Vector Pass output in Cycles is wrong when camera focal length is animated #69731

New Issue

Maria · 2019-09-10T20:40:52+02:00

Maria commented

2019-09-10 20:40:52 +02:00

System Information
Operating system: MacOSX 10.12.6
Graphics card: NVIDIA GeForce GT 750M 2048 MB

Blender Version
Originally detected for: Blender 2.79 (5bd8ac9)
Still broken for daily build downloaded on Sept 10, 2019: 2.81 alpha (6be3348505)

Short description of error
Vector pass ignores motion due animation of camera focal length. In case of multiple sources of motion, motion due to focal length is ignored, while the other motion is output (i.e. sum motion is wrong).

Exact steps for others to reproduce the error
Vector pass should contain screen space X, Y displacement of primitives in the current frame to their respective locations in the next frame. The actual vector pass output fails to account for motion caused by the change to camera focal length. E.g. vector pass output for the square_focallength_exr.blend is zero, but it should be similar to the vector pass output for the blend with camera simply moving forward square_cammotion_exr.blend. We have examined the output of the vector pass by first writing to EXR files using the following steps:

open blend
set output file path
execute "import bpy; bpy.ops.render.render(animation=False, write_still=True)" in python console.
examine EXR file using python library available from https://www.openexr.com/, by looking at channels Vector.Z, Vector.W (perplexingly, these correspond to correct vector pass output, except when focal length is changed).

We verified that for both builds above:
a) vector pass output for square_cammotion_exr.blend is as expected,
b) vector pass output for square_focallength_exr.blend is zero (WRONG).

square_focallength_exr.blend
square_cammotion_exr.blend

**System Information** Operating system: MacOSX 10.12.6 Graphics card: NVIDIA GeForce GT 750M 2048 MB **Blender Version** Originally detected for: Blender 2.79 (5bd8ac9) Still broken for daily build downloaded on Sept 10, 2019: 2.81 alpha (6be3348505ce) **Short description of error** Vector pass ignores motion due animation of camera focal length. In case of multiple sources of motion, motion due to focal length is ignored, while the other motion is output (i.e. sum motion is wrong). **Exact steps for others to reproduce the error** Vector pass should contain screen space X, Y displacement of primitives in the current frame to their respective locations in the next frame. The actual vector pass output fails to account for motion caused by the change to camera focal length. E.g. vector pass output for the square_focallength_exr.blend is zero, but it should be similar to the vector pass output for the blend with camera simply moving forward square_cammotion_exr.blend. We have examined the output of the vector pass by first writing to EXR files using the following steps: 1) open blend 2) set output file path 3) execute "import bpy; bpy.ops.render.render(animation=False, write_still=True)" in python console. 4) examine EXR file using python library available from https://www.openexr.com/, by looking at channels Vector.Z, Vector.W (perplexingly, these correspond to correct vector pass output, except when focal length is changed). We verified that for both builds above: a) vector pass output for square_cammotion_exr.blend is as expected, b) vector pass output for square_focallength_exr.blend is zero (WRONG). [square_focallength_exr.blend](https://archive.blender.org/developer/F7732015/square_focallength_exr.blend) [square_cammotion_exr.blend](https://archive.blender.org/developer/F7732016/square_cammotion_exr.blend)

Maria commented

2019-09-10 20:40:52 +02:00

Added subscriber: @MariaShugrina

Philipp Oeser commented

2020-01-13 11:42:24 +01:00

Added subscribers: @Sergey, @brecht, @lichtwerk

Philipp Oeser commented

2020-01-13 11:42:24 +01:00

Changed status from 'Needs Triage' to: 'Archived'

Philipp Oeser closed this issue

2020-01-13 11:42:24 +01:00

Philipp Oeser self-assigned this 2020-01-13 11:42:24 +01:00

Philipp Oeser commented

2020-01-13 11:42:24 +01:00

Hi there and thx for the report! (sorry this has been lying around for so long)

Well, judging the comment here https://developer.blender.org/diffusion/B/browse/master/intern/cycles/render/camera.cpp$336

/* TODO(sergey): Support perspective (zoom, fov) motion. */

this looks like a known limitation to me (in regards to writing this to the pass -- it renders correctly if you use Motion Blur directly...).

I am not quite sure how much work it would be to support this [Scene::MOTION_PASS and Scene::MOTION_BLUR are handled differently at a couple of places], but really, this is more of a feature request than an actual bug (@Sergey, @brecht: correct me if I am wrong).

Sure, this would be nice to have, but the issue reported here is a request for modified/improved behavior and not a bug in current behavior. Closing as this bug tracker is only for bugs and errors.
For user requests and feedback, please use other channels: https://wiki.blender.org/wiki/Communication/Contact#User_Feedback_and_Requests
For more information on why this isn't considered a bug, visit: https://wiki.blender.org/wiki/Reference/Not_a_bug

Hi there and thx for the report! (sorry this has been lying around for so long) Well, judging the comment here https://developer.blender.org/diffusion/B/browse/master/intern/cycles/render/camera.cpp$336 > /* TODO(sergey): Support perspective (zoom, fov) motion. */ this looks like a known limitation to me (in regards to writing this to the pass -- it renders correctly if you use Motion Blur directly...). I am not quite sure how much work it would be to support this [`Scene::MOTION_PASS` and `Scene::MOTION_BLUR` are handled differently at a couple of places], but really, this is more of a feature request than an actual bug (@Sergey, @brecht: correct me if I am wrong). Sure, this would be nice to have, but the issue reported here is a request for modified/improved behavior and not a **bug** in current behavior. Closing as this bug tracker is only for bugs and errors. For user requests and feedback, please use other channels: https://wiki.blender.org/wiki/Communication/Contact#User_Feedback_and_Requests For more information on why this isn't considered a bug, visit: https://wiki.blender.org/wiki/Reference/Not_a_bug

Maria commented

2020-01-13 16:08:20 +01:00

Hello @lichtwerk , thanks very much for taking a look! Would it be possible to at least add a warning message somewhere? Blender has been often used by the Computer Vision community to render out ground truth optical flow information. For our use case, this information must be precise. Silent output of incorrect values could mean incorrectly reported results, or (as was the case with me) many hours of debugging. It was by mere chance that I discovered fov was the cause of odd behavior.

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Vector Pass output in Cycles is wrong when camera focal length is animated #69731