Multiple textures in GLSL - only one works

Question

My problem is getting more than one texture accessible in a GLSL shader. Here's what I'm doing:

Shader:

uniform sampler2D sampler0;
uniform sampler2D sampler1;
uniform float blend;
void main( void )
{
    vec2 coords = gl_TexCoord[0];
    vec4 col = texture2D(sampler0, coords);
    vec4 col2 = texture2D(sampler1, coords);
    if (blend > 0.5){
        gl_FragColor = col;
    } else {
        gl_FragColor = col2;
    }
};

So, I simply choose between the two color values based on a uniform variable. Simple enough (this is a test), but instead of the expected behavior, I get all black when blend <= 0.5.

OpenGL code:

m_sampler0location = m_shader.FindUniform("sampler0");
m_sampler1location = m_shader.FindUniform("sampler1");
m_blendlocation = m_shader.FindUniform("blend");

glActiveTexture(GL_TEXTURE0);
glEnable(GL_TEXTURE_2D);
m_extensions.glUniform1iARB(m_sampler0location, 0);
glBindTexture(GL_TEXTURE_2D, Texture0.Handle);  
glActiveTexture(GL_TEXTURE1);
glEnable(GL_TEXTURE_2D);
m_extensions.glUniform1iARB(m_sampler1location, 1);
glBindTexture(GL_TEXTURE_2D, Texture1.Handle);
glBegin(GL_QUADS);
    //lower left
    glTexCoord2f(0, 0);
    glVertex2f(-1.0, -1.0);
    //upper left
    glTexCoord2f(0, maxCoords0.t);
    glVertex2f(-1.0, 1.0);
    //upper right
    glTexCoord2f(maxCoords0.s, maxCoords0.t);
    glVertex2f(1.0, 1.0);
    //lower right
    glTexCoord2f(maxCoords0.s, 0);
    glVertex2f(1.0, -1.0);
glEnd()

The shader is compiled and bound before all this. All the sanity checks in that process indicate that it goes ok. As I said, the value of col in the shader program reflects fragments from a texture; the value of col2 is black. The texture that is displayed is the last active texture - if I change the last glBindTexture to bind Texture0.Handle, the texture changes. Fixed according to Bahbar's reply.

As it is, the scene renders all black, even if I add something like gl_FragColor.r = blend; as the last line of the shader. But, if I comment out the call glActiveTexture(GL_TEXTURE1);, the shader works again, and the same texture appears in both sampler0 and sampler1.

What's going on? The line in question, glActiveTexture(GL_TEXTURE1);, seems to work just fine, as evidenced by a subsequent glGetIntegerv(GL_ACTIVE_TEXTURE, &anint). Why does it break everything so horribly? I've already tried upgrading my display drivers.

Answer 1

When compiling your shader to test, I found two errors:

1. coords should be assigned the st portion of the 4-component gl_TexCoord, e.g.

   vec2 coords = gl_TexCoord[0].st;
   

2. The shader should not end with a semicolon.

Are you checking anywhere in your main program that shader compiles correctly? You may want to look at GL_COMPILE_STATUS via glGetShader and glGetShaderInfoLog.

Answer 2

It sounds like your glActiveTexture call does not work. Are you sure you set up the function pointer correctly ?

Verify by calling glGetIntegerv(GL_ACTIVE_TEXTURE, &anint) after having called your glActiveTexture(GL_TEXTURE1).

Also glEnable(GL_TEXTURE_2D) are not useful. The shader itself specifies what texture units to use, and which target of each unit to "enable".

---

Edit to add:

Well, your new situation is peculiar. The fact that you can't even show red is weird, in particular (well, did you set alpha to 1 just to make sure ?).

That said, you should restore GL_TEXTURE0 as the glActiveTexture after you're done setting the texture unit 1 (i.e. after the glBindTexture call).

Answer 3

Just to help other people who might be interested in using multiple textures and will feel hopeless after days searching for an answer. I found that you need to call

glUseProgram(program);

GLuint t1Location = glGetUniformLocation(program, "tex1");
GLuint t2Location = glGetUniformLocation(program, "tex2");

glUniform1i(t1Location, 0);
glUniform1i(t2Location, 1);

In that order for it to work (glUseProgram is the last instruction for 99% of the sample code I've found online). Now it may be only the case for me and not affect anyone else on Earth, but just in case I thought I'd share.

Answer 4

Quite late reply, but for anybody encountering this - I encountered same problem and after short fiddling, I realized that calling glActiveTexture(GL_TEXTURE0) fixes the issue. It seems something down the line gets confused if active texture unit is not 'reset' to zero. This could be system-dependent behavior; I'm running 64bit Archlinux with Mesa 8.0.4.

