pyopengl-users

[PyOpenGL-Users] using glGetUniformLocation

[Ben De L. <bd...@gm...>]

I have this simple test program distilled from a more complex code
base, I am using glGetUniformLocation incorrectly but I am not sure
what the correct syntax is.

Any hits are appreciated!

from OpenGL import GL
from OpenGL import GLU
from OpenGL import GLUT
import sys
simpleShader = '''
#version 140
uniform int test;

void main()
{ ;
}
'''
def init():
   # select clearing color
   GL.glClearColor (0.0, 0.0, 0.0, 0.0)

   # initialize viewing values
   GL.glMatrixMode(GL.GL_PROJECTION)
   GL.glLoadIdentity()
   GL.glOrtho(0.0, 1.0, 0.0, 1.0, -1.0, 1.0)
   shader = GL.glCreateShader(GL.GL_VERTEX_SHADER)
   GL.glShaderSource(shader, simpleShader)
   GL.glCompileShader(shader)
   print GL.glGetShaderInfoLog(shader)

   program = GL.glCreateProgram()
   GL.glAttachShader(program, shader)
   GL.glLinkProgram(program)

   loc = GL.glGetUniformLocation(program, "test")
   print loc


def display():
    GL.glClear (GL.GL_COLOR_BUFFER_BIT)
    GL.glColor3f (1.0, 1.0, 1.0)
    GL.glBegin(GL.GL_POLYGON)
    GL.glVertex3f (0.25, 0.25, 0.0)
    GL.glVertex3f (0.75, 0.25, 0.0)
    GL.glVertex3f (0.75, 0.75, 0.0)
    GL.glVertex3f (0.25, 0.75, 0.0)
    GL.glEnd()
    GL.glFlush ();



GLUT.glutInit(sys.argv)
GLUT.glutInitDisplayMode(GLUT.GLUT_SINGLE | GLUT.GLUT_RGB)
GLUT.glutInitWindowSize(250, 250)
GLUT.glutInitWindowPosition(100, 100)
GLUT.glutCreateWindow("hello")
init()
GLUT.glutDisplayFunc(display)
GLUT.glutMainLoop()

Re: [PyOpenGL-Users] using glGetUniformLocation

[Ian M. <geo...@gm...>]

Firstly, you probably don't want to specify the version in the vertex shader
unless you're sure of your system.  I also don't know what happens if you
don't specify anything under the main function.  Try using: "void main {
gl_Position = ftransform(); }".  As to your problem, I presume the uniform
location is returning -1?  You need to use the shader before you can query
values from it.  I think it's glUseProgram()

Re: [PyOpenGL-Users] using glGetUniformLocation

[Gijs <in...@bs...>]

Apart from needing to call glUseProgram before you can ask its uniform 
location, your vertex or fragment shader also needs to use the variables 
in the shader itself. If you don't use them, the shader-compiler will 
optimize the program so that the variable doesn't exist (since it's not 
used anyway, it wouldn't really matter), thereby saving memory. The 
uniform location of a shader variable which doesn't exist or is not used 
is -1.

Not defining the version of the shader afaik doesn't really matter on 
Windows or Linux, except for special cases, however on Mac OS X you 
hardly can do without. Mac OS X is quite strict in things like that. You 
need to define the extensions you use as well, even though Windows and 
Linux both have no problem if you don't specify them. (I think it does 
give you a warning of some sorts though).

Regards, Gijs

PS: You don't have to put GLUT. or GL. in front of every call to glut or 
the GL. Since you imported them, you have direct access to the functions 
defined in glut and the GL.


On 8/26/09 20:39 , Ian Mallett wrote:
> Firstly, you probably don't want to specify the version in the vertex 
> shader unless you're sure of your system.  I also don't know what 
> happens if you don't specify anything under the main function.  Try 
> using: "void main { gl_Position = ftransform(); }".  As to your 
> problem, I presume the uniform location is returning -1?  You need to 
> use the shader before you can query values from it.  I think it's 
> glUseProgram()
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