Week09 Lab Exercise

CS4815 Week09 Lab Exercise
Lab Objective: We will complete our work on the B´ezier curve drawing program. We will
consider two tasks today: 1) drawing in the lines that connect the points so that we have a
better idea of how the curve drawing performs; and, b) dragging some of the points of the
curve and watch the curve be updated.
Here’s a quick summary of the tasks:
❶ Copy the main source file for this week’s lab
❷ Modify the program so that we can see the lines that connect the control points
❸ Modify the program so that we can select control points and drag them around the
screen; the B´ezier drawing should update
❹ Submit your completed program using the handin command
handin -m cs4815 -p w09
In Detail
❶ This week’s lab, although based on B´ezier curves again, is independent of last week’s.
Therefore you may use as a starting point the starting code of last week’s lab. However, if
you were able to complete last week’s lab, for the sake of a more complete program you may
want to add to that instead of making two sets of incompatible modifications to the same
initial file.
When I began thinking about this lab I thought that it would be nice to have you read in
an arbitrary set of points from a file via the argc and argv command-line variables and have
you compute the curve based on this set of points. This would be in place of the hard-coded
4 points of the array ctrlPts in the function display(). However, when I sat down to do
the preliminary work for it I soon realised that it would be too complicated to explain to
people who were only now learning C++ . So this might be good news for you. Maybe.
Eventhough I am not going to have you base your program on reading in an arbitrary
set of points I want you to modify the bezier.cc program so that it behaves in this way.
By this I mean that you should remove from the function displayFcn() the initialisatino of
the array of points, ctrlPts and its associated size nCtrlPts, and make both of these global
variables. That is, I want your program to “fake” reading points from a file by declaring the
points as a global array; if you had more expertise in C++ then it would be possible for you
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to write the function that would open a file and fill this array. You should add a few more
points to the array to make the drawings more interesting; 6-ish would be an ok number of
points.
There is one more change that you should make to the code that relates to faking reading
the points from a file. At the top of the program the world-coordinate clipping window
dimensions are hard-coded based on the 4 points given. You should generalise this process
by writing a function that goes through all of the points of the array – more than 4 of them,
please! – and initialises the four extremes. This will be important later.
❷ You should faintly draw in the lines that connect all of your control points. You should
set these lines up as a single OpenGL primitive declaration, as we have seen in a previous
lab. That is, only one glBegin() “command” should apply over all the points / lines. I will
leave it to your own aesthetic judgement for the colouring, thickness, options, etc. But just
remember that sledge-hammers can hurt.
❸ You should now modify your program so that it responds to mouse left clicks when within
a “reasonable” distance (radius) of a point. In this case the point should be “selected” and
you should be able to drag it and update the B´ezier curve. The idea of a reasonable distance
should be based on the scaling factors so that with more spread out points there should not
be a need for such a tight radius around the points.
❹ Using the handin command given at the top of the lab sheet please submit your lab
exercise by the usual deadline.
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