Archive for the How To Category

Using ARRAY_BUFFER’s in PyOpenGL

Posted in How To, Programming, PyGLy with tags , , , , on 2013/02/16 by Adam Griffiths

I’m in the process of converting my code from Pyglet GL to PyOpenGL.
In doing so, my VBO objects stopped rendering.

It turns out the problem is glVertexAttribPointer.
The Pyglet GL version takes the last parameter (offset) as a number. I set this to 0 for arrays with no offset.

glVertexAttribPointer( in_position, 3, GL_FLOAT, GL_FALSE, 0, 0)

It seems that PyOpenGL expects the pointer value by absolute instead of relative.
The solution is to pass None instead of 0.

glVertexAttribPointer( in_position, 3, GL_FLOAT, GL_FALSE, 0, None)

If you have an actual offset to pass, you need to convert to a ctypes c_void_p (void*).

glVertexAttribPointer( in_position, 3, GL_FLOAT, GL_FALSE, 0, ctypes.c_void_p(offset))

Rotating a vector by a quaternion in GLSL

Posted in How To, Programming with tags , , , , on 2013/02/11 by Adam Griffiths

I’ve found numerous code samples which produce erroneous results in my shaders.

The functions I’ve found to work are the following:

vec4 multQuat(vec4 q1, vec4 q2)
return vec4(
q1.w * q2.x + q1.x * q2.w + q1.z * q2.y - q1.y * q2.z,
q1.w * q2.y + q1.y * q2.w + q1.x * q2.z - q1.z * q2.x,
q1.w * q2.z + q1.z * q2.w + q1.y * q2.x - q1.x * q2.y,
q1.w * q2.w - q1.x * q2.x - q1.y * q2.y - q1.z * q2.z

vec3 rotate_vector( vec4 quat, vec3 vec )
vec4 qv = multQuat( quat, vec4(vec, 0.0) );
return multQuat( qv, vec4(-quat.x, -quat.y, -quat.z, quat.w) ).xyz;

Source from here:

And this version which is optimised:

vec3 rotate_vector( vec4 quat, vec3 vec )
return vec + 2.0 * cross( cross( vec, ) + quat.w * vec, );

Using gl_VertexID without any VBOs.

Posted in How To, Programming with tags , , , on 2013/02/03 by Adam Griffiths

Normally, gl_VertexID is the index of the currently rendered vertex element.

When you provide indices (GL_ELEMENT_ARRAY_BUFFER), gl_VertexID should match the value provided. Ie, the gl_VertexID value can repeat if you render the same element multiple times.


I say “should”, because this is NOT correct when you haven’t provided any VBO data.

This means, that if you are rendering using nothing but a shader and a GL_ELEMENT_ARRAY_BUFFER, the gl_VertexID will NOT match the indices you provide, and will instead be a sequence from 0 -> N.

The solution is to use a single VBO and use glDrawArrays.


Personally, I consider this a bug.

Adobe Air hanging when using SDK but not when packaged

Posted in How To with tags , , , , , on 2013/01/30 by Adam Griffiths

I just had an issue during the development of an Adobe AIR Application.
When using Flash Pro CS 6, the application hang in certain situations for 5 odd seconds.

I found a few peculiar quirks:

  • Packaged builds didn’t have the problem.
  • Moving the project to a different directory fixed the problem.
  • Clean checkouts of the source didn’t have the problem
  • Re-creating the project in Flash Pro CS 6 didn’t fix the problem.

The console was printing the following messages

Breakpoint not set; No executable code at line ###

It turns out the problem was old breakpoints that don’t exist anymore.
There’s not trivial way to remove all the breakpoints (the button was greyed out!).
The easiest way I found (on OS-X) was to do the following:

vi ~/Library/Application\ Support/Adobe/Flash\ CS6/en_US/Configuration/Debugger/AsBreakpoints.xml

Remove all the breakpoint entries in the XML.
The file should look similar to this:

<?xml version="1.0"?>
<flash_breakpoints version="1.0">

This resolved the issue.

Getting Cocos 1.x / Kobold2D to work with the latest CocosBuilder

Posted in Development, How To, Programming, Rant with tags , , , , , on 2012/03/30 by Adam Griffiths

CocosBuilder is a brilliant tool that helps you rapidly develop Cocos2D applications.

But the latest versions require the Cocos 2.x branch.

Some of us are stuck with Cocos 1.x for the time being. So let’s figure out how to get things going.

Continue reading

Adventures in Kivy

Posted in How To, Programming, Rant with tags , , , , , , on 2012/03/22 by Adam Griffiths

So here is a semi-live blog as I delve into Kivy.

Mission #1: Attempt to resize the default window

Ok let’s start at the start.

We first create an object that inherits from App. Done.

Ok, and the examples all return a widget from the App’s build() method. So we don’t over-ride __init__… ok… thats… unique.

