openscenegraph OSG

i am using openscenegraph for rendering .flt files.Using osgviewer i can walk thru .flt file. my model(.flt) is forest terrain.
my problem is, i am seeing white line between two trees.what may be the problem?
is it problem of visual database? i have a image showing the problem.i have marked the problem in red. Please help me to find out the solution.
Thanks in advance.

Link to the image

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This has nothing to do with “OpenGL on Linux”.