Question

I would like to be able to combine two textures in a GLSL fragment shader. I am currently using PyOpenGL and everything i’ve done up to this point using shaders has worked fine.

I am running into difficulty when I try to access multiple textures from the fragment shader, for instance, the following shader displays the correct texture minus the blue pixels:

uniform sampler2D my_texture1;
uniform sampler2D my_texture2;
void main()
{
    vec4 color1 = texture2D(my_texture1, gl_TexCoord[1].st);
    vec4 color2 = texture2D(my_texture2, gl_TexCoord[2].st);
    if (color1.b > 0.8)
        discard;
    gl_FragColor = color1;
}

but

uniform sampler2D my_texture1;
uniform sampler2D my_texture2;
void main()
{
    vec4 color1 = texture2D(my_texture1, gl_TexCoord[1].st);
    vec4 color2 = texture2D(my_texture2, gl_TexCoord[2].st);
    if (color2.b > 0.8)
        discard;
    gl_FragColor = color2;
}

results in a blank screen.

I have a feeling that the problem might lie in how i am passing the texture uniforms to the shader but cant for the life of me work out why the first texture works but the second doesn’t. Below is the full program.

from OpenGL.GL import *
from OpenGL.GLU import *
from OpenGL.GLUT import *
from Image import *
from OpenGL.GL.shaders import *

ESCAPE = '\033'
global size
size = 512

def drawQuad(B,T,L,R):
    glBegin(GL_QUADS)
    glMultiTexCoord2f(GL_TEXTURE1, 0.0, 0.0); glMultiTexCoord2f(GL_TEXTURE2, 0.0, 0.0); glVertex3f(B, L,  1.0);       ## Bottom Left Of The Texture and Quad
    glMultiTexCoord2f(GL_TEXTURE1, 1.0, 0.0); glMultiTexCoord2f(GL_TEXTURE2, 1.0, 0.0); glVertex3f( T, L,  1.0);       ## Bottom Right Of The Texture and Quad
    glMultiTexCoord2f(GL_TEXTURE1, 1.0, 1.0); glMultiTexCoord2f(GL_TEXTURE2, 1.0, 1.0); glVertex3f( T,  R,  1.0);       ## Top Right Of The Texture and Quad
    glMultiTexCoord2f(GL_TEXTURE1, 0.0, 1.0); glMultiTexCoord2f(GL_TEXTURE2, 0.0, 1.0); glVertex3f(B,  R,  1.0);       ## Top Left Of The Texture and Quad
    glEnd()

def InitGL(Width, Height):
    print "Vendor:   " + glGetString(GL_VENDOR)
    print "Renderer: " + glGetString(GL_RENDERER)
    print "OpenGL Version:  " + glGetString(GL_VERSION)
    print "Shader Version:  " + glGetString(GL_SHADING_LANGUAGE_VERSION)

    if not glUseProgram:
        print 'Missing Shader Objects!'
        sys.exit(1)

    global program
    program = compileProgram(
        compileShader('''
                void main()
                {
                    gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;
                    gl_TexCoord[1] = gl_MultiTexCoord1;
                    gl_TexCoord[2] = gl_MultiTexCoord2;
                }
        ''',GL_VERTEX_SHADER),
        compileShader('''
                uniform sampler2D my_texture1;
                uniform sampler2D my_texture2;
                void main()
                {
                    vec4 color1 = texture2D(my_texture1, gl_TexCoord[1].st);
                    vec4 color2 = texture2D(my_texture2, gl_TexCoord[2].st);
                    if (color1.b > 0.8)
                        discard;
                    gl_FragColor = color1;
                }
    ''',GL_FRAGMENT_SHADER),
    )

