Question

I am learning with opengl-tutorial.org, I’m using QT Too, When I move the mouse, the MVP matrix is updated, it works well. But the screen is not showing up the changes until I press the Alt Key. I don’t know why, is it an issue of QT or OpenGL?

InitGL Function:

programId = this->createPorgram(":/glsl/VertexShader.glsl",":/glsl/FragmentShader.glsl");
if(programId==-1){
    emit blewUpApplication();
    return;
}

glClearColor(0.0,0.0,0.0,1.0);
glEnable(GL_CULL_FACE);
glEnable(GL_MULTISAMPLE); // Enables multisampling for a better look
//glEnable(GL_BLEND); // Enable textures with alpha channel and alpha channels colors
glEnable(GL_TEXTURE_2D);
glEnable(GL_ALPHA_TEST);
glEnable(GL_DEPTH_TEST);
glDepthFunc(GL_LESS);
// Bla bla bla, the rest does not matters.

PaintGL Function:

glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); // Clears the screen and paint the bg with the color specified on glClearColor

float currT = float(tmr.elapsed() / 1000.0f);
if((currT-lastTime)>=1.0f){
    lastTime+=1.0f;
}
deltaTime = (float)(currT-lastTime);

glm::mat4 Project = this->getMatrixProjection();
glm::mat4 View = this->getMatrixView();
glm::mat4 Model = glm::mat4(1.0f);
glm::mat4 MVP = Project * View * Model;

glUseProgram(programId);

GLuint MatrixId = glGetUniformLocation(programId,"MVP");
glUniformMatrix4fv(MatrixId,1,GL_FALSE,glm::value_ptr(MVP));

glActiveTexture(cursor_texture);
glBindTexture(GL_TEXTURE_2D,cursor_texture);

glUniform1i(glGetUniformLocation(programId,"textura"),cursor_texture);
// Draw or triangles

glEnableVertexAttribArray(0);
glEnableVertexAttribArray(1);   

glBindBuffer(GL_ARRAY_BUFFER,trianglebuffer[0]);
glVertexAttribPointer(0,3,GL_FLOAT,GL_FALSE,0,(void*)0);


glBindBuffer(GL_ARRAY_BUFFER,trianglebuffer[1]);
glVertexAttribPointer(1,2,GL_FLOAT,GL_FALSE,0,(void*)0);


glDrawArrays(GL_TRIANGLES,0,12*3);

glDisableVertexAttribArray(0);
glDisableVertexAttribArray(1);

glUseProgram(0);

Mouse Move Function:

mousepos[0] = evt->x();
    mousepos[1] = evt->y();
    horzAn += mouseSpeed * deltaTime * float(this->width()/2 - mousepos[0]);
    vertAn += mouseSpeed * deltaTime * float(this->height()/2 - mousepos[1]);

    CmDirect = glm::vec3(cos(horzAn) * sin(vertAn), sin(vertAn), cos(horzAn) * cos(vertAn));
    glm::vec3 RgtVc = glm::vec3(sin(horzAn - 3.14f/2.0f),0,cos(horzAn - 3.14f/2.0f));
    UpVc = glm::cross(RgtVc,CmDirect);

    Proj = glm::perspective(Zoom,4.0f/3.0f,0.1f,100.0f);
    Vw = glm::lookAt(PosCm,PosCm+CmDirect,UpVc);
    this->cursor().setPos(suppose_x,suppose_y);

Shaders:

// Fragment
#version 330 core
out vec3 color;
in vec2 thcoord;
in vec2 UV;
uniform sampler2D d_textura;
void main(){
color = texture(d_textura,thcoord).rgb;
}
// Vertex
#version 330 core
layout(location = 0) in vec3 vertexPosition_modelspace;
layout(location = 1) in vec2 tcoord;
out vec2 thcoord;
uniform mat4 MVP;
void main(){
vec4 v = vec4(vertexPosition_modelspace,1);
thcoord=tcoord;
gl_Position = MVP * v;
}

I know that the matrix operation is working good because the effects change when I press the alt key (only one time happens).

Answer 1

You should add update request to the end of your mouseMoveEvent handler for Qt to know that widget needs repaint. update() or repaint() should do the trick.