TChart coordinate systems
lazarus / 55-TAChart

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wp · 2012-06-18 10:37

I am a bit confused about the TAChart coordinate systems. The
documentation reads:
Quote- Axis coordinates (known in some other applications as object
coordinates) -- this is the "raw" coordinate values obtained from the
data...
- Graph coordinates (aka world coordinates) are converted from the
axis coordinates using axis transformation, such as logarithmic scale.
Graph coordinates are common for all objects in the chart.
What confuses me is the phrase "world coordinates" attributed to the
second point, "graph coordinates".

In my view, "world coordinates" refer to the data in their own units,
and "object" and "world" coordinates are the same. So, if my data
range between 1 and 10 meters, these numbers are the "world
coordinates". When I want to plot these data logarithmically I apply a
log transform and get numbers between lg(1)=0 and lg(1)=1 which are
plotted in the diagram. Therefore, I would call the values 1...10
"axis coordinates" because that's what's written on the axis, and the
numbers 0...1 "graph coordinates" because that is what is actually
plotted. This is in agreement with the doc, but the phrase "world
coordinates" should not be where it is.

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Ask · 2012-06-18 14:54

Traditionally, in geometry and computer graphics "world coordinates"
refer
to a global coordinate system common for all objects,
as opposed to "object" or "local" coordinates.

Quotehttp://en.wikipedia.org/wiki/Coordinate_system#Change_of_coordina
tesQuotehttp://www.cse.ohio-state.edu/~gurari/course/cis681/cis681Ch8.
htmlQuotehttp://www.jimbrooks.org/web/opengl/tutorials/#Coordinate%20S
ystems
(or just google for "world coordinates").

Anyway, although I am quite confident in this meaning,
I am not a native English speaker.
So if you think this terminology is incorrect, I can drop it.

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wp · 2012-06-18 21:07

I know "object" and "local" coordinates mainly from CAD. Maybe the
confusion lies in the sentence
Quote"world coordinates" refer to a global coordinate system common
for all objects
Of course, after transformations there is only one coordinate system
which is applied to all objects. Therefore, you call this "world"
coordinates. To me, on the other hand, the "world" refers to the data
BEFORE transformations - as I said above to data in meters for
example, not to their logarithms.

QuoteI am not a native English speaker.
I'm not either. Maybe a native English speaker out there could
comment.