# Julia, Pyglet, and a GPU

A Julia fractal. Note the smooth, anti-aliased rendering. The color is picked as function of the estimated function to the Julia boundary.

Pyglet is a very good Python module, wrapping OpenGL and providing access to mouse and keyboards events. The API is really easy to learn, well-documented, with that "clean & lean" feel to it. I started to use Pyglet for work, and I positively love it.

So, Pyglet, allows you to summon the power of your graphic card using Python. And those days, even a puny low-end graphic card packs a lot of power : GPU, massively parallel computing. Although it's been around for years now, I never bothered tinkering with GPU, lack of time, other interests. Now that I feel comfortable with OpenGL, Python and Pyglet, however, harnessing this GPU goodness require very little effort. With years of delay, let's jump in that band-wagon already !

For a first try, I went for the done, done again, and overdone : fractals. More precisely, Julia fractals. You can see this fractal as function to iterate for each pixel of a picture, that takes as input the coordinate of the pixel.

$Z_n$ is a complex variable, with $Z_0$ being the coordinate of the pixel in the complex plane, and $C$ a complex constant. This function is iterated until $|Z_N|<2$ or a maximum count of iterations is reached. You then color the pixel depending on the number of iterations spent. Consider visiting the Wikipedia page about Julia fractals, there is a lot to learn about it there.

And you get a nice picture, a fractal. What happen if you play with the parameter $C$ ? The Julia fractal is fairly intensive to compute, rendering it in real-time requires some significant engineering... or a GPU, as the computations for each pixel are completely parallel. So what I did is to render the Julia fractal as a fragment shader, and control the $C$ parameter with the mouse. The code for this turns out to be dead simple, assuming you are familiar with the OpenGL API


import pyglet
from pyglet.gl import *

class JuliaWindow(pyglet.window.Window):
def __init__(self):
super(JuliaWindow, self).__init__(caption = 'julia', width = 512, height = 512)
self.C = (-0.70176, -0.3842)
)

def on_mouse_motion(self, x, y, dx, dy):
self.C = (6. * ((float(x) / window.width) - .5), 6 * ((float(y) / window.height) - .5))

def on_draw(self):
glMatrixMode(GL_PROJECTION)
glOrtho(-1., 1., 1., -1., 0., 1.)

glMatrixMode(GL_MODELVIEW)

glVertex2i(-1, -1)
glTexCoord2i(-2, -2)
glVertex2f(1, -1)
glTexCoord2i(2, -2)
glVertex2i(1, 1)
glTexCoord2i(2, 2)
glVertex2i(-1, 1)
glTexCoord2i(-2, 2)
glEnd()

window = JuliaWindow()
pyglet.app.run()



This code create a window, and display a quad polygon that fills the window. The texture for the quad is a procedural one, computed by a shader. Here, I use a neat utility class that encapsulate OpenGL API for the shaders : shader.py, by Tristam MacDonald. The $C$ parameter of the fractal  is controlled by the mouse position in the window.

Of course, most of the magic happens inside the shaders. The vertex shader,   julia.v.glsl, is not exactly rocket science.


#version 110

varying vec2 uv;

void
main() {
gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
uv = vec2(gl_MultiTexCoord0);
}



All the actual job is done in the fragment shader, julia.f.glsl. Basically, I count how much iterations are required to have $|Z_N|<2$, and use this to pick a color. $C$ can be controlled from outside the shader, as uniforn variable. For sake of simplicity, I hard-coded the color scheme. One nice improvement would be to pick the color from a 1D texture, itself procedurally generated.


#version 110

varying vec2 uv;

int max_iter_count;
uniform vec2 C;

int iter_count;
vec2 Z, dZ;
vec2 sqr_Z;
float sqr_norm_Z, sqr_norm_dZ;

void
main() {
max_iter_count = 1024;

Z = uv;
dZ = vec2(1., 0.);

for(iter_count = 0; iter_count < max_iter_count; ++iter_count) {
sqr_Z = vec2(Z.x * Z.x, Z.y * Z.y);
if (sqr_Z.x + sqr_Z.y > 1e7)
break;

dZ = vec2(
2. * (Z.x * dZ.x - Z.y * dZ.y) + 1.,
2. * (Z.x * dZ.y + Z.y * dZ.x));
Z = vec2(
Z.x * Z.x - Z.y * Z.y,
2. * Z.x * Z.y) + C;
}

sqr_norm_Z = Z.x * Z.x + Z.y * Z.y;
sqr_norm_dZ = dZ.x * dZ.x + dZ.y * dZ.y;
vec4 color0 = vec4(1.0, 0.0, 0.0, 1.0);
vec4 color1 = vec4(1.0, 1.0, 0.0, 1.0);

gl_FragColor = mix(
color0,
color1,
sqrt(sqrt(sqr_norm_Z / sqr_norm_dZ) * .5 * log(sqr_norm_Z))
);
}



Note that here I use a variant on the classical Julia : I render a distance to the boundary of the fractal, the trick being from the amazing Inigo Quilez. It gives me anti-aliasing at a much cheaper cost than super-sampling. A few minutes of coding, and I got that instant enlightenment, with a feeling of achievement on the top of it. Those GPU are indeed great little pieces of silicon !

