. Thus, unless the size of the y dimension of the playfield is an exact multiple of the graphpaper squares the plotting is incorrect. This is illustrated by the graph to the left, the plot of which should pass precisely through the corners of the squares of the graph paper.
Once again, really a very minor bug.
http://squeakland.org/project.jsp?http://berty/GraphBug.005.pr
squeakland@lists.squeakfoundation.org