Blendend (blendend v0.3.0)

blendend is an Elixir wrapper around the blend2d 2D graphics engine.

Core concepts

Canvas

The main drawing surface in blendend is a canvas (Blendend.Canvas resource).

A canvas owns:

• an internal pixel buffer (32-bit RGBA, with alpha)
• a blend2d context that stores the current graphics state: stroke width, current style, transform matrix, composition mode, etc.

Everything we draw goes through a canvas. Most blendend modules take a canvas as their first argument and either fill or stroke geometry onto it.

There is no retained scene or display list. If we want a new frame, clear the canvas and draw everything again.

We can snapshot a canvas as:

• raw PNG / QOI data (Blendend.Canvas.to_png/1, to_qoi/1)
• files via helpers like Blendend.Canvas.save/2

Geometry and paths

blendend exposes both simple shapes and complex paths:

• simple primitives are available via:

  • Blendend.Canvas.Fill (rectangles, circles, ellipses, boxes, pies)
  • Blendend.Canvas.Stroke (lines, rectangles, circles etc. but stroked instead of filled)

• complex shapes live in Blendend.Path:

  • commands such as move_to/3, line_to/3, quad_to/5, cubic_to/7, arc_quadrant_to/5, add_circle/4
  • utilities to inspect and deform paths: vertex_count/1, vertex_at/2, set_vertex_at/5, hit_test/3,4, equal?/2, fit_to/2, debug helpers

A path is a sequence of lines and curves. We can fill or stroke paths just like simple shapes.

Styles, colors, gradients, patterns

Whenever we fill or stroke a geometry, we apply a style blendend has:

• solid colors (Blendend.Style.Color)
• gradients (Blendend.Style.Gradient)
• patterns based on images (Blendend.Style.Pattern)

Styles can carry alpha. Alpha means how strongly this layer should show up when blended with the existing canvas. When we draw, pixels are blended with whatever is already in the canvas using a chosen composition operator (e.g. :src_over, :multiply, :screen ...).

Stroke rendering can be tuned with:

• caps (:butt, :round, :square, ...)
• joins (:miter, :round, :bevel, ...)
• stroke_width, stroke_color

The low–level style parsing lives in the NIF layer; at the Elixir level we pass keyword lists like:

Canvas.Fill.rect(canvas, 10, 10, 100, 40,
  color: Color.rgb!(240, 240, 255),
  comp_op: :multiply
)