Resolution based render artifact

[SonyBoy]

I have an animation that seems to have some sort of resolution based render artifacting going on that I can't trace. The attached image shows the same example frame rendered at 75 percent and then at 100 percent of final rez (1355 x 1080), and in the full rez there is an overbright rectangle appearing either on the curtain or in the mirror's surface. There are about 15 frames of my animation where these show up, intermittently. It's hard to tell which, if any, specific parts of the calculation that are the problem -- the artifacts seem to appear mostly in GI, reflection and SSS passes, however since they're appearing in a reflection it's extra hard to be sure and turning these calculations off still doesn't get rid of them, it only dims them. Since resolution is causing it, memory would obviously be a guess but I have 128Gb installed and while these frames are slow (about an hour each), the scene is not heavy and I've rendered 300-400 frames of the same scene that don't display this issue. Been all over various settings (embree on/off, color mapping, messing with lights and shader settings) to see if it's just some bad math that a 0.001 change might fix, but so far rendering at lower than full rez seems to be the only fix, which obviously isn't a fix at all. Anyone got any ideas?

[hermit.crab]

Best you send the scene to support@chaosgroup.com so we can take a look. Mention this thread in the e-mail along with your V-Ray and host platform versions.

[SonyBoy]

In the end I traced this to some bad/duplicate faces on the curtain and/or the fact that I was using viewport subdivision for smoothing. Cleaning up the curtain geo and using Vray Open Subdiv instead got rid of it. I've been trying to wean myself off the bad habit of viewport subdiv for a long time, but it's just so convenient as a workflow option. BUT, it's still really odd as the problem was ONLY occuring at certain resolutions, as previously stated. I even tried cheating the renderer by setting my resolution higher then using the VFB to output at 75 percent, giving me my actual target resolution, and this still had artifacts. Seems like anywhere near my target resolution of 1355 x 1080 had the problem. Odd.