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GPencil: Texture Image Alpha pass through grease pencil strokes
Confirmed, NormalPublicKNOWN ISSUE

Description

System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce GTX 1080/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 441.87

Blender Version
Broken: version: 2.83.1, branch: master, commit date: 2020-06-25 09:47, hash: rB8289fc688b3e
Worked: (newest version of Blender that worked as expected)

Short description of error
Context: Using texture image to make some kind of 2d fur on top of a GPencil Object

The Image Texture used to make some fur is mostly transparent and the brush used to make the colored part is a texture paint brush with Cloud Mask (it was made entirely in Blender with Blender tools).

When on top of the GPencil object, the part of the Image Texture that is only half transparent(between 99%-1% transparent), makes the GPencil color disappear, even when moving the center point behind the one of the fur images.

On the contrary, when on top of another Image Texture (imported to be used as camera mapping), the Image Texture of the fur doesn't impact the transparency of that one.
Notice that the Image texture boundary, even from the imported one, has problems with clipping and always makes a white line appear.

Exact steps for others to reproduce the error

  • Open attached .blend file
  • Zoom in and out to notice the alpha transparency influencing the GP object

Thanks for your help!

Event Timeline

I'm struggling to understand the role of each object to the descripition of the problem.
A simplified file is highly recommended.

But I notice that there is something strange about the material of the Fur_Image mesh object.

I am not very familiar with this area. It may not be a good practice to mix transparent meshes with Grease Pencil objects.

I'm marking the Grease Pencil team to take a look.

Antonio Vazquez (antoniov) renamed this task from Texture Image Alpha pass through Grease Pencil to GPencil: Texture Image Alpha pass through grease pencil strokes.Jul 11 2020, 9:08 AM

@Clément Foucault (fclem) Could you take a look?

Clément Foucault (fclem) changed the task status from Needs Triage to Confirmed.Jul 13 2020, 6:47 PM
Clément Foucault (fclem) changed the subtype of this task from "Report" to "Known Issue".

This is the issue of compositing EEVEE foreground elements with Gpencil in the background. the AA patern does not match perfectly and uncover the scene background.

Workaround: If you disable EEVEE AntiAliasing and GPencil AntiAliasing it fixes the issue but you loose the antialiasing. Render to twice the resolution and downsample in post processing to reduce antialiasing.

I would consider this known limitation as we don't have any planned fix for this.

Ok thanks for the infos and workaround @Clément Foucault (fclem)

Further notes :

  • EEVEE with material blend mode Alpha Blend (default with import image as plane) has different problem : Texture mesh appears behind the GPencil object.
  • Cycles, The alpha completely holdout the Grease pencil

Like the OP, I also noticed this whilst mixing Image Textures and GPencil. Then when trying to reproduce it with the smallest amount of steps, I realised that it doesn't just affect Image Textures but anything that passes in front of a GPencil object apart from other GPencil objects.
Thought I'd post this screenshot to illustrate the problem a bit more clearly. It's black GP filled square, between two 3D meshes, a blue plane and a red UV sphere (both with Emission shaders):


So I understand there's no plan to fix this cos it might not be that simple, but doesn't that seriously limit possibilities if GPencil objects can never be behind 3D meshes? (at least without having to render at twice the size (btw thanks for that workaround))

I agree with Kevin'g comment above - this issue seems like a pretty serious limitation, as the one significant competitive advantage Blender Grease Pencil has over 2D animation software is the ability to combine 3D elements in a 2D scene; using textured 3D meshes with alpha transparency is a very powerful tool. I understand if this issue is not high priority and might not get fixed in the near future, but in my opinion it should not be classified as known limitation. Please let me know if I'm missing something. I would appreciate if someone from the dev team can post a point of view on this.

I found a slightly better workaround for the anti-aliasing issue. If you put the Gpencil objects and 3d objects on two different View Layers then put them back together in the compositor with a Z Combine node (enable Use Alpha and disable Anti-Alias Z), you will still need to set EEVEE's filter size to 0, but you can keep the Grease Pencial anti-aliasing on. So not perfect but in some cases it might be enough.

@Adrian Th (adrianth) I think that's not exactly the same problem, in your case you just need to switch Alpha Blend to Alpha Hashed in the material settings.