Perhaps the widget is the window if it is returned from the build() method?

Widget is not the window. Widget returned from build() is added to the window. So… where is the window created?

Window docs state the window has a constructor that lets you set the window size. Ok, cool. But we don’t create the window… so.. we can’t use the constructor… sigh.

Ok, Window has a static accessor.

kivy.core.window.Window = None

No comment on what is stored here but I assume it’s the instantiated window. It doesn’t mention when it is set… handy!

Only 1 window allowed. Ok… weird. Well… let’s try that.

import kivy.core.window.Window
AttributeError: 'module' object has no attribute 'window'


Let’s scour the code.

Screen capture of Kivy window module

No file……… how far does this rabbit hole go.

Ok, let’s check out the file in the window module.

class WindowBase(EventDispatcher):
 '''WindowBase is a abstract window widget, for any window implementation.

So… window base is in there. Feel free to scream now.

So we can get this by importing kivy.core.window and just leaving the class off the end.

import kivy.core.window
def build(self):
    print kivy.core.window.Window.size
    return None
(800, 600)

Ok sweet, thats our window.

So let’s change its size.

def build(self):
    print kivy.core.window.Window.size
    kivy.core.window.Window.size = (1024, 768)
    print kivy.core.window.Window.size
    return None
File "/Users/adamgriffiths/Workspace/VirtualEnvs/progress_quest/lib/python2.7/site-packages/kivy/core/window/", line 243, in _set_size
 if super(WindowBase, self)._set_size(size):
 AttributeError: 'super' object has no attribute '_set_size'


Ok, so after some googling we find this post about the relationship between the App and Window classes.

His solution is to pass the size as a command line parameter……….. LAME. That kinda throws the whole configuration settings thing out the window.

Looking through the Kivy code we find this block inside their base window class’ __init__ method.

if 'fullscreen' not in kwargs:
    fullscreen = Config.get('graphics', 'fullscreen')
        if fullscreen not in ('auto', 'fake'):
        fullscreen = fullscreen.lower() in ('true', '1', 'yes', 'yup')
    kwargs['fullscreen'] = fullscreen
if 'width' not in kwargs:
    kwargs['width'] = Config.getint('graphics', 'width')
if 'height' not in kwargs:
    kwargs['height'] = Config.getint('graphics', 'height')

Ok, so we can try and set it via the global config object.

So let’s add that to our App class.

def build_config(self, config):
    config.setdefault('graphics', 'width', 1024)
    config.setdefault('graphics', 'height', 768)
    config.set('graphics', 'height', 1024)
    config.set('graphics', 'width', 768)

Run the code and……

(800, 600)

……. sigh

So let’s put some debug INSIDE kivy and see what is happening. We’ll also force it to pull the ‘graphics’ setting and ignore kwargs.

if True: #'width' not in kwargs:
    kwargs['width'] = Config.getint('graphics', 'width')
print kwargs['width']
if True: #'height' not in kwargs:
    kwargs['height'] = Config.getint('graphics', 'height')
print kwargs['height']

And the output…


…. calm blue ocean…. calm blue ocean….

Ok… google…

Hmm.. ok, this post on the kivy-users forum (because documentation is for lusers) shows a similar method.

They use the Config module directly instead of the config object passed to App….. because….. they’re not the same? Well.. let’s try it anyway.

from kivy.config import Config
Config.set('graphics', 'width', 1024)
Config.set('graphics', 'height', 768)

And our output

(800, 600)

Ok, weird because the window HAS ACTUALLY CHANGED SIZE.

Whatever. We’re all living in crazy-ville atm so let’s just pretend everything is normal.

So let’s move that inside our build_config() method.

def build_config(self, config):
    Config.set('graphics', 'height', 1024)
    Config.set('graphics', 'width', 768)

And the output

[INFO ] Kivy v1.1.1
[INFO ] [Logger ] Record log in /Users/adamgriffiths/.kivy/logs/kivy_12-03-22_40.txt
[INFO ] [Factory ] 102 symbols loaded
[INFO ] [Text ] using <pygame> as text provider
[INFO ] [Loader ] using <pygame> as thread loader
[INFO ] [Window ] using <pygame> as window provider
[WARNING] [Window ] Unable to use <pygame> as windowprovider
[CRITICAL] [Window ] Unable to find any valuable Window provider at all!
Fatal Python error: (pygame parachute) Segmentation Fault
Abort trap: 6

(╯°□°)╯︵ ┻━┻

Install GCC with XCode 4.3

Posted in How To with tags , , , , on 2012/03/21 by Adam Griffiths

XCode 4.3 removed GCC from the default installation. To get it back simply do the following:

  1. Run XCode
  2. Select XCode -> Preferences -> Downloads -> Command Line Tools -> Install

Voila! GCC is now installed and libraries such as PIL will once again install.

%d bloggers like this: