Kiva

Kiva is a 2D vector drawing interface providing functionality similar to Quartz, Cairo, the Qt QPainter interface, the 2D drawing routines of OpenGL , the HTML5 Canvas element and many other similar 2D vector drawing APIs. Rather than re-implementing everything, Kiva is a Python interface layer that sits on top of many different back-ends which are in fact provided by some of these libraries, depending on the platform, GUI toolkit, and capabilities of the system.

This approach permits code to be written to the Kiva API, but produce output that could be rendered to a GUI window, an image file, a PDF file, or a number of other possible output formats without any (or at least minimal) changes to the image generation code.

Kiva is the base drawing layer of the Chaco plotting library, and is what is responsible for actually drawing pixels on the screen. Developers interested in writing code that renders new plots or other graphical features for Chaco will need to be at least passingly familiar with the Kiva drawing API.

The most important Kiva backend is the Agg or “Image” backend, which is a Python extension module which wraps the C++ Anti-grain geometry drawing library into a Python extension and exposes the Kiva API. The Agg renders the vector drawing commands into a raster image which can then be saved as a standard image format (such as PNG or JPEG) or copied into a GUI window. The Agg backend should be available on any platform, and should work even if there is no GUI or windowing system available.

Kiva Concepts

This section gives a whirlwind tour of the concepts involved with drawing with Kiva.

The Graphics Context

The heart of the Kiva drawing API is the “graphics context”, frequently abbreviated as gc in code. The graphics context holds the current drawing state (such as pen and fill colors, font state, and affine transformations to be applied to points) and provides methods for changing the state and performing drawing actions.

In many common use-cases (such as writing renderers for Chaco), you will be provided a graphics context by other code, but it is straight-forward to create your own graphics context:

from kiva.image import GraphicsContext

gc = GraphicsContext((400, 400))

This is an graphics context for the Agg or “image” backend which has a size of 400x400 pixels. If instead we wanted to draw into a Qt QPainter drawing context in a QWidget called my_qwidget we would use:

from kiva.qpainter import GraphicsContext

gc = GraphicsContext((400, 400), parent=my_qwidget)

Other Kiva backends have similar methods of creating a graphics context, and each may take somewhat different arguments to the constructor, depending on the requirements of the backend.

Once you have a graphics context, you can use it to draw vector graphics. For example, the following code will draw a translucent gray line from (100, 100) to (100, 200):

gc.move_to(100, 100)
gc.line_to(100, 200)
gc.set_stroke_color((0.5, 0.5, 0.5, 0.5))
gc.stroke_path()

For many of the backends, you can save the rendered image out as an image file using the save() method:

gc.save("my_line.png")

Kiva is numpy-aware, and has a number of methods that allow you to pass numpy arrays of points to draw many things in one operation, with loops being performed in C where possible:

from numpy import empty, linspace, random

# Nx2 array of points
pts = empty(shape=(20, 2), dtype=float)
pts[:, 0] = linspace(100, 200, 20)
pts[:, 1] = random.uniform(size=20)*100 + 100

gc.lines(pts)
gc.stroke_path()

Coordinate Model

Kiva uses mathematical axes direction conventions as opposed to framebuffer axes conventions. In other words, the origin is always at the bottom left of the screen, and the positive y axis goes up from bottom to top; as opposed to screen coordinates which typically have the origin at the top left and the positive y axis goes down from top to bottom.

Additionally, for backends that produce raster images, the coordinates represent the corner of pixels, rather than the center of pixels. This has consequences when rendering thin lines. Compare the two lines in this example, for instance:

from kiva.image import GraphicsContext

gc = GraphicsContext((200, 100))

gc.move_to(40, 35)
gc.line_to(160, 35)

gc.move_to(40, 65.5)
gc.line_to(160, 65.5)

gc.set_stroke_color((0.0, 0.0, 0.0))
gc.stroke_path()

gc.save("pixel_coordinates.png")

Notice that the line on the bottom (the first of the two lines) is fuzzier because it is drawn along the boundary of the pixels, while the other line is drawn through the center of the pixels:

../_images/pixel_coordinates.png

The Coordinate Transform Matrix

The Kiva API allows arbitrary affine transforms to be applied to the graphics context during drawing. The API provides convenience methods for common transformations, such as rotation and scaling:

from numpy import empty, linspace, random, pi
from kiva.image import GraphicsContext

# Nx2 array of points
pts = empty(shape=(20, 2), dtype=float)
pts[:, 0] = linspace(100, 200, 20)
pts[:, 1] = random.uniform(size=20)*100 + 100

gc = GraphicsContext((400, 400))

# draw a simple graph
gc.move_to(100, 200)
gc.line_to(100, 100)
gc.line_to(200, 100)
gc.set_stroke_color((0.5, 0.5, 0.5, 0.5))
gc.stroke_path()

gc.lines(pts)
gc.set_stroke_color((1.0, 0.0, 0.0, 0.5))
gc.stroke_path()

# translate by 100 pixels in the x direction
gc.translate_ctm(100, 0)

# rotate by 45 degrees
gc.rotate_ctm(pi/4.0)

# scale by 1.5 in the x direction
gc.scale_ctm(1.5, 1.0)

# now draw in the transformed coordinates
gc.move_to(100, 200)
gc.line_to(100, 100)
gc.line_to(200, 100)
gc.set_stroke_color((0.5, 0.5, 0.5, 0.5))
gc.stroke_path()

gc.lines(pts)
gc.set_stroke_color((0.0, 0.0, 1.0, 0.5))
gc.stroke_path()

gc.save('transformed_lines.png')
../_images/transformed_lines.png

If desired, the user can also supply their own transformations directly.

