OpenGL. Cannot draw multiple shapes. New init clears previously drawn shape - opengl

Question

I'm trying to draw some squares within squares. I have two functions, initRedSquare() and initBlueSquare(). They have different buffers and seperate vao's which I just added before posting the question here.
Individually bothe get drawn. But I want the blue to show up over the red. When the blue square is made it removes the red square completely.
And I have to draw many more shapes after these two also.
Code :
void initRedSquare( GLfloat val )
{
GLfloat vertices[] =
{
+0.0f, +0.0f,
+1.0f, +0.0f, +0.0f,
(+0.8f - val), (+0.8f - val),
+1.0f, +0.0f, +0.0f,
(-0.8f + val), (+0.8f - val),
+1.0f, +0.0f, +0.0f,
(-0.8f + val), (-0.8f + val),
+1.0f, +0.0f, +0.0f,
(+0.8f - val), (-0.8f + val),
+1.0f, +0.0f, +0.0f,
};
// // Specifiy the vertices for a triangle
// vec2 vertices[3] = {
// vec2( -1, 0 ), vec2( -0.8, 1 ), vec2( -0.6, 0 )
// };
// Create a vertex array object
GLuint vao[1];
glGenVertexArrays( 1, vao );
glBindVertexArray( vao[0] );
// Create and initialize a buffer object
GLuint vertexbuffer;
glGenBuffers( 1, &vertexbuffer );
glBindBuffer( GL_ARRAY_BUFFER, vertexbuffer );
glBufferData( GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW );
// Load shaders and use the resulting shader program
GLuint program = InitShader( "vshader21.glsl", "fshader21.glsl" );
glUseProgram( program );
// Initialize the vertex position attribute from the vertex shader
GLuint vPosition_location = glGetAttribLocation( program, "vPosition" );
glEnableVertexAttribArray( vPosition_location );
glVertexAttribPointer( vPosition_location, 2, GL_FLOAT, GL_FALSE, sizeof(float) * 5,
BUFFER_OFFSET(0) );
GLuint vColor_location = glGetAttribLocation( program, "vColor" );
glEnableVertexAttribArray( vColor_location );
glVertexAttribPointer( vColor_location, 3, GL_FLOAT, GL_FALSE, sizeof(float) * 5,
BUFFER_OFFSET(sizeof(float) * 2) );
GLushort indices[] =
{
0, 1, 2,
0, 3, 4,
0, 2, 3,
0, 4, 1
};
GLuint indexBuffer;
glGenBuffers( 1, &indexBuffer );
glBindBuffer( GL_ELEMENT_ARRAY_BUFFER, indexBuffer );
glBufferData( GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW );
//glClearColor( 1.0, 1.0, 1.0, 1.0 ); // white background
}
void initBlueSquare( GLfloat val )
{
GLfloat vertices[] =
{
+0.0f, +0.0f,
+0.0f, +0.0f, +1.0f,
(+0.8f - val), (+0.8f - val),
+0.0f, +0.0f, +1.0f,
(-0.8f + val), (+0.8f - val),
+0.0f, +0.0f, +1.0f,
(-0.8f + val), (-0.8f + val),
+0.0f, +0.0f, +1.0f,
(+0.8f - val), (-0.8f + val),
+0.0f, +0.0f, +1.0f,
};
// // Specifiy the vertices for a triangle
// vec2 vertices[3] = {
// vec2( -1, 0 ), vec2( -0.8, 1 ), vec2( -0.6, 0 )
// };
// Create a vertex array object
GLuint vao[2];
glGenVertexArrays( 1, vao );
glBindVertexArray( vao[1] );
// Create and initialize a buffer object
GLuint vertexbuffer2;
glGenBuffers( 1, &vertexbuffer2 );
glBindBuffer( GL_ARRAY_BUFFER, vertexbuffer2 );
glBufferData( GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW );
// Load shaders and use the resulting shader program
GLuint program = InitShader( "vshader21.glsl", "fshader21.glsl" );
glUseProgram( program );
// Initialize the vertex position attribute from the vertex shader
GLuint vPosition_location = glGetAttribLocation( program, "vPosition" );
glEnableVertexAttribArray( vPosition_location );
glVertexAttribPointer( vPosition_location, 2, GL_FLOAT, GL_FALSE, sizeof(float) * 5,
BUFFER_OFFSET(0) );
GLuint vColor_location = glGetAttribLocation( program, "vColor" );
glEnableVertexAttribArray( vColor_location );
glVertexAttribPointer( vColor_location, 3, GL_FLOAT, GL_FALSE, sizeof(float) * 5,
BUFFER_OFFSET(sizeof(float) * 2) );
GLushort indices[] =
{
0, 1, 2,
0, 3, 4,
0, 2, 3,
0, 4, 1
};
GLuint indexBuffer2;
glGenBuffers( 1, &indexBuffer2 );
glBindBuffer( GL_ELEMENT_ARRAY_BUFFER, indexBuffer2 );
glBufferData( GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW );
//glClearColor( 1.0, 1.0, 1.0, 1.0 ); // white background
}
void
display( void )
{
//glClear( GL_COLOR_BUFFER_BIT ); // clear the window
//glDrawArrays( GL_TRIANGLES, 0, 6 ); // draw the points
glDrawElements(GL_TRIANGLES, 12, GL_UNSIGNED_SHORT, 0);
glFlush();
}
void
keyboard( unsigned char key, int x, int y )
{
switch ( key ) {
case 033:
exit( EXIT_SUCCESS );
break;
}
}
int
main( int argc, char **argv )
{
glutInit( &argc, argv );
glutInitDisplayMode( GLUT_RGBA );
glutInitWindowSize( 500, 500 );
glutCreateWindow( "Assignment 2" );
glewExperimental=GL_TRUE;
glewInit();
initRedSquare(0.2f);
glutDisplayFunc( display );
glutKeyboardFunc( keyboard );
initBlueSquare(0.3f);
glutDisplayFunc( display );
glutKeyboardFunc( keyboard );
glutMainLoop();
return 0;
}
Vertex Shader :
attribute vec4 vPosition;
attribute vec3 vColor;
varying vec3 finalColor;
void
main()
{
gl_Position = vPosition;
finalColor = vColor;
}
Fragment Shader :
varying vec3 finalColor;
void
main()
{
gl_FragColor = vec4( finalColor, 1.0 );
}

Answer 1

The way you're program is structured will not work out.
First, and most important, glutDisplayFunc( display ); does not call the draw function directly, it just tells glut which function should be called whenever something should be drawn. So this function has to be able to draw ALL the content at once, in your case it will require two draw-calls with the correct state set.
To do this you'll have to store the buffers/vaos that are generated in your init functions somewhere outside of these functions and draw both objects:
void display( void )
{
glClear( GL_COLOR_BUFFER_BIT );
glBindVertexArray(red_vao);
glDrawElements(GL_TRIANGLES, 12, GL_UNSIGNED_SHORT, 0);
glBindVertexArray(blue_vao);
glDrawElements(GL_TRIANGLES, 12, GL_UNSIGNED_SHORT, 0);
glFlush();
}
, where red_vao and blue_vao are the vao handles generated in the corresponding init function.