C++ SDL opengles2 GLSL - blending czyli mieszanie tekstur

// program rysujacy dwie rozne tekstury
#include <SDL2/SDL.h>
#include <SDL2/SDL_image.h>
#include <SDL2/SDL_opengles2.h>
#include<iostream>
using namespace std;

#include "math.cpp"

SDL_Window * window;

GLuint vTextureShader;
GLuint fTextureShader;
GLuint vDoubleTextureShader;
GLuint fDoubleTextureShader;

GLuint textureProgram;
GLuint doubleTextureProgram;

GLuint textures[ 2 ];
static GLfloat vertexes[ 6 ];

// SHADERS //////////////////////////////////////////////////////
const char * vTextureSource =
"attribute vec4 position;\n"
"attribute vec2 texcoord;\n"
"varying vec2 Texcoord;\n"
"void main()\n"
"{\n"
"gl_Position = position;\n"
"Texcoord = texcoord;\n"
"}\n";

const char * fTextureSource =
"precision mediump float;\n"
"uniform sampler2D texture;\n"
"varying vec2 Texcoord;\n"
"void main()\n"
"{\n"
"gl_FragColor = texture2D(texture, Texcoord);\n"
"}\n";

const char * fDoubleTextureSource =
"precision mediump float;\n"
"uniform sampler2D texture1;\n"
"uniform sampler2D texture2;\n"
"varying vec2 Texcoord;\n"
"void main()\n"
"{\n"
"vec4 color1 = texture2D(texture1, Texcoord);\n"
"vec4 color2 = texture2D(texture2, Texcoord);\n"
"gl_FragColor = color1*0.5 + color2*0.5;\n"
"}\n";

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
    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 );
   
    SDL_DisplayMode DM;
    SDL_GetCurrentDisplayMode( 0, & DM );
   
    window = SDL_CreateWindow(
    "Load PNG x2 and draw textures",
    SDL_WINDOWPOS_UNDEFINED, SDL_WINDOWPOS_UNDEFINED,
    int( DM.w ), int( DM.h ),
    SDL_WINDOW_SHOWN |
    SDL_WINDOW_OPENGL | SDL_WINDOW_FULLSCREEN );
    cout << "created window size is a [" << DM.w << "x" << DM.h << "]\n";
    SDL_GL_CreateContext( window );
}

void initShaders()
{
    vTextureShader = glCreateShader( GL_VERTEX_SHADER );
    fTextureShader = glCreateShader( GL_FRAGMENT_SHADER );
    vDoubleTextureShader = glCreateShader( GL_VERTEX_SHADER );
    fDoubleTextureShader = glCreateShader( GL_FRAGMENT_SHADER );
   
    glShaderSource( vTextureShader, 1, & vTextureSource, NULL );
    glShaderSource( fTextureShader, 1, & fTextureSource, NULL );
    glShaderSource( vDoubleTextureShader, 1, & vTextureSource, NULL );
    glShaderSource( fDoubleTextureShader, 1, & fDoubleTextureSource, NULL );
   
    glCompileShader( vTextureShader );
    glCompileShader( fTextureShader );
    glCompileShader( vDoubleTextureShader );
    glCompileShader( fDoubleTextureShader );
   
    textureProgram = glCreateProgram();
    glAttachShader( textureProgram, vTextureShader );
    glAttachShader( textureProgram, fTextureShader );
   
    doubleTextureProgram = glCreateProgram();
    glAttachShader( doubleTextureProgram, vDoubleTextureShader );
    glAttachShader( doubleTextureProgram, fDoubleTextureShader );
   
    glLinkProgram( textureProgram );
    glLinkProgram( doubleTextureProgram );
}

void setVertexes( vec2f screenSize, vec2f v1, vec2f v2, vec2f v3 )
{
    vertexes[ 0 ] = v1.x /( screenSize.x / 2.f );
    vertexes[ 1 ] = v1.y /( screenSize.y / 2.f );
    vertexes[ 2 ] = v2.x /( screenSize.x / 2.f );
    vertexes[ 3 ] = v2.y /( screenSize.y / 2.f );
    vertexes[ 4 ] = v3.x /( screenSize.x / 2.f );
    vertexes[ 5 ] = v3.y /( screenSize.y / 2.f );
}

void drawTriangle( vec2f screenSize, vec2f v1, vec2f v2, vec2f v3, GLuint texture )
{
    glUseProgram( textureProgram );
    setVertexes( screenSize, v1, v2, v3 );
   
