Move model in OpenGL by translate matrix instead of glTranslate() - c++

Question

My goal is moving object by arrow keys pressing. For this purpose in callback I create translate matrix from some offset and multiply it to world matrix. However it doesn't work - cube doesn't move by pressing keys. I also noticed using glTranslate() directly with setted offset works good but looks like crutch. I mean, I should use only translation matrices for any transformation of model.
Where is the problem in my code? How to fix it? Why glTranslate() works good?
Minimal code example:
glm::mat4 mWorldMatrix;
glm::mat4 mViewMatrix;
glm::mat4 mProjMatrix;
void onKeyCallback(GLFWwindow*, int key, int scan, int action, int mods)
{
switch (key)
{
case GLFW_KEY_UP:
{
auto translationMatrix = glm::translate(glm::mat4{}, glm::vec3{ 0, 1, 0 });
mWorldMatrix = mWorldMatrix * translationMatrix;
break;
}
case GLFW_KEY_DOWN:
{
auto translationMatrix = glm::translate(glm::mat4{}, glm::vec3{ 0, -1, 0 });
mWorldMatrix = mWorldMatrix * translationMatrix;
break;
}
case GLFW_KEY_LEFT:
{
auto translationMatrix = glm::translate(glm::mat4{}, glm::vec3{ -1, 0, 0 });
mWorldMatrix = mWorldMatrix * translationMatrix;
break;
}
case GLFW_KEY_RIGHT:
{
auto translationMatrix = glm::translate(glm::mat4{}, glm::vec3{ 1, 0, 0 });
mWorldMatrix = mWorldMatrix * translationMatrix;
break;
}
}
}
int main()
{
glfwInit();
const int weight = 640;
const int height = 480;
auto mWindow = glfwCreateWindow(weight, height, "TesT", nullptr, nullptr);
glfwMakeContextCurrent(mWindow);
glShadeModel(GL_SMOOTH);
glEnable(GL_DEPTH_TEST);
glDepthFunc(GL_LEQUAL);
mWorldMatrix = glm::mat4{ 1.0f };
mViewMatrix = glm::lookAt(glm::vec3{ 0, 0, -1 },
glm::vec3{ 0, 0, 0 },
glm::vec3{ 0, 1, 0 });
mProjMatrix = glm::perspective(glm::radians(45.0f),
static_cast<float>(weight) / height,
0.1f,
100.0f);
glfwSetKeyCallback(mWindow, onKeyCallback);
while (!glfwWindowShouldClose(mWindow)) {
glClearColor(0.5, 0.5, 0.5, 0);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glViewport(0,0, weight, height);
auto modelViewMatrix = mViewMatrix * mWorldMatrix;
glMatrixMode(GL_MODELVIEW);
glLoadMatrixf(static_cast<const float*>(glm::value_ptr(modelViewMatrix)));
glMatrixMode(GL_PROJECTION_MATRIX);
glLoadMatrixf(static_cast<const float*>(glm::value_ptr(mProjMatrix)));
Cube cube{ glm::vec3{0.5, 0.5, 0.5}, 1 }; //cube with center in {0.5} and side length 1
auto vertices = cube.soup(); //vector of vertex
glTranslatef(0 /* + offset.x*/, 0/* + offset.y*/, -5); //Setting offset here is work good
glBegin(GL_TRIANGLES);
for (const auto& vertex : vertices)
{
glColor3f(vertex.position.x, vertex.position.y, vertex.position.z);
glVertex3f(vertex.position.x, vertex.position.y, vertex.position.z);
}
glEnd();
glfwSwapBuffers(mWindow);
glfwWaitEvents();
}
glfwTerminate();
return 0;
}

Answer 1

Note, that drawing by glBegin/glEnd sequences and the fixed function matrix stack and fixed function. See Fixed Function Pipeline and Legacy OpenGL.
Read about Vertex Specification and Shader for a state of the art way of rendering.
The projection matrix should be applied to the projection matrix stack and the model view matrix to the model view matrix stack.
There are 2 issues.
GL_PROJECTION_MATRIX is not a valid enum constant for glMatrixMode and will cause a GL_INVALID_ENUM error. The valid enum constant for the projection matrix mode is GL_PROJECTION. GL_PROJECTION_MATRIX would be used for reading the current projection matrix by glGetFloatv.
If you want to apply additional transformation to the model, then you have to choose the GL_MODELVIEW matrix before. If the GL_PROJECTION matrix is "selected", this state is kept until it is changed again explicitly.
glMatrixMode(GL_MODELVIEW);
glLoadMatrixf(static_cast<const float*>(glm::value_ptr(modelViewMatrix)));
glMatrixMode(GL_PROJECTION); // <----- `GL_PROJECTION` instead of `GL_PROJECTION_MATRIX`
glLoadMatrixf(static_cast<const float*>(glm::value_ptr(mProjMatrix)));
// [...]
glMatrixMode(GL_MODELVIEW); // <-------- specify `GL_MODELVIEW`
glTranslatef(0 /* + offset.x*/, 0/* + offset.y*/, -5); //Setting offset here is work good

Answer 2

Because in every loop you set the cube to the original position. You should define these objects outside the loop. Then you can mutate them in the while loop.
In openGL lessons at the university I had very much use of a youtube series from "The Cherno". He made openGL course.