[libGD] #100 [Task opened] spurious horizontal line drawn by gdImageFilledPolygon; fix attached

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A new Flyspray task has been opened.  Details are below.

User who did this - Ducky Sherwood (duckysherwood)

Attached to Project - libGD
Summary - spurious horizontal line drawn by gdImageFilledPolygon; fix attached
Task Type - Bug Report
Category - General
Status - Unconfirmed
Assigned To - 
Operating System - All
Severity - Low
Priority - Normal
Reported Version - 2.0.35
Due in Version - Undecided
Due Date - Undecided
Details - There is a bug in gdImageFilledPolygon such that you can get spurious horizontal lines.  I've attached fix for it and a test case that demonstrates it.

gdImageFilledPolygon works by iteratively drawing horizontal lines between the borders of the polygon.  To do this, it interpolates the x-value for a given y-value, based on the positions of two connected vertices.  Then, it sorts the x-values (for that given y-value) and draws lines between the first and second x-value, third and fourth, etc.

(Why does it sort?  To deal with cases where the polygon is shaped such that there is a gap in the horizontal line.  Imagine, for example, a "V" shape -- a horizontal slice near the top would give two colored pieces with a non-colored piece in between them.)

Unfortunately, it is possible to get conditions where you have an *odd* number of x-values that intersect a given y-value.  Thus when 
gdImageFilledPolygon tries to draw a horizontal line between x[i] and x[i+1], the x[i+1] is garbage (usually 0 for me, YMMV).

To fix this, I checked for odd-numbers of intercepts.  If it was odd,
I added an intercept, making x[i+1] = x[i].  This means drawing a spurious dot, but that seemed safer than taking one intercept away.



More information can be found at the following URL:
http://bugs.libgd.org/?do=details&task_id=100

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