# Multiple pages PDF documents with Matplotlib

Matplotlib, what is that ? It is a software package to make plots, yet another one... but a really good one. Since Matplotlib is a Python module, plots are described in Python, rather than a (usually clumsy) custom language. So a script using Matplotlib can harness the full power of Python and its nice modules like Numpy. Say, reading compressed data file, doing some fancy statistics, etc. Moreover, Matplotlib is rather complete, providing a wide diversity of plots and render targets such as PNG, PDF, EPS. Finally, the quality of the plots is up to the standard of scientific publication. Originally a Gnuplot user, it's been a couple of years with Matplotlib and me, and I am very happy with it. Perfect ? Well, documentation for Matplotlib is a little bit scattered. I'm working on a tutorial, a Sisyphus-grade task... Rather than waiting for completing that looooong tutorial, a short recipe today !

# The problem

The problem : you want to have several plots on a single figure, you want to fit them on a A4 format, and you want to have this as a PDF document. The latter constraint, PDF document, is welcome, as the recipe is for PDF output only. It works for Matplotlib 1.1.x, and it mostly works for Matplotlib 1.0.x. It's a two step trick.

# First step: multiple pages & PDF

Only one Matplotlib back-end seems to support multiple pages, the PDF back-end. We have to explicitly instantiate that back-end and tells it that we want to go multiple pages. Here's how it's done

from matplotlib.backends.backend_pdf import PdfPages

pdf_pages = PdfPages('my-fancy-document.pdf')


Now, for each new page we want to create, we have to create a new Figure instance. It's simple, one page, one figure. We want a A4 format, so we can eventually print our nice document, for instance. That's not too hard, we have just to specify the proper dimensions and resolutions when creating a Figure. Once we are done with a page, we have to explicitly tell it to the back-end. In this example, the script creates 3 pages.

from matplotlib import pyplot as plot
from matplotlib.backends.backend_pdf import PdfPages

# The PDF document
pdf_pages = PdfPages('my-fancy-document.pdf')

for i in xrange(3):
# Create a figure instance (ie. a new page)
fig = plot.figure(figsize=(8.27, 11.69), dpi=100)

# Plot whatever you wish to plot

# Done with the page
pdf_pages.savefig(fig)

# Write the PDF document to the disk
pdf_pages.close()


# Second step: multiple plots on a figure

One A4 page is a lot of space, we can squeeze a lot of plots in one single figure. Recent Matplotlib versions (1.x.x and later) have now a nice feature to easily layout plots on one figure : grid layout.

plot.subplot2grid((5, 2), (2, 0), rowspan=1, colspan=1)


The first parameter of subplot2grid is the size of the grid. The second parameter is the coordinate of one element of the grid. That element will be filled with a plot. Finally, we can specify the span of the plot, so that its spans over several elements of the grid. This gives a great deal of control over the layout, without too much headaches. There is a small gotcha however: the coordinates are in row, col order. Here's an example of subplot2grid in action.

import numpy
from matplotlib import pyplot as plot

# Prepare the data
t = numpy.linspace(-numpy.pi, numpy.pi, 1024)
s = numpy.random.randn(2, 256)

#
# Do the plot
#
grid_size = (5, 2)

# Plot 1
plot.subplot2grid(grid_size, (0, 0), rowspan=2, colspan=2)
plot.plot(t, numpy.sinc(t), c= '#000000')

# Plot 2
plot.subplot2grid(grid_size, (2, 0), rowspan=3, colspan=1)
plot.scatter(s[0], s[1], c= '#000000')

# Plot 2
plot.subplot2grid(grid_size, (2, 1), rowspan=3, colspan=1)
plot.plot(numpy.sin(2 * t), numpy.cos(0.5 * t), c= '#000000')

# Automagically fits things together
plot.tight_layout()

# Done !
plot.show()


Grid layout in action

Note the tight_layout call, which pack all the plots within a figure automatically, and does a nice job at it. But it is in Matplotlib 1.1.x only... With Matplotlib 1.0.x I don't know yet how to do it. Why bothering ? Because say, at work, you might have to deal with Linux distributions are providing rather outdated versions of software in their official packages repositories, and even the unofficial ones are lagging. Yes, CentOS, I'm looking at you !

# Putting it together

Now, we can combine subplot2grid with the multiple page trick, and get multiple pages PDF documents which look good.

import numpy

from matplotlib import pyplot as plot
from matplotlib.backends.backend_pdf import PdfPages

# Generate the data
data = numpy.random.randn(7, 1024)

# The PDF document
pdf_pages = PdfPages('histograms.pdf')

# Generate the pages
nb_plots = data.shape[0]
nb_plots_per_page = 5
nb_pages = int(numpy.ceil(nb_plots / float(nb_plots_per_page)))
grid_size = (nb_plots_per_page, 1)

for i, samples in enumerate(data):
# Create a figure instance (ie. a new page) if needed
if i % nb_plots_per_page == 0:
fig = plot.figure(figsize=(8.27, 11.69), dpi=100)

# Plot stuffs !
plot.subplot2grid(grid_size, (i % nb_plots_per_page, 0))
plot.hist(samples, 32, normed=1, facecolor='#808080', alpha=0.75)

# Close the page if needed
if (i + 1) % nb_plots_per_page == 0 or (i + 1) == nb_plots:
plot.tight_layout()
pdf_pages.savefig(fig)

# Write the PDF document to the disk
pdf_pages.close()


A preview of the PDF document you should get from the example. Do not mind the ugly antialiasing, it's me who fumbled with the PDF to PNG conversion.

As you can see, a little bit of arithmetic is used to have the proper number of pages and create them at the right moment. It's a minimal example, but by now, you got the recipe.