CODEWITHSUNDEEP
c++openglglut
InternetUser
InternetUser

Reputation: 1

C++ opengl intersecting glScissor

A Project I am working on involves me using glScissor, in some cases i need to perform a scissor on an area twice (or more), with the goal of only rendering what is within both scissor boxes.

The issue im running into is that the second scissor box just overrides the previous one, meaning only the last box set is used instead of both.

I have tried existing solutions such as setting scissor1, push matrix, enable scissor_test, set scissor2, disable scissor_test, popmatrix, disable scissor_test. As proposed here: glScissor() call inside another glScissor() I could not get these to produce any difference, I had also tried glPushAttrib instead of matrix but still no difference.

Here is an example program I wrote for scissor testing, its compiled by g++ and uses freeglut, the scissoring takes place in display():

/*
Compile: g++ .\scissor.cpp -lglu32 -lfreeglut -lopengl32

*/

#include <GL/gl.h>//standard from mingw, already in glut.h - header library
#include <GL/glu.h>//standard from mingw, already in glut.h - utility library
#include <GL/glut.h>//glut/freeglut - more utilities, utility tool kit

void display();
void reshape(int, int);
void timer(int);

void init(){
    glClearColor(0, 0, 0, 1);
}

int main(int argc, char **argv){
    glutInit(&argc, argv);//init glut
    glutInitDisplayMode(GLUT_RGB | GLUT_DOUBLE);//init display mode, add double buffer mode

    //init window
    glutInitWindowPosition(200, 100);//if not specified, it will display in a random spot
    glutInitWindowSize(500, 500);//size


    //create window
    glutCreateWindow("Window 1");

    //give glut a function pointer so it can call that function later
    glutDisplayFunc(display);
    glutReshapeFunc(reshape);
    glutTimerFunc(0, timer, 0);//call certain function after a specified amount of time
    init();


    glutMainLoop();//once this loop runs your program has started running, when the loop ends the program terminates

}



float xPos = -10;
int state = 1;//1 = right, -1 = left

//our rendering happens here
void display(){
    //clear previous frame
    glClear(GL_COLOR_BUFFER_BIT);//pass in flag of frame buffer
    //draw next frame below
    glLoadIdentity();//reset rotations, transformations, ect. (resets coordinate system)
    //we are using a model view matrix by default

    //TEST
    glEnable(GL_SCISSOR_TEST);
    glScissor(0, 0, 100, 1000);
    
    glPushMatrix();
    glEnable(GL_SCISSOR_TEST);
    glScissor(50, 0, 1000, 1000);
    
    //assuming both scissors intersect, we should only see the square between 50 and 100 pixels
    

    //draw
    
    glBegin(GL_QUADS);//every set of 3 verticies is a triangle
    //GL_TRIANGLES = 3 points
    //GL_QUADS = 4 points
    //GL_POLYGON = any amount of points
    
    glVertex2f(xPos, 1);//the 2 is the amount of args we pass in, the f means theyr floats
    glVertex2f(xPos, -1);
    glVertex2f(xPos+2, -1);
    glVertex2f(xPos+2, 1);

    glEnd();//tell opengl your done drawing verticies
    
    
    glDisable(GL_SCISSOR_TEST);
    glPopMatrix();
    glDisable(GL_SCISSOR_TEST);


    //display frame buffer on screen
    //glFlush();
    glutSwapBuffers();//if double buffering, call swap buffers instead of flush
}




//gets called when window is reshaped
void reshape(int width, int hight){
    //set viewport and projection
    //viewport is a rectangle where everything is drawn, like its the window
    glViewport(0, 0, width, hight);

    //matrix modes: there is model view and projection, projection has depth
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();//reset current matrix after changing matrix mode
    gluOrtho2D(-10, 10, -10, 10);//specify 2d projection, set opengl's coordinate system
    glMatrixMode(GL_MODELVIEW);//change back to model view


}


//this like makes a loop
void timer(int a){
    glutPostRedisplay();//opengl will call the display function the next time it gets the chance
    glutTimerFunc(1000/60, timer, 0);

    //update positions and stuff
    //this can be done here or in the display function
    switch(state){
        case 1:
            if(xPos < 8)
                xPos += 0.15;
            else
                state = -1;
            break;
        case -1:
            if(xPos > -10)
                xPos -= 0.15;
            else
                state = 1;
            break;
    }
}

I tried following example solutions, such as push/pop matrix/attrib, but couldnt get anything to work

Upvotes: 0

Views: 339

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473577

There is no first or second scissor box. There is just the scissor box. You can change the scissor box and that change will affect subsequent rendering. But at any one time, there is only one.

What you want is to use the stencil buffer to discard fragments outside of an area defined by rendering certain values into the stencil buffer.

Upvotes: 3

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