Brush Fill for grease pencil

Blender addon - Grease pencil flat brush that act like a 2D brush for fill materials

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/!\ Need external modules (see section below How to get opencv and shapely modules)
Disclaimer : This addon is a WIP and highly experimental.
It's to be considered as a workaround until better native blender solution exists for flat coloring.

Description

Paint Grease pencil closed flat stroke with a 2D brush on a plane defined by same projecting option as usual grease pencil. Projection modes "Surface" and "Stroke" are not supported (act as if Origin is selected).

Usage

The UI panel appears once a grease pencil object is selected, in the left toolbar.

The brush fill modal operator is lauched with a button brush fill.
You can bind a shortcut to it by right clicking on the brush fill button > add shortcut
In the addon preferences you can enable an option to bind a default ctrl+shit+F shortcut

click : Normal paint, add grease pencil stroke with current selected materials.

Combining with shift/alt before releasing allow add/sub modes with existing grease pencil strokes. This modes have filter available in user interface (filter what types of grease pencil strokes to affect)

shift+click : Add, paint in additive mode. merge crossed strokes

alt + click : Eraser, paint in substractive mode. merge crossed strokes too

esc or right-click : Leave the modal

[/], X/C, numpad -/+ or mousewheel down/up : change radius of the brush

S/D : Change spacing value

Limitations

Everything happens in the screen space visible part of the viewport that is locked during the modal.
You can't move view during the painting and the brush is clamped to viewport border.

For now :

• Only work in perspective view (tring in ortho view will raise an error message)
• Impossible to erase plain stroke without fill (use native eraser for that)
• No undo-stack
• since everything else is lock, need to leave the modal with right click or esc to perform other actions.
• Impossible to add holes in shapes when erasing
• In additive/substractive mode, closing a draw shape (like a lasso) will englobe everything inside.
• When merging shapes it deletes points informations (thickness/pressure/etc)
• Generally keep in mind that it delete crossed strokes to create a new one (for the same reason, it can messed up with the depth order of your fills).

Important technical note

This addon rely on external python modules :

opencv to use the "paint" in the region screen pixel with circle brush and convert it to polygon > grease pencil strokes

shapely for additive/substractive boolean operation between polygon generated by the brush and existing grease pencil strokes.

The addon will try to auto-install those modules but you might need to run Blender as admin and retry installation even if it fail once
If the modules auto-install fail, read following notes about manual installation

How to get opencv and shapely modules

Note : the package name for opencv is opencv-python, or opencv-contrib-python for extended version.
the module folder will be named cv2

You need to have python installed on your machine to install with Pip:
Detailed infos on module installation here

The local python version need to be the same version as blender python : 3.7.
To check pip python version enter : pip --version or pip -V

If your version is ok, run this lines and go to the Place modules in blender section: pip install opencv-contrib-python pip install shapely

If you have multiple python version installed you can choose like this : On linux python3.7 -m pip install shapely (if you seem to have a permission problem add --user to this line to install for current user only) On windows py -3.7 -m pip install shapely

Details on packages here : opencv: https://pypi.org/project/opencv-python/
shapely : https://pypi.org/project/Shapely/

For windows user there is the alternative solution of the Unofficial Windows Binaries for Python Extension Packages. Download whl of opencv and shapely. In the list choose the last version that is compatible with your system. amd64 for 64bits else get win 32.
Then open a terminal in the folder containing these file and run pip install ThePackage.whl (more info on Wheel installation)

Locate modules

Once installed you need to copy module files from your local python into your blender module folder.
Those file are stored in the python version directory "site-packages".
To easily locate them use : pip show shapely, that will display their location.
ex: On windows it will probably be something like C:/Users/User/AppData/Local/Programs/Python/Python-version/Lib/site-packages

Place modules in blender

Now copy the folder cv2 and shapely in you blender module folder
Don't mind the folders ending with .dist-info
Blender module folder is in temp directory and (there is another in install directory).
To locate it open blender and in the python interactive console and run print(bpy.utils.user_resource('SCRIPTS')) (or use my devtool addon button "print resources filepath" ;) ) Copy the modules in the modules folder inside scripts (if theres no "modules" folder just create it).
The path would be something like .../Blender/2.80/scripts/modules/

Once modules are in this folder you can use them right away (no need to restart blender). Just launch the operator. \o/

Todo:

• Show only fill materials in the combo box.

• Replace the stroke in 3D view (adding a fill to a previous one make it pop to the top)

• Make it orthographic view compatible (if possible with screen coordinates calculations).

• Work with stroke without fill

• Add a curve pressure for tablet pen (impossible to add mapping curve properties)

• add thickness as a scene properties (exposed to UI if any)

• maybe : add a way to handle rapid radius change (like 'F' for classic blender brush.)

• maybe reorder encountered shape list by screen space proximity to stroke starting point (might greatly slow down)

• Handle holes in shapes (detected Hole shapes may goes on a substractive layer on top OR two shapes side by side)

  • need to find best naming convention and handling method...

• find relative brush size and spacing according to region resolution / pixel density

• re-sample shape to get uniform contour points spacing along line (quite hard)

  • Can be use on single line but in that case must manage to keep point properties (and interpolate them when creating in-betweens)...

• re-enable coplanarity check:

  • Fix bug in coplanar check (sometimes not evaluated as coplanar even)
  • maybe divide in two function or condition to check without object matrix applyed.

• coplanar handling: what to do when multiple strokes have coplanar points but not coplanar between them ? need project from view (on centroid or on selected point) need self make coplanar with average normal and point position. might need an option to "straighten" a stroke, rotate it on centroid (not project) so it fit chosen object axis very optional: might be cool quantify "coplanarity" to allow coplanar-tolerance conditions. very optional: align view to selected stroke normal

Todo UI:

• Display thickness

• material selector (to specify what material is gonna be use for the paint)

  • material selector UI select a material that use the fill (if not selected) OR select a predefined brush (that will have a material locked) but there will be different fill. might be cool to create a "material selector" field in UI later. generate stroke (what to do with pressure ?...)

Done:

• 0.3.0 - 2020-09-24:

• feat: Dependencies fetch, try to auto-install modules

• fixes:

  • error if no material exists on active objects.
  • error when trying to evaluate en empty strokes
  • spacing of 1 minimum

• 0.2.0 - 2019-10-13:

  • Material selector : add specific material path to use (automatically added to material of the object if not listed)
    Allow to keep a dedicated material for fills (limitation, Show 'All' scene materials for now, not only grease pencil)

• 0.1.8 - 2019-10-12:

  • In additive mode : Add a shape even when nothing is instersected (boy that was frustrating !)
  • Add undo step on each stroke, can now safely ctrl+Z after leaving modal without losing all last strokes !
  • Changed panel from toolbar (T panel) to sidebar (N panel).

• More precision : Upscale the points coordinates and the empty numpy array before tracing the the circles in the numpy array, then trace and downscale.

• Omit final point in shape on gp stroke creation.

• add pressure control for tablets

• add/substract (erase) stroke - (shapely)

  • handle when shape is contained (and not intersected)
  • Fix shape when substracted on multiple shape simultaneously.
  • add angle warning

• stop without error if not enough element in mouse path list

• reproject stroke on axis plane (if axis locked) or with intersected stroke (first ?)

  • respect stroke placement

• show operation type (+, -, x) if shift, alt