Paths

The basic drawing operations are performed by building a path out of primitive operations, and then performing stroking and/or filling operations with it.

The simplest path operations are move_to() and line_to() which respectively move the current point in the path to the specified point, and add a line to the path from the current point to the specified point.

In addition to the straight line commands, there are 4 arc commands for adding curves to a path: curve_to() which draws a cubic bezier curve, quad_curve_to() which draws a quadratic bezier curve, arc() which draws a circular arc based on a center and radius, and arc_to() which draws a circular arc from one point to another.

Finally, the rect() method adds a rectangle to the path.

In addition there are convenience methods lines(), rects() and line_set() which add multiple lines or rectangles to a path, reading from appropriately shaped numpy arrays.

None of these methods make any change to the visible image until the path is either stroked with stroke_path() or filled with fill_path(). The way these actions are performed depends upon certain state of the graphics context.

For stroking, the graphics context keeps track of the color to use with set_stroke_color(), the thickness of the line with set_line_width(), the way that lines are joined with set_line_join() and set_miter_limit(), and the way that they are ended with set_line_cap(). Lines can also be dashed using the set_line_dash() method which takes a pattern of numbers to use for lengths of on and off, and an optional phase for where to start in the pattern.

Thicknesses:

from kiva.image import GraphicsContext

gc = GraphicsContext((200, 100))

for i in range(5):
    y = 30.5 + i*10
    thickness = 2.0**(i-1)

    gc.move_to(40, y)
    gc.line_to(160, y)
    gc.set_line_width(thickness)
    gc.stroke_path()

gc.save('thicknesses.png')
../_images/thicknesses.png

Joins:

from kiva.api import JOIN_ROUND, JOIN_BEVEL, JOIN_MITER
from kiva.image import GraphicsContext

gc = GraphicsContext((200, 100))
gc.set_line_width(8)

for i, join in enumerate([JOIN_ROUND, JOIN_BEVEL, JOIN_MITER]):
    y = 20 + i*20

    gc.move_to(y, 80)
    gc.line_to(y, y)
    gc.line_to(160, y)
    gc.set_line_join(join)
    gc.stroke_path()

gc.save('joins.png')
../_images/joins.png

Caps:

from kiva.api import CAP_ROUND, CAP_BUTT, CAP_SQUARE
from kiva.image import GraphicsContext

gc = GraphicsContext((200, 100))
gc.set_line_width(8)

for i, cap in enumerate([CAP_ROUND, CAP_BUTT, CAP_SQUARE]):
    y = 30 + i*20

    gc.move_to(40, y)
    gc.line_to(160, y)
    gc.set_line_cap(cap)
    gc.stroke_path()

gc.save('caps.png')
../_images/caps.png

Dashes:

from kiva.image import GraphicsContext

gc = GraphicsContext((200, 100))
dashes = ([6.0, 6.0], [9.0, 3.0], [3.0, 5.0, 9.0, 5.0])
gc.set_line_width(2)

for i, dash in enumerate(dashes):
    y = 30.5 + i*20

    gc.move_to(40, y)
    gc.line_to(160, y)
    gc.set_line_dash(dash)
    gc.stroke_path()

gc.save('dashes.png')
../_images/dashes.png

Before filling a path, the colour of the fill is via the set_fill_color() method, and gradient fills can be done via the set_linear_gradient() and set_radial_gradient() methods. Finally, there are two different fill modes available: even-odd fill and non-zero winding fill

Winding vs. Even-Odd Fill:

from numpy import pi
from kiva.api import FILL, EOF_FILL
from kiva.image import GraphicsContext

gc = GraphicsContext((200, 100))
gc.set_fill_color((0.0, 0.0, 0.0))

gc.move_to(50, 90)
for i in range(1, 6):
    theta = 4*pi/5*i
    x = 50+40*sin(theta)
    y = 50+40*cos(theta)
    gc.line_to(x, y)

gc.fill_path()


gc.move_to(150, 90)
for i in range(1, 6):
    theta = 4*pi/5*i
    x = 150+40*sin(theta)
    y = 50+40*cos(theta)
    gc.line_to(x, y)

gc.eof_fill_path()

gc.save('fill.png')
../_images/fill.png

Text

Text can be rendered at a point by first setting the font to use, then setting the text location using set_text_position() and then show_text() to render the text:

from kiva.api import Font
from kiva.image import GraphicsContext

gc = GraphicsContext((200, 100))

gc.set_font(Font(size=24))
gc.set_text_position(30, 40)
gc.show_text("Hello World")

gc.save('text.png')
../_images/text.png

Text defaults to being rendered filled, but can be rendered with an outline.

Kiva Backends

GUI-capable

Each of these backends can be used to draw the contents of windows in a graphical user interface.

kiva.agg/image

This is a wrapper of the popular Anti-Grain Geometry C++ library. It is the current default backend.

cairo

A backend based on the Cairo graphics library.

celiagg

A newer wrapper of Anti-Grain Geometry which is maintained outside of kiva/enable.

gl

OpenGL drawing. This backend is quite limited compared to others.

qpainter

Qt QPainter drawing. This is only availble with the Qt toolkit.

quartz

macOS Quartz graphics (ie CGContext). This is only available on macOS.

File-only

Each of these backends can be used to create an output file.

pdf

A backend which writes PDF files.

ps

A backend which writes PostScript files.

svg

A backend which writes SVG files.