Grease pencil. Artefacts at beginning of strokes #87984

New Issue

Tomasz Kaye · 2021-05-02T11:25:43+02:00

Tomasz Kaye commented

2021-05-02 11:25:43 +02:00

System Information
Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce RTX 2060/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 457.30

Blender Version
Broken: version: 3.0.0 Alpha, branch: master, commit date: 2021-04-23 20:25, hash: 17fca62fff

Short description of error
Grease pencil strokes drawn with a stylus have unwanted artefacts at their beginning. Especially noticeable at high radius settings. Related to this, the start of a stroke is often narrower than the artist's intention.

Exact steps for others to reproduce the error

Open new document, 2d animation
Set brush strength to 1 (not using pressure)
Set brush radius to 200 (using pressure)
draw several lines with a stylus
zoom in to inspect the start of the lines. There are artefacts, a 'lumpy' line close to the beginning of the stoke (see screenshot)

Notes
When you draw with deformable physical natural media (e.g. a paint brush, felt-tip) adding pressure increases the size of the mark, even when the tip is not moving over the paper. Other drawing programs also work this way. Currently blender doesn't work this way.

I looked at the source code (gpencil_paint.c). Here's what i think is going wrong with the grease pencil strokes:

When the pressure difference between two close adjacent vertices is large this can cause ugly 'lumps' in the line.

Currently, stylus pressure can be very different between the first and second vertex recorded in a grease pencil stroke. This is because there can be a large time difference between the recording of the two points; The first vertex stores a record of the pressure only at the moment the stylus first touches the tablet, subsequent changes of pressure before the stylus begins to move across the tablet are not recorded in the stroke. Only after the stylus starts moving is the second point recorded with the current pressure.

Proposed solution
Any time the stationary stylus tip is touching the tablet, the drawing routine polls the changing tablet pressure continuously. When the detected pressure is higher than the pressure of the last recorded point, the pressure of the last recorded point is overwritten by the newly detected (higher) pressure. This way, the pressure of the first and second points in a stroke is much more likely to be similar, the width of the start of the line will more closely match the artists intention, and the likelihood of artefacts is reduced.

**System Information** Operating system: Windows-10-10.0.18362-SP0 64 Bits Graphics card: GeForce RTX 2060/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 457.30 **Blender Version** Broken: version: 3.0.0 Alpha, branch: master, commit date: 2021-04-23 20:25, hash: `17fca62fff` **Short description of error** Grease pencil strokes drawn with a stylus have unwanted artefacts at their beginning. Especially noticeable at high radius settings. Related to this, the start of a stroke is often narrower than the artist's intention. ![image.png](https://archive.blender.org/developer/F10049657/image.png) **Exact steps for others to reproduce the error** * Open new document, 2d animation * Set brush strength to 1 (not using pressure) * Set brush radius to 200 (using pressure) * draw several lines with a stylus * zoom in to inspect the start of the lines. There are artefacts, a 'lumpy' line close to the beginning of the stoke (see screenshot) **Notes** When you draw with deformable physical natural media (e.g. a paint brush, felt-tip) adding pressure increases the size of the mark, even when the tip is not moving over the paper. Other drawing programs also work this way. Currently blender doesn't work this way. I looked at the source code (gpencil_paint.c). Here's what i think is going wrong with the grease pencil strokes: When the pressure difference between two close adjacent vertices is large this can cause ugly 'lumps' in the line. Currently, stylus pressure can be very different between the first and second vertex recorded in a grease pencil stroke. This is because there can be a large time difference between the recording of the two points; The first vertex stores a record of the pressure only at the moment the stylus first touches the tablet, subsequent changes of pressure before the stylus begins to move across the tablet are not recorded in the stroke. Only after the stylus starts moving is the second point recorded with the current pressure. **Proposed solution** Any time the stationary stylus tip is touching the tablet, the drawing routine polls the changing tablet pressure continuously. When the detected pressure is higher than the pressure of the last recorded point, the pressure of the last recorded point is overwritten by the newly detected (higher) pressure. This way, the pressure of the first and second points in a stroke is much more likely to be similar, the width of the start of the line will more closely match the artists intention, and the likelihood of artefacts is reduced.

