Understanding the importance of the QPainter in QOpenGLWidget's paintEvent function - c++

In overpaint example of qt in paint event if I remove QPaint calls nothing is being drawed.
void GLWidget::paintEvent(QPaintEvent *event)
{
makeCurrent();
glMatrixMode(GL_MODELVIEW);
glPushMatrix();
qglClearColor(qtPurple.dark());
glShadeModel(GL_SMOOTH);
glEnable(GL_DEPTH_TEST);
glEnable(GL_CULL_FACE);
glEnable(GL_LIGHTING);
glEnable(GL_LIGHT0);
glEnable(GL_MULTISAMPLE);
static GLfloat lightPosition[4] = { 0.5, 5.0, 7.0, 1.0 };
glLightfv(GL_LIGHT0, GL_POSITION, lightPosition);
setupViewport(width(), height());
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glLoadIdentity();
glTranslatef(0.0, 0.0, -10.0);
glRotatef(xRot / 16.0, 1.0, 0.0, 0.0);
glRotatef(yRot / 16.0, 0.0, 1.0, 0.0);
glRotatef(zRot / 16.0, 0.0, 0.0, 1.0);
logo->draw();
glShadeModel(GL_FLAT);
glDisable(GL_CULL_FACE);
glDisable(GL_DEPTH_TEST);
glDisable(GL_LIGHTING);
glMatrixMode(GL_MODELVIEW);
glPopMatrix();
//QPainter painter(this); ---> I remove this and nothing
//painter.end(); will be displayed.
}
But I am curious because inside logo->draw(); there are QpenGL calls like "glDrawElements" which I believe shouldn't depend on QPainter.
What I want to understand how QPainter has an effect on this function.
And why it does not being used when we override paintGL which can be seen in this example

Related

How to draw a rhombus with freeglut?

I am trying to draw something like this image of the target output using OpenGL. I am using freeglut. I could draw three polygons, but I don't know how to give the three polygons right positioning. Below I attached the target output and my correct output.
The target output
My output
#include <GL/glut.h>
void init(void)
{
glClearColor(1.0, 1.0, 1.0, 0.0);
glMatrixMode(GL_PROJECTION);
gluOrtho2D(-5.0, 5.0, -5.0, 5.0);
glMatrixMode(GL_MODELVIEW);
}
void drawSquare(void)
{
glBegin(GL_POLYGON);
glVertex2f(0.0f, -1.0f);
glVertex2f(2.0f, 0.0f);
glVertex2f(0.0f, 1.0f);
glVertex2f(-2.0f, 0.0f);
glEnd();
}
void myDraw1(void)
{
glClear(GL_COLOR_BUFFER_BIT);
glLoadIdentity();
glColor3f(1.0, 0.0, 0.0);
drawSquare();
glTranslatef(2.0, 3.0, 0.0);
glRotatef(30, 0.0, 0.0, 1.0);
glColor3f(0.0, 1.0, 0.0);
drawSquare();
glLoadIdentity();
glTranslatef(-2.0, -3.0, 0.0);
glRotatef(-30, 0.0, 0.0, 1.0);
glColor3f(0.0, 0.0, 1.0);
drawSquare();
glFlush();
}
void myDraw2(void)
{
glClear(GL_COLOR_BUFFER_BIT);
glLoadIdentity();
glColor3f(1.0, 0.0, 0.0);
drawSquare();
glPushMatrix();
glTranslatef(2.0, 3.0, 0.0);
glRotatef(30, 0.0, 0.0, 1.0);
glColor3f(0.0, 1.0, 0.0);
drawSquare();
glPopMatrix();
glTranslatef(-2.0, -3.0, 0.0);
glRotatef(-30, 0.0, 0.0, 1.0);
glColor3f(0.0, 0.0, 1.0);
drawSquare();
glFlush();
}
void main(int argc, char** argv)
{
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
glutInitWindowPosition(0, 0);
glutInitWindowSize(600, 600);
glutCreateWindow("Rotate");
init();
glutDisplayFunc(myDraw1);
glutMainLoop();
}
The angle between the polygons is 120°. The center of the polygons is (0, 0). You have to rotate around the point (-2, 0). Translate the polygon in that way that that (-2, 0) is moved to (0, 0) and rotate the polygon:
glRotatef(120.0, 0.0, 0.0, 1.0);
glTranslatef(2.0, 0.0, 0.0);
Note, operations like glTranslatef and glRotatef, specify a matrix and multiply the current matrix by the new specified matrix. Use glPushMatrix/glPopMatrix to save and restore the matrix (push on and pop from matrix stack).
e.g:
void myDraw1(void)
{
glClear(GL_COLOR_BUFFER_BIT);
glLoadIdentity();
glColor3f(1.0, 0.0, 0.0);
glPushMatrix();
glRotatef(90.0, 0.0, 0.0, 1.0);
glTranslatef(2.0, 0.0, 0.0);
drawSquare();
glPopMatrix();
glColor3f(0.0, 1.0, 0.0);
glPushMatrix();
glRotatef(210.0, 0.0, 0.0, 1.0);
glTranslatef(2.0, 0.0, 0.0);
drawSquare();
glPopMatrix();
glColor3f(0.0, 0.0, 1.0);
glPushMatrix();
glRotatef(330.0, 0.0, 0.0, 1.0);
glTranslatef(2.0, 0.0, 0.0);
drawSquare();
glPopMatrix();
glFlush();
}

