Zooming, Bezier Paths and Affine Transforms

FROM : Greg
DATE : Mon Dec 27 08:49:31 2004

Hi,
I am creating a graph view that allows the user to be able to zoom an
area of the x-axis only. This is because a plot can contain up to
around 10,000 samples, so to make it easier in viewing areas of the
graph I need to be able to zoom areas of it. I am having some trouble
trying to display the graph correctly. I can get the correct range of
samples to display from the user drawing a box on the graph to zoom it,
but when it comes to display it correctly, is where my problems occur.

The steps I am using are to translate to the start of the graph area as
there is a margin for axis values and labels.  I then translate the x
axis to the start of the zoomed samples, (_durationStart). I then scale
the X axis by the width and number of zoomed samples
(NSWidth(graphBounds) / (_durationEnd - _durationStart)) and the height
by the number of values in the Y axis. I then translate the graph by
the Y axis minimum value as this might not be set at 0.

I seem to have problems when I use affine transforms as my
understanding of them is very limited. I would appreciate it if someone
could recommend a book or resource on the net to help me better
understand what is happening to the co-ordinate space when I do
translations, scales and rotations. Does anyone know what I am doing
wrong fundamentally?

Thanks,
Greg

           if (_flags.showHeartRate)
           {
               NSAffineTransform *scale = [NSAffineTransform transform];
               NSBezierPath *p = [cur objectForKey:HeartRateKey];
               NSRect pathRect = [p bounds];
               [scale translateXBy:NSMinX(graphBounds)  yBy:NSMinY(graphBounds)];
               [scale translateXBy:-_durationStart yBy:0];
               [scale scaleXBy:NSWidth(graphBounds) / (_durationEnd -
_durationStart)
                           yBy:NSHeight(graphBounds)/(_heartRateMax - _heartRateMin)];
               [scale translateXBy:0 yBy: - _heartRateMin];
               [scale concat];
               [[_heartRateColor colorWithAlphaComponent:1.0 - (i / (c * 1.0))]
set];
               [p setLineWidth:_heartRateStroke];
               [p stroke];
               [scale invert];
               [scale concat];
           }