Bug in HdResampleRawTimeSamples()

[GrangerFX]

Description of Issue

There is a bug in the templated function HdResampleRawTimeSamples that causes errors in motion blur streaks.
HdResampleRawTimeSamples() is called from HdTimeSampleArray::Resample().
The functions are in pxr\imaging\hd\timeSampleArray.h
The bug is in this line:
// Linear blend of neighboring samples.
float alpha = (us[i]-u) / (us[i]-us[i-1]);

Alpha is used to interpolate between vs[i-1] and vs[i] in the next line:
return HdResampleNeighbors(alpha, vs[i-1], vs[i]);

As u gets closer to us[i], alpha gets closer to zero. As u gets closer to us[i-1], alpha gets closer to 1.
This is backwards. Alpha should get closer to 1 as u gets closer to us[i] and closer to 0 as u gets closer to us[i-1].

The fix is as follows:
// Linear blend of neighboring samples.
float alpha = (u-us[i-1]) / (us[i]-us[i-1]);

Steps to Reproduce

Any use of HdTimeSampleArray::Resample() with a different number of samples than count will show an error in the resulting time sample values.

System Information (OS, Hardware)

Any/NA

Package Versions

USD v21.05

Build Flags

Any/NA

[jilliene]

Filed as internal issue #USD-6776

[blevin]

Thanks for calling this out. We checked in a fix to our internal tree today and it should be on the the github dev branch soon after the USD 21.08 release.