How to make qt designer custom QOpenGLWidget widget background be transparent?

I have create a custom qt designer widget which inherit from QOpenGLWidget. Everything is fine except I cannot get the widget background transparent.
Is there a way to fix this issue?
void MyOpenGl::initializeGL()
{
initializeOpenGLFunctions();
glClearColor(0,0,0,0);
glEnable(GL_DEPTH_TEST);
glEnable(GL_LIGHT0);
glEnable(GL_LIGHTING);
glColorMaterial(GL_FRONT_AND_BACK, GL_AMBIENT_AND_DIFFUSE);
glEnable(GL_COLOR_MATERIAL);
}
void MyOpenGl::resizeGL(int w, int h)
{
glViewport(0,0,w,h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
//gluPerspective(45, (float)w/h, 0.01, 100.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
//gluLookAt(0,0,5,0,0,0,0,1,0);
}
void MyOpenGl::paintGL()
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glBegin(GL_TRIANGLES);
glColor3f(1.0, 0.0, 0.0);
glVertex3f(-0.5, -0.5, 0);
glColor3f(0.0, 1.0, 0.0);
glVertex3f( 0.5, -0.5, 0);
glColor3f(0.0, 0.0, 1.0);
glVertex3f( 0.0, 0.5, 0);
glEnd();
}
Do not call glClear() in your MyOpenGL::paintGL() function with GL_COLOR_BUFFER_BIT, that will make to rewrite the entire color buffer to the color set by glClearColor(). You can even omit it completely.