    // add array [vertexes] to edit position
    GLint position = glGetAttribLocation( textureProgram, "position" );
    glEnableVertexAttribArray( position );
    glVertexAttribPointer( position, 2, GL_FLOAT, GL_FALSE, 0, vertexes );
   
    GLint texcoord = glGetAttribLocation( textureProgram, "texcoord" );
    glEnableVertexAttribArray( texcoord );
    glVertexAttribPointer( texcoord, 2, GL_FLOAT, GL_FALSE, 0, vertexes );
   
    glActiveTexture( GL_TEXTURE0 );
    glBindTexture( GL_TEXTURE_2D, texture );
   
    // get the sampler2D from shaders
    GLint textureUniform = glGetUniformLocation( textureProgram, "texture" );
    glUniform1i( textureUniform, 0 ); // texture from shader = GL_TEXTURE0
   
    glDrawArrays( GL_TRIANGLES, 0, 4 );
}

void drawTriangle( vec2f screenSize, vec2f v1, vec2f v2, vec2f v3, GLuint tex1, GLuint tex2 )
{
    glUseProgram( doubleTextureProgram );
    setVertexes( screenSize, v1, v2, v3 );
   
    // add array [vertexes] to edit position
    GLint position = glGetAttribLocation( doubleTextureProgram, "position" );
    glEnableVertexAttribArray( position );
    glVertexAttribPointer( position, 2, GL_FLOAT, GL_FALSE, 0, vertexes );
   
    GLint texcoord = glGetAttribLocation( doubleTextureProgram, "texcoord" );
    glEnableVertexAttribArray( texcoord );
    glVertexAttribPointer( texcoord, 2, GL_FLOAT, GL_FALSE, 0, vertexes );
   
    // TEXTURE 0  
    glActiveTexture( GL_TEXTURE0 );
    glBindTexture( GL_TEXTURE_2D, tex1 );
    GLint textureUniform1 = glGetUniformLocation( doubleTextureProgram, "texture1" );
    glUniform1i( textureUniform1, 0 ); // texture from shader = GL_TEXTURE0
   
    // TEXTURE 1  
    glActiveTexture( GL_TEXTURE1 );
    glBindTexture( GL_TEXTURE_2D, tex2 );
    GLint textureUniform2 = glGetUniformLocation( doubleTextureProgram, "texture2" );
    glUniform1i( textureUniform2, 1 ); // texture from shader = GL_TEXTURE1
   
    glDrawArrays( GL_TRIANGLES, 0, 4 );
}

void loadTexture( GLuint & texture, const char * path )
{
    SDL_Surface * img = IMG_Load( path );
    if( img == NULL )
    {
        cout << "error load texture: " << path << "\n";
        return;
    }
   
    glGenTextures( 1, & texture );
    glBindTexture( GL_TEXTURE_2D, texture );
   
    // now set parameters of texture // texture must be repeat when end of tex
    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST );
    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST );
    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_REPEAT );
    glTexParameteri( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_REPEAT );
   
    // now set the pixels to texture
    glTexImage2D(
    GL_TEXTURE_2D, 0, GL_RGBA, img->w, img->h,
    0, GL_RGBA, GL_UNSIGNED_BYTE,
    img->pixels );
   
    cout << "load texture: " << path << "\n";
}

void loadTextures()
{
    loadTexture( textures[ 0 ], "obraz1.png" );
    loadTexture( textures[ 1 ], "obraz2.png" );
}

int main()
{
    initSDL();
    initShaders();
    loadTextures();
   
    vec2f screen, v1, v2, v3, v4;
    float side = 1024;
    screen = vec2f( 2640, 1200 );
    v1 = vec2f( - side / 2, side / 2 );
    v2 = vec2f( side / 2, side / 2 );
    v3 = vec2f( side / 2, - side / 2 );
    v4 = vec2f( - side / 2, - side / 2 );
   
    while( true )
    {
        // clear screen
        glClearColor( 0.125f, 0.125f, 0.125f, 1.0f );
        glClear( GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT );
       
        drawTriangle( screen, v1, v2, v3, textures[ 0 ] ); // draw normal texture
        drawTriangle( screen, v1, v3, v4, textures[ 0 ], textures[ 1 ] ); // draw blend textures
       
        // draw submit
        SDL_GL_SwapWindow( window );
    }
    return 0;
}