C++ SDL2 opengles GLSL - rysowanie przezroczystych prostokatow

const char * vColorSource =
"attribute vec4 position;\n"
"attribute vec4 color;\n"
"varying vec4 vcolor;\n"
"void main()\n"
"{\n"
"gl_Position = vec4(position.xyz, 1.0);\n"
"vcolor = color;\n"
"}\n";

const char * fColorSource =
"precision mediump float;\n"
"varying vec4 vcolor;\n"
"void main()\n"
"{\n"
"gl_FragColor = vcolor;\n"
"}\n";

GLuint vColorShader;
GLuint fColorShader;
GLuint colorProgram;

void initShader()
{
    vColorShader = glCreateShader( GL_VERTEX_SHADER );
    fColorShader = glCreateShader( GL_FRAGMENT_SHADER );
   
    glShaderSource( vColorShader, 1, & vColorSource, NULL );
    glShaderSource( fColorShader, 1, & fColorSource, NULL );
   
    glCompileShader( vColorShader );
    glCompileShader( fColorShader );
   
    colorProgram = glCreateProgram();
    glAttachShader( colorProgram, vColorShader );
    glAttachShader( colorProgram, fColorShader );
   
    glLinkProgram( colorProgram );
   
}

void drawRectColor( float screenWidth, float screenHeight, float posX, float posY, float width, float height, float red, float green, float blue, float alpha )
{
    // rgba must have a [0 - 256]
    // vertexes of Rect
    // v1 v2
    // v4 v3
    float v1x, v1y, v2x, v2y, v3x, v3y, v4x, v4y;
    v1x = posX - width / 2.f; v1y = posY + height / 2.f;
    v2x = posX + width / 2.f; v2y = posY + height / 2.f;
    v3x = posX + width / 2.f; v3y = posY - height / 2.f;
    v4x = posX - width / 2.f; v4y = posY - height / 2.f;
   
    static GLfloat colors[ 12 ]; // [r,g,b,a]
    static GLfloat vertexes[ 6 ]; // x=[-1,1], y=[-1,1]
    GLint position, color;
    // GLSL uses rgba = [0.0 - 1.0]
    for( int i = 0; i < 4; i++ )
    {
        colors[ i * 4 + 0 ] = red / 256.f;
        colors[ i * 4 + 1 ] = green / 256.f;
        colors[ i * 4 + 2 ] = blue / 256.f;
        colors[ i * 4 + 3 ] = alpha / 256.f;
    };
   
    // draw first triangle
    vertexes[ 0 ] = v1x * 2 / screenWidth; vertexes[ 1 ] = v1y * 2 / screenHeight;
    vertexes[ 2 ] = v2x * 2 / screenWidth; vertexes[ 3 ] = v2y * 2 / screenHeight;
    vertexes[ 4 ] = v3x * 2 / screenWidth; vertexes[ 5 ] = v3y * 2 / screenHeight;
   
    glUseProgram( colorProgram );
    position = glGetAttribLocation( colorProgram, "position" );
    glEnableVertexAttribArray( position );
    glVertexAttribPointer( position, 2, GL_FLOAT, GL_FALSE, 0, vertexes );
    glDrawArrays( GL_TRIANGLES, 0, 4 );
   
    color = glGetAttribLocation( colorProgram, "color" );
    glEnableVertexAttribArray( color );
    glVertexAttribPointer( color, 4, GL_FLOAT, GL_FALSE, 0, colors );
   
    glDrawArrays( GL_TRIANGLES, 0, 4 );
   
    // draw second triangle
    vertexes[ 0 ] = v1x * 2 / screenWidth; vertexes[ 1 ] = v1y * 2 / screenHeight;
    vertexes[ 2 ] = v3x * 2 / screenWidth; vertexes[ 3 ] = v3y * 2 / screenHeight;
    vertexes[ 4 ] = v4x * 2 / screenWidth; vertexes[ 5 ] = v4y * 2 / screenHeight;
   
    position = glGetAttribLocation( colorProgram, "position" );
    glEnableVertexAttribArray( position );
    glVertexAttribPointer( position, 2, GL_FLOAT, GL_FALSE, 0, vertexes );
    glDrawArrays( GL_TRIANGLES, 0, 4 );
   
    color = glGetAttribLocation( colorProgram, "color" );
    glEnableVertexAttribArray( color );
    glVertexAttribPointer( color, 4, GL_FLOAT, GL_FALSE, 0, colors );
   
    glDrawArrays( GL_TRIANGLES, 0, 4 );
   
}

#include <iostream>
using namespace std;
#include <SDL2/SDL.h>
#include <SDL2/SDL_opengles2.h>

#include "shaders.cpp"
SDL_Window * window;
bool programRun;
float screenWidth, screenHeight;

void initSDL()
{
    //program graphics init
    SDL_Init( SDL_INIT_EVERYTHING );
    // We use OpenGL ES 2.0
    SDL_GL_SetAttribute( SDL_GL_CONTEXT_MAJOR_VERSION, 2 );
    SDL_GL_SetAttribute( SDL_GL_CONTEXT_MINOR_VERSION, 0 );
   
    // We want at least 8 bits per color, alpha and depth
    SDL_GL_SetAttribute( SDL_GL_RED_SIZE, 8 );
    SDL_GL_SetAttribute( SDL_GL_GREEN_SIZE, 8 );
    SDL_GL_SetAttribute( SDL_GL_BLUE_SIZE, 8 );
    SDL_GL_SetAttribute( SDL_GL_ALPHA_SIZE, 8 );
    SDL_GL_SetAttribute( SDL_GL_DEPTH_SIZE, 8 );
   
    // get current display settings
    SDL_DisplayMode DM;
    SDL_GetCurrentDisplayMode( 0, & DM );
   
    window = SDL_CreateWindow(
    "Terrain War",
    SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED,
    int( DM.w ), int( DM.h ),
    SDL_WINDOW_OPENGL | SDL_WINDOW_FULLSCREEN );
   
    // and now can use OpenGles 2.0 !! :D
    SDL_GL_CreateContext( window );
   
}

void initProgram()
{
    SDL_DisplayMode DM;
    SDL_GetCurrentDisplayMode( 0, & DM );
   
    screenWidth = int( DM.w );
    screenHeight = int( DM.h );
   
    programRun = true;
}

void drawBackground()
{
    glClearColor( 0.125f, 0.125f, 0.125f, 1.0f );
    glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
}

int main()
{
    initSDL();
    initProgram();
    initShader();
   
    while( programRun )
    {
        drawBackground();
        // draw first rectangle
        drawRectColor(
        screenWidth, screenHeight,
        0, 0, 256, 256, // x, y, width, height
        128, 48, 48, 128 // rgba
        );
       
        drawRectColor(
        screenWidth, screenHeight,
        128, 128, 256, 256,
        48, 128, 48, 128
        );
        SDL_GL_SwapWindow( window );
    };
   
    return 0;
}