How to use spot light in opengl

I am trying to learn how to use a spot light in OpenGL. I wish to shine a spot (torch) light from a point marked (lighpos), and shown as grey dot, onto the side of a GLUT teapot centered at (possph) in 3D space. I have attached an example where I have tried to do this but I cannot get the light to focus on the teapot. I am expecting a almost touch light focus based on the parameters I have tried to use.
Could someone point out what I have missed / mistake I have made.
Thanks
Stuart
#include <windows.h>
#include <GL/glut.h>
#include <stdio.h>
GLfloat lighpos[] = { -300., 200., 250., 1.0 }; // Location of light
GLfloat possph[] = { -50., 350., 150. }; // Position of teapot
GLfloat ligdir[] = { 250., 150., -100. }; // Direction from light to teapot
void init(void) {
glClearColor(0.0, 0.0, 0.0, 0.0);
glShadeModel(GL_SMOOTH);
glEnable(GL_COLOR_MATERIAL);
glEnable(GL_DEPTH_TEST);
glLightfv(GL_LIGHT0, GL_POSITION, lighpos);
glLightfv(GL_LIGHT0, GL_SPOT_DIRECTION, ligdir);
glLightf(GL_LIGHT0, GL_SPOT_CUTOFF, 10.0);
GLfloat ambientLight0[] = { 0.25, 0.25, 0.25, 1.0 };
glLightfv(GL_LIGHT0, GL_AMBIENT, ambientLight0);
glLightModelfv(GL_LIGHT_MODEL_AMBIENT, ambientLight0);
GLfloat diffuseLight0[] = { 1.0, 1.0, 1.0, 1.0 };
glLightfv(GL_LIGHT0, GL_DIFFUSE, diffuseLight0);
GLfloat specularLight0[] = { 1.0, 1.0, 1.0, 1.0 };
glLightfv(GL_LIGHT0, GL_SPECULAR, specularLight0);
glLightf(GL_LIGHT0, GL_SPOT_EXPONENT, 128.0f);
glEnable(GL_LIGHTING);
glEnable(GL_LIGHT0);
}
void drawAxis() {
glBegin(GL_LINES);
glColor3f(1.0, 0.0, 0.0); glVertex3f(-500., 0.0, 0.0); glVertex3f(500., 0.0, 0.0);
glColor3f(0.0, 1.0, 0.0); glVertex3f(0.0, -500., 0.0); glVertex3f(0.0, 500., 0.0);
glColor3f(0.0, 0.0, 1.0); glVertex3f(0.0, 0.0, -500.); glVertex3f(0.0, 0.0, 500.);
glEnd();
}
void display(void) {
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glColorMaterial(GL_FRONT_AND_BACK, GL_AMBIENT_AND_DIFFUSE);
drawAxis();
glPushMatrix();
glPointSize(10);
glColor3f(1.0, 1.0, 1.0);
glBegin(GL_POINTS);
glVertex3d(lighpos[0], lighpos[1], lighpos[2]);
glEnd();
glPopMatrix();
glColor3f(1., 0., 0.);
glPushMatrix();
glTranslated(possph[0], possph[1], possph[2]);
glutSolidTeapot(100.);
glPopMatrix();
glutSwapBuffers();
}
void reshape(int w, int h) {
glViewport(0, 0, (GLsizei)w, (GLsizei)h);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glFrustum(-1.0, 1.0, -1.0, 1.0, 4, 3000.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(1500.0, 1500.0, 1500.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
}
int main(int argc, char** argv) {
glutInit(&argc, argv);
glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB | GLUT_DEPTH);
glutInitWindowPosition(0.0, 0.0);
glutInitWindowSize(800, 800);
glutCreateWindow("spotlight");
init();
glutDisplayFunc(display);
glutReshapeFunc(reshape);
glutMainLoop();
return 0;
}
Fixed function pipeline illumination, which is what you are using in your code, performs lighting calculations only at the vertices. If your spotlight illuminates only a few or a single vertex you'll not get a pleasing spotlight effect. One possible solution would be to highly refine your models' meshes. The better, much more efficient and elegant solution is to drop using the fixed function pipeline and implement a illumination fragment shader, so that lighting is calculated for each pixel.

linux openGL depth_test not working

I tried a lot of methods on the Internet but it still not working.
now i think about if the trouble is related to my OS (ubuntu).
void init(void){
glClearColor(1.0, 1.0, 1.0, 0.0);
glShadeModel(GL_SMOOTH);
glEnable(GL_DEPTH_TEST);
glClearDepth(1.0);
glDepthFunc(GL_LESS);
glFrontFace(GL_CCW);
glCullFace(GL_BACK);
glEnable(GL_CULL_FACE);
init2();
}
void reshape(int w, int h){
int t = min (w,h);
glViewport (0, 0, t, t);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrtho(-100.0, 100.0, -100.0, 100.0, 1.1, 200.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
gluLookAt(40.0, 40.0, 100.0, 0.0, 0.0, 0.0, 0.0, 1.0, 0.0);
}
Two functions above are from my code..
glClear(GL_COLOR_BUFFER_BIT|GL_DEPTH_BUFFER_BIT) must be called at the beginning of every frame to clear both the color buffer and the depth buffer.
Other than that, you should post more details. What are you seeing? How are you requesting the OpenGL context? Could you post the whole draw loop?

