The SamplerRect worklet had an error where it was possible that the
invSpacing stack-local values could be used uninitialized. On the first
iteration of the loop in the SamplerRect operation, it calls LocateCell,
which is supposed to set invSpacing. Under most conditions it does, but
if one of the ray directions is 0 (which can happen with axis-aligned
views), one of the invSpacing dimensions was skipped, leaving the value
to whatever garbage happened to be on the stack. Later, the invSpacing
value was used to interpolate a scalar, which under some circumstances
could cause an array index error when looking up a color in the color
map.
This fix changes the condition for when the ray direction is 0 to still
initialize invSpacing.
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