Answer 5

Here's a basic GLUT example (written on OS X, adapt as needed) that generates two checkerboard textures, loads a shader with two samplers and combines them by tinting each (one red, one blue) and blending. See if this works for you:

#include <stdio.h>
#include <stdlib.h>

#include <GLUT/glut.h>
#include <OpenGL/gl.h>
#include <OpenGL/glu.h>

#define kTextureDim 64

GLuint t1;
GLuint t2;

/* adapted from the red book */
GLuint makeCheckTex() {
    GLubyte image[kTextureDim][kTextureDim][4]; // RGBA storage

    for (int i = 0; i < kTextureDim; i++) {
        for (int j = 0; j < kTextureDim; j++) {
            int c = ((((i & 0x8) == 0) ^ ((j & 0x8)) == 0))*255;
            image[i][j][0]  = (GLubyte)c;
            image[i][j][1]  = (GLubyte)c;
            image[i][j][2]  = (GLubyte)c;
            image[i][j][3]  = (GLubyte)255;
        }
    }

    GLuint texName;
    glGenTextures(1, &texName);    
    glBindTexture(GL_TEXTURE_2D, texName);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, kTextureDim, kTextureDim, 0, GL_RGBA, GL_UNSIGNED_BYTE, image);

    return texName;
}

void loadShader() {
#define STRINGIFY(A) #A

    const GLchar* source = STRINGIFY(

                                     uniform sampler2D tex1;
                                     uniform sampler2D tex2;

                                     void main() {
                                         vec4 s1 = texture2D(tex1, gl_TexCoord[0].st);
                                         vec4 s2 = texture2D(tex2, gl_TexCoord[0].st + vec2(0.0625, 0.0625));
                                         gl_FragColor = mix(vec4(1, s1.g, s1.b, 0.5), vec4(s2.r, s2.g, 1, 0.5), 0.5);
                                     }

                                     );

    GLuint program = glCreateProgram();
    GLuint shader = glCreateShader(GL_FRAGMENT_SHADER);
    glShaderSource(shader, 1, &source, NULL);
    glCompileShader(shader);

    GLint logLength;
    glGetShaderiv(shader, GL_INFO_LOG_LENGTH, &logLength);
    if (logLength > 0) {
        GLchar* log = (GLchar*)malloc(logLength);
        glGetShaderInfoLog(shader, logLength, &logLength, log);
        printf("Shader compile log:\n%s\n", log);
        free(log);
    }

    glAttachShader(program, shader);  
    glLinkProgram(program);

    glGetProgramiv(program, GL_INFO_LOG_LENGTH, &logLength);
    if (logLength > 0) {
        GLchar* log = (GLchar*)malloc(logLength);
        glGetProgramInfoLog(program, logLength, &logLength, log);
        printf("Program link log:\n%s\n", log);
        free(log);
    }

    GLuint t1Location = glGetUniformLocation(program, "tex1");
    GLuint t2Location = glGetUniformLocation(program, "tex2");

    glUniform1i(t1Location, 0);
    glUniform1i(t2Location, 1);

    glUseProgram(program);
}


void init()
{
    glClearColor(0.0, 0.0, 0.0, 0.0);
    glEnable(GL_DEPTH_TEST);
    glShadeModel(GL_FLAT);

    t1 = makeCheckTex();
    t2 = makeCheckTex();

    loadShader();
}


void display()
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glLoadIdentity();

    glActiveTexture(GL_TEXTURE0);
    glEnable(GL_TEXTURE_2D);
    glBindTexture(GL_TEXTURE_2D, t1);

    glActiveTexture(GL_TEXTURE1);
    glBindTexture(GL_TEXTURE_2D, t2);

    glBegin(GL_QUADS);
    //lower left
    glTexCoord2f(0, 0);
    glVertex2f(-1.0, -1.0);
    //upper left
    glTexCoord2f(0, 1.0);
    glVertex2f(-1.0, 1.0);
    //upper right
    glTexCoord2f(1.0, 1.0);
    glVertex2f(1.0, 1.0);
    //lower right
    glTexCoord2f(1.0, 0);
    glVertex2f(1.0, -1.0);
    glEnd();

    glutSwapBuffers();
}


void reshape(int w, int h)
{
    glViewport(0, 0, w, h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(-2, 2, -2, 2, -2, 2);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
}


int main(int argc, char **argv)
{
    glutInit(&argc, argv);

    glutInitDisplayMode(GLUT_DOUBLE | GLUT_DEPTH | GLUT_RGBA);

    glutInitWindowSize(512, 512);
    glutInitWindowPosition(0, 0);

    glutCreateWindow("GLSL Texture Blending");

    glutReshapeFunc(reshape);
    glutDisplayFunc(display);
    glutIdleFunc(display);

    init();

    glutMainLoop();
    return 0;
}

Hopefully the result will look something like this (you can comment out the glUseProgram call to see the first texture drawn without the shader):