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student    : yusui
graded by  : Damian Isla
problem set: ivscan
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Renders 1st test (triangle)   (0-3): 2
Renders 2nd test (rgb)        (0-2): 2
Renders 3rd test (cube)       (0-2): 1.5
Renders 4th test (interpen)   (0-2): 0.5
Renders 5th test (ellipse)    (0-2): 0
Renders 6th test (geekball)   (0-2): 1
Exactitude                    (0-2): 2
total                        (0-15): 9
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notes:

There are a few serious problems with your renderer:

1) Most seriously, your ivscan crashed when running the last two of
   the above tests (ellipse and geek ball). The cause is an assertion
   error. The place in which this error occurs would suggest to me
   that you're taking edges off of the active edge list at the wrong
   time.

2) There were certain occasions (e.g. sometimes when rendering the
   cube.iv) where a crack would appear between two polygons that
   should have been flush and/or when some strange artifacts appear
   around the corners of the models.

3) Interpen.iv does not work correctly -- there is some sort of
   problem with your visibility calculation. In fact, I think if you
   flipped the sign on your z-buffer entries it would probably work
   correctly.

And more subtle problems:

4) You were not correctly intializing the EdgeRec, in that you were
   not correctly adjusting the starting value of X to account for the
   start of the edge being off-pixel-center. i.e. the initial
   value of xcurr does NOT equal xstart, but rather xstart + epsilon
   (finding epsilon is the trick). Because what you REALLY want is the
   value of X at the intersection of the edge with the next scan line. 
   Note that it is not JUST X that should be adjusted in this way, but 
   also color and depth (and inverse depth).

5) The above also counts for when you're interpolating along a
   scan-line (in RenderScanLine()). For each pixel that you light up,
   you want to take the values of color and depth at the CENTER of the
   pixel you're considering. So, if the starting X of an edge does not
   happen to be at the center of a pixel, you need to adjust the
   starting values of h and color to find their correctly interpolated 
   values at the center of the first pixel.
   
   The formula for this 1-D case is:

   hstart  = e1->hstart + (pixelCenter_X - e1->xcurr) * 
			  (e1->hcurr - e2->hcurr) / (e1->xcurr - e2->xcurr)

   and likewise for color.
   Where pixelCenter_X is the X-coordinate of the center of the first
   pixel you're lighting up, e1 is the first edge and e2 is the second
   edge.


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