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Color discrepancy in highlights between the 3D viewport render and the Offline Render under certain view-transforms
Closed, ResolvedPublic


System Information
Windows 10 - GTX 770

Blender Version
Broken: 2.79b, f4dc9f9d68b (the current official release at the time of reporting, and tested in several others 2.79 builds with equal results)

Depending on the behaviour of the selected view-transform, there is a color discrepancy between the live render in the 3D viewport and the offline render from the UV/Image Editor. This is a Color Management bug not related to the already known alpha issues causing discrepancies between this two viewers, I clarify just in case they might be confused.

This difference is only notably visible in "over-exposed" conditions, when CMed values approach and/or reach [1.0,1.0,1.0] in their RGB triplet on scene values that have one channel at near 0.0. For the purpose of making the difference clearly noticeable, this tests have been performed with an extremely bright Sun lamp on pure colors and white, and can be seen under the following view-transforms in Blender's default Color Management settings:

-False Color

Note that when using the official Filmic release by Troy Sobotka this issue is not present and both viewers match accordingly on all view-transforms (4th screen-capture example with False Color using official Filmic). Issue is present in Blender's implementation of Filmic (plus that RRT transform it ships with)

After consulting with him about it he suggested the problem was about GPU Allocation variables. Far from my area of knowledge so I'm not in position to confirm this.

It occurs both rendering with GPU and CPU.

This first capture is just a reference of how the scene looks like under normal lighting conditions.

Sun Lamp intensity increase to 200k to make the differences obvious:

Viewport UP - Offline DOWN - Filmic view-transform (Blender's)

Viewport UP - Offline DOWN - RRT view-transform

Viewport UP - Offline DOWN - False Color view-transform (Blender's)

Viewport UP - Offline DOWN - Filmic view-transform (Official Troy's Filmic release)

Exact steps for others to reproduce the error

  1. Open the attached .blend file in Blender (for convenience, Load UI).
  2. Press F12 for an offline render.
  3. Press Shift + Z in the viewport for Render view.

Difference should be apparent as in the screen-captures presented when using either Filmic, False Color, or RRT transforms.

This results can be also spotted by using lower Sun intensities and increasing the exposure value (in the Color Management panel), as long as the initial values are high enough to reach the point in which the discrepancy becomes apparent, which are the very highlights, and shows the following behaviour that you can test from the unmodified blend file:

With e. g. Filmic set as view-transform, and Exposure set to "-10", both viewers match. Going through the image with the color picker and looking at the CMed values you can see that as you increase the Exposure and the CMed values in the RGB triplet start approaching [1.0,1.0,1.0] at about -8.5 stops and above, Filmic's desaturation kicks in seemingly properly on the Offline render, but seems to "cap" prior of reaching fully desaturated over-exposed results in the active viewport. Similar effects can be notice at nearby exposure levels on False Color and RRT, not on other view-transforms (that I could notice).

EDIT: Added some clarity and fixed a typo

Event Timeline

This is an easy fix I believe in the broken GPU allocation.

Line 82 should be changed from:

allocationvars: [-15, 6]


allocationvars: [-12.473931188, 12.526068812]

Props to @Jonatan Mercado (jonimercado) for the testing, and @Joaquín Kierbel (Jk) for the eyes to spot it.

@Brecht Van Lommel (brecht) easy one line patch here for 2.8.