OpenGL Picking selects on wrong places

I'm trying to recognize a drawn object on a mousPressEvent in OpenGL in Qt with picking.
I did some research but wasn't able to find the problem.
Clearly it recognizes something (because the return value of glRenderMode(GL_RENDER) is often an integer > 0), but not necessarily when I click on an object.
I think gluPerspective is the problem right here, but i just don't know how to resolve it.
mousePressEvent:
void WorldView::mousePressEvent(QMouseEvent *e)
{
GLuint buff[256];
GLint hits;
GLint view[4];
//Buffer to store selection data
glSelectBuffer(256, buff);
//Viewport information
glGetIntegerv(GL_VIEWPORT, view);
//Switch to select mode
glRenderMode(GL_SELECT);
//Clear the name stack!
glInitNames();
//Restric viewing volume
glMatrixMode(GL_PROJECTION);
glPushMatrix();
glLoadIdentity();
//Restrict draw area
gluPickMatrix(e->x(), e->y(), 1.0, 1.0, view);
gluPerspective(40.0f, (GLfloat)view[2]/(GLfloat)view[3], 1.0, 100.0);
//Draw the objects onto the screen
glMatrixMode(GL_MODELVIEW);
//Draw only the names in the stack
paintGL();
//Back into projection mode to push the matrix
glMatrixMode(GL_PROJECTION);
glPopMatrix();
hits = glRenderMode(GL_RENDER);//number of recognized objects
printf("\n%d\n",hits);
//Back to modelview mode
glMatrixMode(GL_MODELVIEW);
}
Draw function:
void WorldView::paintGL ()
{
this->dayOfYear = (this->dayOfYear+1);
this->hourOfDay = (this->hourOfDay+1) % 24;
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glLoadIdentity();
// store current matrix
glMatrixMode( GL_MODELVIEW );
glPushMatrix( );
gluLookAt(camPosx ,camPosy ,camPosz,
camViewx,camViewy,camViewz,
camUpx, camUpy, camUpz );
//Draw Axes
glDisable( GL_LIGHTING );
glBegin(GL_LINES);
glColor3f(1.0, 0.0, 0.0);
glVertex3f(0.0, 0.0, 0.0);
glVertex3f(10.0, 0.0, 0.0);
glColor3f(0.0, 1.0, 0.0);
glVertex3f(0.0, 0.0, 0.0);
glVertex3f(0.0, 10.0, 0.0);
glColor3f(0.0, 0.0, 1.0);
glVertex3f(0.0, 0.0, 0.0);
glVertex3f(0.0, 0.0, 10.0);
glEnd();
//Draw objects we want to pick
glPushName(0);
glBegin(GL_TRIANGLES);
glVertex3d(1,1,1);
glVertex3d(2,3,2);
glVertex3d(5,2,2);
glEnd();
glPopName();
glPushName(1);
glBegin(GL_TRIANGLES);
glVertex3d(7,-5,1);
glVertex3d(10,3,2);
glVertex3d(10,2,2);
glEnd();
glPopName();
glPushName(2);
glBegin(GL_TRIANGLES);
glVertex3d(1,-5,7);
glVertex3d(2,3,9);
glVertex3d(5,2,9);
glEnd();
glPopName();
}
EDIT1: Maybe completing the code could help?
Initializer:
void WorldView::initializeGL ()
{
this->dayOfYear = 0;
this->hourOfDay = 0;
// Initialize QGLWidget (parent)
QGLWidget::initializeGL();
glShadeModel(GL_SMOOTH);
// Black canvas
glClearColor(0.0f,0.0f,0.0f,0.0f);
// Place light
glEnable( GL_LIGHTING );
glEnable( GL_LIGHT0 );
glEnable(GL_DEPTH_TEST);
GLfloat light0_position [] = {0.1f, 0.1f, 0.1f, 0.1f};
GLfloat light_diffuse []={ 1.0, 1.0, 1.0, 1.0 };
glLightfv ( GL_LIGHT0, GL_POSITION, light0_position );
glLightfv ( GL_LIGHT0, GL_DIFFUSE, light_diffuse );
}
resizer:
void WorldView::resizeGL ( int width, int height )
{
if ((width<=0) || (height<=0))
return;
//set viewport
glViewport(0,0,width,height);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
//set persepective
//change the next line order to have a different perspective
GLdouble aspect_ratio=(GLdouble)width/(GLdouble)height;
gluPerspective(40.0f, aspect_ratio, 1.0, 100.0);
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();
}
Use bullet raycast and not gl_Select which is way too slow and unwieldy. This will also make you get away from calling paintGL manually and other glCalls...in qt mousepressevent. Dont do this!