OpenGL showing blank screen. Maybe due to shaders? - c++

Question

My OpenGL ES application isn't working. I'm using SDL for windowing management, and it holds the context. After looking around, I noticed that the vertex shader and the fragment shader showed up as 0 on the debugger. Even the program was 0. Could this be a reason? I followed my shader compiling and linking code to a template that was previously made.
If it is, what is wrong? Here is the code:
GLuint ShaderHelper::compileShader(GLenum type, std::string fileName) {
std::string fileContents;
std::ifstream fin;
std::string path;
// Getting the necessary path...
// These are abstractions for getting file contents
// from the main bundle.
if (type == GL_VERTEX_SHADER) {
FileOpener opener;
path = opener.retriveFileFromBundle(fileName, "vsh");
} else if (type == GL_FRAGMENT_SHADER) {
FileOpener opener;
path = opener.retriveFileFromBundle(fileName, "fsh");
} else {
std::cout << "ERROR: Invalid shader type at filename " << fileName << std::endl;
exit(1);
}
fin.open(path);
if (!fin.is_open()) {
std::cout << "ERROR: Failed to open file " << fileName << std::endl;
exit(1);
}
// Retrieving the string from the file...
while (!fin.eof()) {
char CLine[255];
fin.getline(CLine, 255);
std::string line = CLine;
fileContents = fileContents + line;
fileContents = fileContents + " \n";
}
fin.close();
// I'm creating these variables because a pointer is needed for
// glShaderSource
GLuint shaderHandle = glCreateShader(type);
const GLint shaderStringLength = (GLint)fileContents.size();
const GLchar *shaderCString = fileContents.c_str();
glShaderSource(shaderHandle, 1, &shaderCString, &shaderStringLength);
glCompileShader(shaderHandle);
return shaderHandle;
}
void ShaderHelper::linkProgram(std::vector<GLuint *> shaderArray) {
program = glCreateProgram();
for (int i = 0; i < shaderArray.size(); i++) {
glAttachShader(program, *shaderArray[i]);
}
glLinkProgram(program);
}
void ShaderHelper::addUniform(uniform_t uniform) {
std::string name = uniform.name;
uniforms[name] = uniform;
// With that step done, we need to assign the location...
uniforms[name].location = glGetUniformLocation(program, uniforms[name].name.c_str());
}
EDIT: After suggestions, I ran my code through glError(). I fixed an error, but I still got a blank screen. I'm no longer getting 0 as my shader values. I set glClearColor to a white image, and it's just appearing pure white. I adjusted numbers in the MV matrix and projection matrix, but there's still nothing at all. I disabled face culling, but still nothing. Also, shaders are compiling and linking fine. So now what?

Answer 1

The dreaded blank screen can be caused by a variety of problems:
Your context is not created correctly or you're not presenting your scene properly. Does changing your clear color to something else than black show anything?
Your transformation matrix is wrong. How are you setting up the position of the "camera"? Are you using something like GLM to set up a matrix? If so, have a look at glm::perspective() and glm:::lookAt(). Make sure you're passing the matrix to the shader and that you're using it to set gl_Position in your vertex shader.
The geometry you're trying to display is facing away from the viewer. Try glDisable(GL_CULL_FACE). If it works, reverse the order of your vertices.
An OpenGL call is failing somewhere. Make sure you check glGetError() after every call. I usually have something like the following:
struct gl_checker
{
~gl_checker()
{
const auto e = glGetError();
assert(e == GL_NO_ERROR);
}
};
template <class F>
inline auto gl_call(F f) -> decltype(f())
{
gl_checker gc;
return f();
}
which can be used like this:
gl_call([]{ glDisable(GL_CULL_FACE); });
Your shaders fail to compile or link. Have a look at glGetShaderiv() with GL_COMPILE_STATUS and glGetProgramiv() with GL_LINK_STATUS. If they report an error, have a look at glGetShaderInfoLog() and glGetProgramInfoLog().
As for the partial code you provided, I see nothing strictly wrong with it. Providing the shaders and a smaller, complete program might help with finding the problem.