Ink Custom Brush Builder

Custom brushes in the Android Ink API are built around the BrushFamily hierarchy:

BrushFamily
├── InputModel (BrushFamily.InputModel.SlidingWindowModel or PASSTHROUGH_MODEL)
└── BrushCoat[] (one or more rendering layers)
    ├── BrushTip (shape: scaleX, scaleY, cornerRounding, rotationDegrees, slantDegrees, pinch)
    │   └── BrushBehavior[] (dynamic response to stylus or touch input)
    │       └── Tree: TargetNode(input = [Operators](input = SourceNode))
    └── BrushPaint (visual appearance)
        ├── SelfOverlap (ANY, ACCUMULATE, or DISCARD)
        ├── TextureLayer[] (BrushPaint.TilingTexture or BrushPaint.StampingTexture)
        └── BrushPaint.ColorFunction[] (OpacityMultiplier or ReplaceColor)

Create a Brush by combining a custom BrushFamily with a Compose Color, size, and epsilon:

val brush = Brush.createWithComposeColor(
    family = myCustomFamily,
    color = Color.Black,
    size = 10f,
    epsilon = 0.1f,
)

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Version and experimental API requirements :

• Requires androidx.ink 1.1.0-alpha03 and higher : In 1.0.0 (Stable) and 1.1.0-alpha01-alpha02, BrushFamily(...), BrushCoat, BrushTip, BrushPaint, BrushBehavior, and androidx.ink.brush.behavior.* nodes were @RestrictTo(LIBRARY_GROUP) (in 1.0.0 stable, custom brushes can only be loaded from binary proto streams using BrushFamily.decode). Starting in 1.1.0-alpha03 and higher , programmatic custom brush constructors graduated to public API (BrushPaint.TilingTexture, BrushPaint.StampingTexture, BrushPaint.ColorFunction, BrushFamily.InputModel.SlidingWindowModel, and all behavior nodes).

• No @OptIn required in 1.1.0-alpha03 and higher : All public custom brush classes and ink-storage BrushFamily serialization APIs (BrushFamily.encode and decode, AndroidBrushFamilySerialization, BrushFamilyDecodeCallback) are unannotated in 1.1.0-alpha03 and higher (ExperimentalInkCustomBrushApi is @RestrictTo(LIBRARY_GROUP) in 1.1.0-alpha05 and higher, and is only required on BrushFamily serialization in 1.0.0 stable).

Workflow and task routing

Follow these conditional instructions based on the requested custom brush task:

• Step 1 (Dependencies) : When setting up an Android project for programmatic custom brushes, ask user permission before editing build files and verify that androidx.ink:ink-brush, androidx.ink:ink-brush-compose, androidx.ink:ink-storage, and androidx.ink:ink-nativeloader use version 1.1.0-alpha03 or higher in gradle/libs.versions.toml and app/build.gradle.kts.

• Step 2 (Hierarchy, tip shape, paint, and input smoothing) : When defining BrushFamily and multi-coat BrushCoat layers, configuring BrushTip geometry (scaleX, scaleY, cornerRounding, rotationDegrees, slantDegrees, pinch), setting BrushPaint properties (SelfOverlap, ColorFunction), or smoothing stroke input with BrushFamily.InputModel.SlidingWindowModel, you MUST follow brush-structure.md. For baseline Brush creation or 1.0.0 resource loading, review Brush APIs.

• Step 3 (Dynamic stylus and touch behaviors) : When making a brush respond dynamically to stylus pressure, tilt, speed, direction, or procedural noise using BrushBehavior node trees (SourceNode, DampingNode, NoiseNode, ResponseNode, TargetNode, PolarTargetNode), you MUST follow brush-behaviors.md.

• Step 4 (Textures and serialization) : When adding BrushPaint.TilingTexture or BrushPaint.StampingTexture layers, implementing TextureBitmapStore, or encoding and decoding custom BrushFamily definitions (BrushFamily.encode and decode, AndroidBrushFamilySerialization, BrushFamilyDecodeCallback), you MUST follow textures-and-serialization.md. Inform the user which texture image files (such as app/src/main/res/drawable-nodpi/pencil_grain.png) to add to their project and register in TextureBitmapStore.

• Step 5 (Complete brush recipes) : When building a complete pressure pen, calligraphy nib, shading pencil, watercolor wash, or multi-coat shadow brush from scratch, start from the copy-ready implementations in examples.md.

• Compose drawing surface, eraser, or persistence : For Compose drawing surface setup, InProgressStrokes wet and dry rendering, geometry-based erasing, or Room stroke persistence, suggest the ink-app-builder skill.

Antipatterns

• NEVER use raw ink.proto.* builders to construct brushes---always use the public Kotlin constructors (BrushFamily, BrushCoat, BrushTip, BrushPaint, BrushBehavior from androidx.ink.brush.*).

• NEVER manually scale stroke width in a touch listener for pressure or tilt sensitivity---use BrushBehavior mapping SourceNode.Source.NORMALIZED_PRESSURE or TILT_IN_RADIANS to TargetNode.Target.SIZE_MULTIPLIER or WIDTH_MULTIPLIER.

• NEVER use manual Canvas bitmap stamping or BitmapShader for textured brushes---use BrushPaint.TilingTexture or BrushPaint.StampingTexture with a TextureBitmapStore.

• NEVER use BlendMode.SRC_OVER on the final (or only) TextureLayer ---use BlendMode.MODULATE, DST_IN, DST_OUT, SRC_ATOP, or SRC_IN so edge anti-aliasing and OPACITY_MULTIPLIER behaviors aren't overridden.

• NEVER serialize BrushFamily as JSON, Gson, Moshi, or Parcelable ---always use BrushFamily.encode and BrushFamily.decode or AndroidBrushFamilySerialization from androidx.ink.storage.

• NEVER implement manual Bézier (quadTo, cubicTo) smoothing ---configure BrushFamily.InputModel.SlidingWindowModel for input smoothing and DampingNode for behavior smoothing.

• NEVER use Random.nextFloat in draw loops for organic stroke wobble---attach a NoiseNode varied over stroke distance in BrushBehavior.