Tomasz Kaye commented

2021-05-02 11:25:43 +02:00

Added subscriber: @info-27

Falk David commented

2021-05-03 10:25:51 +02:00

Added subscriber: @filedescriptor

Falk David commented

2021-05-03 10:25:51 +02:00

Changed status from 'Needs Triage' to: 'Needs User Info'

Falk David commented

2021-05-03 10:25:51 +02:00

@info-27 Are you using Wintab or Windows Ink? Could you try both of them and see if they give different results?

Mentioning #87499 here as this task could be related.

@info-27 Are you using Wintab or Windows Ink? Could you try both of them and see if they give different results? Mentioning #87499 here as this task could be related.

Tomasz Kaye commented

2021-05-03 11:36:51 +02:00

whether using wintab or windows ink i get unwanted artefacts.

(today the artefacts seem less severe, maybe the problem is made worse when cpu is taxed with other processes)

whether using wintab or windows ink i get unwanted artefacts. ![image.png](https://archive.blender.org/developer/F10051031/image.png) (today the artefacts seem less severe, maybe the problem is made worse when cpu is taxed with other processes)

Nicholas Rishel commented

2021-05-03 23:44:24 +02:00

Added subscriber: @PrototypeNM1

Nicholas Rishel commented

2021-05-03 23:44:24 +02:00

I think the analysis in Notes is mostly accurate; this looks more like a grease pencil issue than Wintab/WinInk.

Similar artifacts occur if you keep changing pressure without changing position mid-stroke.

I think the analysis in **Notes** is mostly accurate; this looks more like a grease pencil issue than Wintab/WinInk. Similar artifacts occur if you keep changing pressure without changing position mid-stroke.

Falk David commented

2021-05-04 08:49:45 +02:00

Changed status from 'Needs User Info' to: 'Confirmed'

Falk David commented

2021-05-04 08:49:45 +02:00

After taking another look at the painting code, I think you guys are right. Working on a potential fix.

Falk David commented

2021-05-04 09:11:09 +02:00

So there are two things happening here:

We do not trigger a drawing event when just the pressure changed.
We do not add points if they have the same position as the last point.

Now the question is, why is it done this way. I hacked together a patch that ignores the two points above (always add a point if the pressure changes).
Here is what this would look like (move the pen, stop, change pressure, move again):

As you can see, that does not look very good. Basically, when we get points that are directly on top/very close to each other, the grease pencil engine does not handle it very well. And I guess that makes sense because a lot of it has to do with the segments between the points. If those have a length of zero then you get problems (as shown in the image).

But I think we can do better. My idea would be: Instead of creating new points when the pressure changes but the position is the same, change the pressure of the current point in the buffer. Or even better, only change it if the pressure got greater. Because otherwise, you could "shrink" the point you are currently drawing on, which doesn't really make much sense painting-wise.
That should in theory solve the issue.

So there are two things happening here: - We do not trigger a drawing event when just the pressure changed. - We do not add points if they have the same position as the last point. Now the question is, why is it done this way. I hacked together a patch that ignores the two points above (always add a point if the pressure changes). Here is what this would look like (move the pen, stop, change pressure, move again): ![image.png](https://archive.blender.org/developer/F10052508/image.png) As you can see, that does not look very good. Basically, when we get points that are directly on top/very close to each other, the grease pencil engine does not handle it very well. And I guess that makes sense because a lot of it has to do with the segments between the points. If those have a length of zero then you get problems (as shown in the image). But I think we can do better. My idea would be: Instead of creating new points when the pressure changes but the position is the same, change the pressure of the current point in the buffer. Or even better, only change it if the pressure got greater. Because otherwise, you could "shrink" the point you are currently drawing on, which doesn't really make much sense painting-wise. That should in theory solve the issue.

Tomasz Kaye commented

2021-05-04 09:14:56 +02:00

that was exactly my suggestion 🙂

Falk David commented

2021-05-04 09:30:42 +02:00

Well... Apparently, I need glasses

Falk David commented

2021-05-04 10:13:33 +02:00

Added a patch that implements your idea @bitbutter. From my testing, it works quite well. If you can test the patch for yourself and see if it fixes the problem, that would be great.

Philipp Oeser removed the

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Grease pencil. Artefacts at beginning of strokes #87984