Grease pencil opacity bug when dealing with line thickness relative to screen #80862

New Issue

Elias · 2020-09-17T00:01:28+02:00

Elias commented

2020-09-17 00:01:28 +02:00

System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: Intel(R) HD Graphics 620 Intel 4.5.0 - Build 26.20.100.7925

Blender Version
Broken: version: 2.90.0, branch: master, commit date: 2020-08-31 11:26, hash: 0330d1af29
Worked: (newest version of Blender that worked as expected)

Short description of error
When a line is small enough relative to the viewport or camera, it seems to lower in opacity, and the interpolation between points can cause artifacts

Exact steps for others to reproduce the error
attached is a .blend file showing an extreme case, where two grease pencil points are at different sizes, and depending on how zoomed in you are, the point on the right becomes transparent, and it causes a very visible gradient between the points. Note, both points are at a strength of 1, any opacity change isn't expected.

This artifact shows up in renders, as pictured below.

opacity thickness bug.blend

**System Information** Operating system: Windows-10-10.0.18362-SP0 64 Bits Graphics card: Intel(R) HD Graphics 620 Intel 4.5.0 - Build 26.20.100.7925 **Blender Version** Broken: version: 2.90.0, branch: master, commit date: 2020-08-31 11:26, hash: `0330d1af29` Worked: (newest version of Blender that worked as expected) **Short description of error** When a line is small enough relative to the viewport or camera, it seems to lower in opacity, and the interpolation between points can cause artifacts **Exact steps for others to reproduce the error** attached is a .blend file showing an extreme case, where two grease pencil points are at different sizes, and depending on how zoomed in you are, the point on the right becomes transparent, and it causes a very visible gradient between the points. Note, both points are at a strength of 1, any opacity change isn't expected. This artifact shows up in renders, as pictured below. ![opacity thickness.png](https://archive.blender.org/developer/F8890441/opacity_thickness.png) [opacity thickness bug.blend](https://archive.blender.org/developer/F8890426/opacity_thickness_bug.blend)

Elias commented

2020-09-17 00:01:28 +02:00

Added subscriber: @razzzmatazzz30297

blender-admin commented

2020-09-17 00:01:28 +02:00

#84909 was marked as duplicate of this issue

Germano Cavalcante commented

2020-09-18 20:49:15 +02:00

Added subscriber: @mano-wii

Germano Cavalcante commented

2020-09-18 20:49:15 +02:00

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

Germano Cavalcante commented

2020-09-18 20:49:15 +02:00

Perhaps when the size of a point is smaller than that of a pixel, its opacity is overwritten to represent that it is less visible.
But in fact, this opacity should not generate this gradient.

Perhaps when the size of a point is smaller than that of a pixel, its opacity is overwritten to represent that it is less visible. But in fact, this opacity should not generate this gradient.

Jamell Moore commented

2020-12-10 22:58:15 +01:00

Added subscribers: @fclem, @JamellMoore

Jamell Moore commented

2020-12-10 22:58:15 +01:00

I have been reviewing the GPencil engine/shader code and can see this is not a bug but intended by design.

In gpencil_common_lib.glsl L:343 it states that line thickness is clamped and the opacity reduced when the thickness is small to avoid aliasing artifacts.

I have removed the opacity modulation and tested the in the viewport and cannot see any artifacts. @fclem do you think the opacity modulation is safe to remove? If not the bug status should be removed from this task.

Bug-#80862-GPencil-Opacity-Small-Stroke-Thickness-FIX-TEST.mp4

I have been reviewing the GPencil engine/shader code and can see this is not a bug but intended by design. In gpencil_common_lib.glsl L:343 it states that line thickness is clamped and the opacity reduced when the thickness is small to avoid aliasing artifacts. I have removed the opacity modulation and tested the in the viewport and cannot see any artifacts. @fclem do you think the opacity modulation is safe to remove? If not the bug status should be removed from this task. [Bug-#80862-GPencil-Opacity-Small-Stroke-Thickness-FIX-TEST.mp4](https://archive.blender.org/developer/F9500896/Bug-#80862-GPencil-Opacity-Small-Stroke-Thickness-FIX-TEST.mp4)

Clément Foucault commented

2020-12-10 23:40:52 +01:00

The opacity modulation is here to avoid strong aliasing of subpixel lines. A better way would be to make this opacity fade inside the pixel shader, so after interpolation.

Jamell Moore commented

2020-12-11 00:57:25 +01:00

@fclem I thought that was the case. Isn't the sub-pixel antialiasing defined in ScenePencil->smaa_threshold sufficient in preventing these artifacts?

Clément Foucault commented

2020-12-11 05:07:52 +01:00

Nope it isn't enough. SMAA is only working on lines that are at least 1 pixel large. But we don't want to clamp actual visual thickness to 1 so we fade based on the line size.