    #bmp texture 1
    image = open("rgb.bmp")
    ix = image.size[0]
    iy = image.size[1]
    image = image.tostring("raw", "RGBX", 0, -1)
    glActiveTexture(GL_TEXTURE1)
    global my_texture1
    my_texture1 = glGenTextures(1)
    glBindTexture(GL_TEXTURE_2D, my_texture1) 
    glPixelStorei(GL_UNPACK_ALIGNMENT,1)
    glTexImage2D(GL_TEXTURE_2D, 0, 3, ix, iy, 0, GL_RGBA, GL_UNSIGNED_BYTE, image)
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST)
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST)
    glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_DECAL)
    glGenerateMipmap(GL_TEXTURE_2D)

    #bmp texture 2
    image = open("rgb2.bmp")
    ix = image.size[0]
    iy = image.size[1]
    image = image.tostring("raw", "RGBX", 0, -1)
    glActiveTexture(GL_TEXTURE2)
    global my_texture2
    my_texture2 = glGenTextures(1)
    glBindTexture(GL_TEXTURE_2D, my_texture2) 
    glPixelStorei(GL_UNPACK_ALIGNMENT,1)
    glTexImage2D(GL_TEXTURE_2D, 0, 3, ix, iy, 0, GL_RGBA, GL_UNSIGNED_BYTE, image)
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST)
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST)
    glTexEnvf(GL_TEXTURE_ENV, GL_TEXTURE_ENV_MODE, GL_DECAL)
    glGenerateMipmap(GL_TEXTURE_2D)

def DrawGLScene():
    global frame, testvar, my_texture1,my_texture2
    glEnable(GL_TEXTURE_2D)
    glBindTexture(GL_TEXTURE_2D, 0)
    glUseProgram(program)
    myUniformLocation1 = glGetUniformLocation(program, "my_texture1")
    glUniform1i(myUniformLocation1, 1)
    myUniformLocation2 = glGetUniformLocation(program, "my_texture2")
    glUniform1i(myUniformLocation2, 2)
    glViewport(0, 0, size,size)
    glClearDepth(1.0)
    glClearColor (0.0, 0.0, 0.0, 1.0)
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT)
    glMatrixMode(GL_PROJECTION)
    glLoadIdentity()  
    glOrtho(-1, 1, -1, 1, -30.0, 30.0)
    glMatrixMode(GL_MODELVIEW)
    glLoadIdentity() 
    glEnable(GL_DEPTH_TEST)
    drawQuad(-1.0,1.0,-1.0,1.0)
    glutSwapBuffers()

def keyPressed(*args):
    global texturenumber, shadernumber, frame
    # If escape is pressed, kill everything.
    if args[0] == ESCAPE:
        sys.exit()

def main():
    global window
    glutInit(sys.argv)
    glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE | GLUT_DEPTH)
    glutInitWindowSize(size,size)
    glutInitWindowPosition(0, 0)
    window = glutCreateWindow("Multitexturing")
    glutDisplayFunc(DrawGLScene)
    glutIdleFunc(DrawGLScene)
    glutKeyboardFunc(keyPressed)
    InitGL(size,size)
    glutMainLoop()

if __name__ == "__main__":
    print "Press 'ESC' key to quit."
    main()

Answer 1

You actually unbind the second texture before using it:

def DrawGLScene():
    global frame, testvar, my_texture1,my_texture2
    glEnable(GL_TEXTURE_2D)
    glBindTexture(GL_TEXTURE_2D, 0)
    [...]

After your initialization, texture unit 1 has my_texture1 assigned and texture unit 2 my_texture2. The active texture is still texture 2. So by calling glBindTexture(GL_TEXTURE_2D, 0) you unbind the texture from the active texture unit 2.

What you shoud do is this:

def DrawGLScene():
    global frame, testvar, my_texture1,my_texture2
    glActiveTexture(GL_TEXTURE1)
    glBindTexture(GL_TEXTURE_2D, my_texture1) 
    glActiveTexture(GL_TEXTURE2)
    glBindTexture(GL_TEXTURE_2D, my_texture2)
    [...]

You could also simply remove the last four lines (after global frame, testvar, my_texture1,my_texture2) since your init routine takes care of this. However, if you bind and unbind any other textures in your code you have to do the texture unit activation and texture binding as above.