[bug #13181] NSAffineTransform order of operations

[anonymous]

URL:
  <http://savannah.gnu.org/bugs/?func=detailitem&item_id=13181>

                 Summary: NSAffineTransform order of operations
                 Project: GNUstep
            Submitted by: None
            Submitted on: Wed 05/25/2005 at 02:44
                Category: None
                Severity: 3 - Normal
              Item Group: Bug
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open

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Details:

I'm not sure if this is a bug or ignorance. The following bit of code in a
graphing application is used to convert data coordinates to screen
coordinates:

        [toScreenCoordinates release];
        toScreenCoordinates = [[NSAffineTransform transform] retain];   // start
fresh
        // the order of operations is important for matrix algebra!
        [toScreenCoordinates scaleXBy: (viewRect.size.width /
dataBounds.size.width)
                           yBy: (viewRect.size.height / 
dataBounds.size.height)];
        [toScreenCoordinates translateXBy: -dataBounds.origin.x
                                   yBy: -dataBounds.origin.y];

It works correctly under OSX but not under GNUstep. The strange part is this:
When I swap the last two operations the code behaves identically for OSX and
GNUstep, unfortunately it is the undesired behavior. For GNUstep it makes no
difference what order the translation and scaling are done, it behaves the
same. For OSX, one way does the transform I need and the other does not. (The
comment line is a reminder to myself not to get them out of order again.)

The reverse transformation, screen to data, works equally well for both OSX
and GNUstep. It requires that the translation be performed before the
scaling.

        [toDataCoordinates release];   // start fresh
        toDataCoordinates = [[NSAffineTransform transform] retain];
        [toDataCoordinates translateXBy: dataBounds.origin.x
                                                                yBy: 
dataBounds.origin.y];
        [toDataCoordinates scaleXBy: (dataBounds.size.width / 
viewRect.size.width)
                                                        yBy: 
(dataBounds.size.height / viewRect.size.height)];

Mark Tracy
tracy454@concentric.net







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