I am trying to simulate a cube bouncing between walls. For this purpose, I have constructed 2 vertex array objects, one of them representing the cube and the other representing walls. Firstly, I created walls. I am drawing walls with glDrawElements call since I have used index (element) buffers to create it. I have also created a cube model, but I am not using index buffer for it, so I will use glDrawArrays call to draw it as far as I know. My walls are showing up nicely, but I cannot figure out how to draw the cube (at this step, I am not even trying to animate the cube, I just want to draw it) This is what my code looks like:

#include "Cube.h"

#include <cstring>

GLuint vao[2];

void init()
{
    cube();

    GLuint program = InitShader( "vshader.glsl", "fshader.glsl" );
    glUseProgram( program );
    GLuint vPosition = glGetAttribLocation(program, "vPosition");
    glGenVertexArrays(2, vao);

    glBindVertexArray(vao[0]);
    GLuint cube_buffer;
    glGenBuffers(1, &cube_buffer);
    glBindBuffer(GL_ARRAY_BUFFER, cube_buffer);
    glBufferData(GL_ARRAY_BUFFER, sizeof(cubePoints), cubePoints, GL_STATIC_DRAW);
    glEnableVertexAttribArray(vPosition);
    glVertexAttribPointer(vPosition, 4, GL_FLOAT, GL_FALSE, 0, (void*)0);

    const float mid_value  = 0.0f;
    const float far_value  = 0.35f;
    const float near_value = 1.0f;
    const float positions[60] = {
        /// positions               /// colors
        mid_value, -near_value, 1.0, 0.0, 0.0, /// 0
        near_value, -near_value, 1.0, 0.0, 0.0, /// 1
        near_value, mid_value, 0.0, 1.0, 0.0, /// 2
        near_value, near_value, 0.0, 1.0, 0.0, /// 3
        mid_value, near_value, 1.0, 1.0, 0.0, /// 4
        -near_value, near_value, 1.0, 1.0, 0.0, /// 5
        -near_value, mid_value, 0.0, 0.0, 1.0, /// 6
        -near_value, -near_value, 0.0, 0.0, 1.0, /// 7
        far_value, -far_value, 1.0, 0.0, 0.0, /// 8
        far_value, far_value, 0.0, 1.0, 0.0, /// 9
        -far_value, far_value, 1.0, 1.0, 0.0, /// 10
        -far_value, -far_value, 0.0, 0.0, 1.0 /// 11
    };

    unsigned int indices[36] = {
        7,11,0,
        0,8,1,
        1,8,2,
        2,9,3,
        3,9,4,
        4,10,5,
        5,10,6,
        6,11,7,
        11,0,8,
        8,2,9,
        9,4,10,
        10,6,11,
    };

    glBindVertexArray(vao[1]);
    GLuint room_buffer;
    glGenBuffers(1, &room_buffer);
    glBindBuffer(GL_ARRAY_BUFFER, room_buffer);
    glBufferData(GL_ARRAY_BUFFER, 60 * sizeof(float), positions, GL_STATIC_DRAW);
    glEnableVertexAttribArray(0);
    glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, sizeof(float) * 5, (void*)0);  /// positions
    glEnableVertexAttribArray(1);
    glVertexAttribPointer(1, 3, GL_FLOAT, GL_FALSE, sizeof(float) * 5, (void*)(2* sizeof(float)));  /// colors

    GLuint index_buffer;
    glGenBuffers(1, &index_buffer);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, index_buffer);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, 36 * sizeof(unsigned int), indices, GL_STATIC_DRAW);

    glClearColor(1.0, 1.0, 1.0, 1.0);
}

void display()
{
    glClear(GL_COLOR_BUFFER_BIT);
    glDrawElements(GL_TRIANGLES, 36, GL_UNSIGNED_INT, nullptr);


    // glDrawArrays(...); --> how to use this, or should I really use this here?

    glutSwapBuffers();
}

void idle() {
    glutPostRedisplay();
}

void reshape(int width, int height) {
    glViewport(0, 0, width, height);
}

int main( int argc, char **argv )
{
    glutInit( &argc, argv );
    glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE);
    glutInitWindowSize( 600, 600 );

    glutCreateWindow( "Bouncing Cube" );
    glewExperimental = GL_TRUE;
    glewInit();

    init();

    glutDisplayFunc(display);
    glutIdleFunc(idle);
    glutReshapeFunc(reshape);

    glutMainLoop();
}

In this code, cubePoints is filled by cube() method in another translation unit. I am not exactly sure how to draw cube on the screen, and how to use glDrawArrays here? Should I bind or unbind anything to draw it? I feel like I am not using vertex array objects properly. Below is what my walls look like:

I simply want cube to appear on the far side, where the white rectangle is located at. Is it possible to call glDrawArrays with glDrawElements?

CodePudding user response：

You must bind the Vertex Array Object before the "draw" call. The drawing instruction uses the data stored in the currently bound Vertex Array Object:

void display()
{
    glClear(GL_COLOR_BUFFER_BIT);

    glBindVertexArray(vao[0]);
    glDrawElements(GL_TRIANGLES, 36, GL_UNSIGNED_INT, nullptr);

    glBindVertexArray(vao[1]);
    glDrawArrays(...);

    glutSwapBuffers();
}