GLSL Texture Function Returning Only Black For Sky Box

Question

I'm attempting to generate a sky box for my OpenGL scene by using GLSL to texture quads. However, the sky box simply becomes black when I attempt to use the texture to generate the sky box color. The sky box works when I manually set the color, so I have basically narrowed the problem down to something being wrong with how I'm setting up the texture. I also messed around with a bunch of different eyeDirection vector values for the texture function, and I still get only a black square.

Here is my Fragment Shader:

#version 450 compatibility

layout(binding=0) uniform samplerCube currTexture;

smooth in vec3 eyeDirection;

out vec4 fragmentColor;

void main() {
    fragmentColor = texture(currTexture, eyeDirection);
    //fragmentColor = vec4(eyeDirection, 1.0);
}

Here is my vertex shader:

#version 450

uniform mat4 projection;
uniform mat4 modelView;

in vec4 aPosition;

smooth out vec3 eyeDirection;

void main() {
    mat3 inverseModelView = inverse(mat3(modelView));
    vec3 unprojected = (inverse(projection) * aPosition).xyz;
    eyeDirection = inverseModelView * unprojected;
    // eyeDirection = aPosition.xyz;

    //gl_Position = aPosition.xyww;
    gl_Position = new vec4(aPosition.x, aPosition.y, 1.0, aPosition.w );
} 

Here is where I initialize the texture:

// initializes all the necessary texture values
void TrainView::initTextures() {
    // loading in texture maps
    SDL_Surface* xPos = IMG_Load("SkyBoxXpos.png");
    SDL_Surface* xNeg = IMG_Load("SkyBoxXneg.png");
    SDL_Surface* yPos = IMG_Load("SkyBoxYpos.png");
    SDL_Surface* yNeg = IMG_Load("SkyBoxYneg.png");
    SDL_Surface* zPos = IMG_Load("SkyBoxZpos.png");
    SDL_Surface* zNeg = IMG_Load("SkyBoxZneg.png");

    if (!xPos) 
        printf("IMG_Load: %s\n", IMG_GetError());
    if (!xNeg) 
        printf("IMG_Load: %s\n", IMG_GetError());
    if (!yPos) 
        printf("IMG_Load: %s\n", IMG_GetError());
    if (!yNeg) 
        printf("IMG_Load: %s\n", IMG_GetError());
    if (!zPos) 
        printf("IMG_Load: %s\n", IMG_GetError());
    if (!zNeg) 
        printf("IMG_Load: %s\n", IMG_GetError());
        // handle error

    glGenTextures(1, &skyBoxTexture);
    glActiveTexture(GL_TEXTURE0);
    glEnable(GL_TEXTURE_CUBE_MAP);
    glBindTexture(GL_TEXTURE_CUBE_MAP, skyBoxTexture);

    glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);
    glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
    glTexParameteri(GL_TEXTURE_CUBE_MAP, GL_TEXTURE_WRAP_R, GL_CLAMP_TO_EDGE);

    // files are 24-bit bmp files
    glTexImage2D(GL_TEXTURE_CUBE_MAP_POSITIVE_X, 0, GL_RGBA, xPos->w, xPos->h, 0, GL_RGBA, GL_UNSIGNED_BYTE, xPos->pixels);
    glTexImage2D(GL_TEXTURE_CUBE_MAP_NEGATIVE_X, 0, GL_RGBA, xNeg->w, xNeg->h, 0, GL_RGBA, GL_UNSIGNED_BYTE, xNeg->pixels);
    glTexImage2D(GL_TEXTURE_CUBE_MAP_POSITIVE_Y, 0, GL_RGBA, yPos->w, yPos->h, 0, GL_RGBA, GL_UNSIGNED_BYTE, yPos->pixels);
    glTexImage2D(GL_TEXTURE_CUBE_MAP_NEGATIVE_Y, 0, GL_RGBA, yNeg->w, yNeg->h, 0, GL_RGBA, GL_UNSIGNED_BYTE, yNeg->pixels);
    glTexImage2D(GL_TEXTURE_CUBE_MAP_POSITIVE_Z, 0, GL_RGBA, zPos->w, zPos->h, 0, GL_RGBA, GL_UNSIGNED_BYTE, zPos->pixels);
    glTexImage2D(GL_TEXTURE_CUBE_MAP_NEGATIVE_Z, 0, GL_RGBA, zNeg->w, zNeg->h, 0, GL_RGBA, GL_UNSIGNED_BYTE, zNeg->pixels);

    glBindTexture(GL_TEXTURE_CUBE_MAP, NULL);

}

Here is where I attempt to draw/render the sky box:

// handles everything for drawing the sky box
void TrainView::drawSkyBox(){

    glUseProgram(skyBoxShader);
    glActiveTexture(GL_TEXTURE0);

    glBindTexture(GL_TEXTURE_CUBE_MAP, skyBoxTexture);

    int loc = glGetUniformLocation(skyBoxShader, "modelView");

    GLfloat mvFl[16], projFl[16];

    glGetFloatv(GL_MODELVIEW_MATRIX, mvFl);

    glUniformMatrix4fv(loc, 1, GL_FALSE, mvFl);

    loc = glGetUniformLocation(skyBoxShader, "projection");

    glGetFloatv(GL_PROJECTION_MATRIX, projFl);
    glUniformMatrix4fv(loc, 1, GL_FALSE, projFl);

    loc = glGetUniformLocation(skyBoxTexture, "currTexture");
    glUniform1i(loc, 0);

    // not sure if this is necessary or done as intended by opengl
    GLuint sampler;
    glGenSamplers(1, &sampler);
    glBindSampler(0, sampler);

    glDisable(GL_DEPTH_TEST);
    glDisable(GL_LIGHTING);

    glBegin(GL_QUADS);
        glVertex3f(-1.0, -1.0, 0.0);
        glVertex3f(1.0, -1.0, 0.0);
        glVertex3f(1.0, 1.0, 0.0);
        glVertex3f(-1.0, 1.0, 0.0);
    glEnd();

    glUseProgram(NULL);
    glEnable(GL_DEPTH_TEST);
    glEnable(GL_LIGHTING);
}

Answer 1

You are enabling mipmapped sampling for your cube texture here:

glTexParameteri(GL_TEXTURE_CUBE_MAP,
                GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR);

However, you are not creating mipmaps for your texture. In spec language, this means that your texture is not "mipmap complete", which in turn makes it not "texture complete". The result of sampling an incomplete texture is BLACK.

The easiest way to create mipmaps is by calling glGenerateMipmap() after your texture data has been specified, i.e. after all the glTexImage2D() calls for the 6 sides:

glGenerateMipmap(GL_TEXTURE_CUBE_MAP);

If you don't need mipmaps, you can simply set the value of the filter parameter to not use mipmaps:

glTexParameteri(GL_TEXTURE_CUBE_MAP,
                GL_TEXTURE_MIN_FILTER, GL_LINEAR);

What you attempted to do with creating a sampler object is not going to help here. Sampler objects are useful if you want to sample the same texture with multiple different sampling attributes. Say you wanted to sample the same texture with both GL_LINEAR and GL_NEAREST in the same shader, you could do that by creating two sampler objects for this texture. I don't think it's a very common use case, but there are situations where it comes in handy, and it was impossible to do before sampler objects were introduced.