Drawing an inverted triangle on top of a rectangle with OpenGL - c++

Question

This is the goal:
I can get the inverted triangle with:
GLfloat vertices[][4] = {
{-0.75, 0.5, 0.0, 1.0},
{0.75, 0.5, 0.0, 1.0},
{0.0, -0.75, 0.0, 1.0}};
and some of the rectangle with:
GLfloat vertices[][4] = {
{-0.9, 0.0, 0.0, 1.0},
{0.9, 0.0, 0.0, 1.0},
{-0.9, -0.9, 0.0, 1.0}};
When I try to add the fourth vertex ({0.0, -0.9, 0.0, 1.0}), it loses its form and creates a skinny rectangle in the top left like this:
When I try to draw them both at the same time, I get a black screen.
How can I draw that goal image?
full code:
#include <stdlib.h>
#include <GL/glew.h>
#include <GL/freeglut.h>
#define BUFFER_OFFSET(offset) ((GLvoid *) offset)
GLuint buffer = 0;
GLuint vPos;
GLuint program;
void init()
{
// Three vertexes that define a triangle.
GLfloat vertices[][ 4 ] =
{
{-0.75, -0.5, 0.0, 1.0},
{0.75, -0.5, 0.0, 1.0},
{0.0, 0.75, 0.0, 1.0}
};
glGenBuffers( 1, &buffer );
glBindBuffer( GL_ARRAY_BUFFER, buffer );
glBufferData( GL_ARRAY_BUFFER, sizeof( vertices ), vertices, GL_STATIC_DRAW );
// OpenGL vertex shader source code
const char* vSource =
{
"#version 330\n"
"in vec4 vPos;"
"void main() {"
" gl_Position = vPos * vec4(1.0f, 1.0f, 1.0f, 1.0f);"
"}"
};
// OpenGL fragment shader source code
const char* fSource =
{
"#version 330\n"
"out vec4 fragColor;"
"void main() {"
" fragColor = vec4(0.8, 0.8, 0, 1);"
"}"
};
// Declare shader IDs
GLuint vShader, fShader;
// Create empty shader objects
vShader = glCreateShader( GL_VERTEX_SHADER );
fShader = glCreateShader( GL_FRAGMENT_SHADER );
glShaderSource( vShader, 1, &vSource, NULL );
glShaderSource( fShader, 1, &fSource, NULL );
glCompileShader( vShader );
glCompileShader( fShader );
// Create an empty shader program object
program = glCreateProgram();
// Attach vertex and fragment shaders to the shader program
glAttachShader( program, vShader );
glAttachShader( program, fShader );
// Link the shader program
glLinkProgram( program );
vPos = glGetAttribLocation( program, "vPos" );
// Specify the background color
glClearColor( 0, 0, 0, 1 );
}
void reshape( int width, int height )
{
// Specify the width and height of the picture within the window
glViewport( 0, 0, width, height );
}
void display()
{
// Clear the window with the background color
glClear( GL_COLOR_BUFFER_BIT );
// Activate the shader program
glUseProgram( program );
glBindBuffer( GL_ARRAY_BUFFER, buffer );
glVertexAttribPointer( vPos, 4, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET( 0 ) );
glEnableVertexAttribArray( vPos );
// Start the shader program
glDrawArrays( GL_TRIANGLES, 0, 3 );
// Refresh the window
glutSwapBuffers();
}
void main( int argc, char *argv[] )
{
glutInit( &argc, argv );
glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE );
glutCreateWindow( argv[ 0 ] );
glewInit();
init();
glutDisplayFunc( display );
glutReshapeFunc( reshape );
glutMainLoop();
}