AGG && SDL_Lock_texture

#include "SDL.h"
#include "agg_renderer_base.h"
#include "agg_pixfmt_rgba.h"
#include "agg_scanline_u.h"
#include "agg_rasterizer_scanline_aa.h"
#include "agg_renderer_scanline.h"
#include "agg_path_storage.h"
#include "agg_conv_stroke.h"

enum
{
    frame_width = 800,
    frame_height = 600
};

int main(int argc, char * argv[])
{
    //initialize SDL
    //--------------
    atexit(SDL_Quit);
    SDL_Init(SDL_INIT_VIDEO);
    SDL_Window* win = SDL_CreateWindow
    (
        "SDL&AGG",
        SDL_WINDOWPOS_CENTERED,
        SDL_WINDOWPOS_CENTERED,
        frame_width,
        frame_height,
        0
    );
    SDL_Renderer* ren = SDL_CreateRenderer
    (
        win,
        -1,
        0
    );

    //initialize AGG with memory allocated by SDL
    #if SDL_BYTEORDER == SDL_BIG_ENDIAN
    #define AGG_RGBA32_BYTE_ORDER agg::pixfmt_rgba32
    #define RMASK 0xff000000
    #define GMASK 0x00ff0000
    #define BMASK 0x0000ff00
    #define AMASK 0x000000ff
    #else
    #define AGG_RGBA32_BYTE_ORDER agg::pixfmt_abgr32
    #define RMASK 0x000000ff
    #define GMASK 0x0000ff00
    #define BMASK 0x00ff0000
    #define AMASK 0xff000000
    #endif
/*
    SDL_Surface* surf = SDL_CreateRGBSurface
    (
        0,
        frame_width,
        frame_height,
        32,
        RMASK,
        GMASK,
        BMASK,
        AMASK
    );
*/
//  SDL_LockSurface(surf);
    void *tex_pixels = NULL;
    int tex_pitch = -1;

    SDL_Texture* tex = SDL_CreateTexture
    (
        ren,
        SDL_PIXELFORMAT_RGBA8888,
        SDL_TEXTUREACCESS_STREAMING,
        frame_width,
        frame_height
    );

    SDL_LockTexture
    (
        tex,
        NULL,
        &tex_pixels,
        &tex_pitch
    );

    agg::rendering_buffer rbuf //rendering buffer object
    (
        (unsigned char*)tex_pixels,
        frame_width,
        frame_height,
        frame_width * 4
    );

    AGG_RGBA32_BYTE_ORDER pixf(rbuf);
    agg::renderer_base<AGG_RGBA32_BYTE_ORDER> rbase(pixf);
        //clear tex_pixels
    rbase.clear(agg::rgba8(0, 0, 255, 255));

    agg::scanline_u8 scanline;
    agg::rasterizer_scanline_aa<> ras;

    agg::renderer_scanline_aa_solid
    <
        agg::renderer_base
        <
            AGG_RGBA32_BYTE_ORDER
        >
    > rscan(rbase);

    //create agg paths
    agg::path_storage path;
    path.move_to(10, 10);
    path.line_to(frame_width-10, frame_height-10);
    path.line_to(frame_width-10, 10);
    path.line_to(10, frame_height-10);

    agg::conv_stroke<agg::path_storage> stroke(path);
    stroke.width(8.2);
    stroke.line_cap(agg::round_cap);
    stroke.line_join(agg::round_join);
    ras.add_path(stroke);
    rscan.color(agg::rgba8(75, 150, 225, 255));

    //draw paths to tex_pixels
    agg::render_scanlines(ras, scanline, rscan);

    //create more agg paths
    path.remove_all();
    path.line_to(frame_width-50, frame_height-50);
    path.line_to(frame_width-50, 50);
    path.line_to(50, frame_height-50);

    ras.add_path(stroke);

    rscan.color(agg::rgba8(200, 150, 50, 255));
    agg::render_scanlines(ras, scanline, rscan);

//  SDL_UnlockSurface(surf);
    SDL_UnlockTexture(tex);

    SDL_RenderCopy
    (
        ren,
        tex,
        NULL,
        NULL
    );
    SDL_RenderPresent(ren);

    //main event loop
    //---------------
    SDL_Event e;
    SDL_Point a, b;
    while(1) //loop until user quits
    {
        if(SDL_PollEvent(&e))
        {
            switch(e.type)
            {
                case SDL_QUIT:
                    return 0;

                case SDL_KEYDOWN:
                    if(e.key.keysym.sym == SDLK_ESCAPE)
                        return(0);

                case SDL_MOUSEBUTTONDOWN:
                    if (e.button.button == SDL_BUTTON_LEFT)
                    {
                        SDL_GetMouseState(&a.x, &a.y);
                        SDL_RenderDrawPoint(ren, a.x, a.y);
//                      path.move_to(a.x, a.y);
                    }
                    break;

                case SDL_MOUSEMOTION:
                    if(e.motion.state & SDL_BUTTON_LMASK)
                    {
                        SDL_GetMouseState(&b.x, &b.y);
                        SDL_RenderDrawLine
                        (
                            ren,
                            a.x,
                            a.y,
                            b.x,
                            b.y
                        );

                        //SDL_LockSurface(surf);
                        SDL_LockTexture
                        (
                            tex,
                            NULL,
                            &tex_pixels,
                            &tex_pitch
                        );

/*                      path.line_to(b.x, b.y);

                        ras.add_path(stroke);
                        rscan.color(agg::rgba8(200, 150, 50, 255));
                        agg::render_scanlines(ras, scanline, rscan);
    */
//                      SDL_UnlockSurface(surf);
                        SDL_UnlockTexture(tex);
                        tex_pixels = NULL;
                        tex_pitch = -1;
                        SDL_RenderCopy
                        (
                            ren,
                            tex,
                            NULL,
                            NULL
                        );
                        SDL_RenderPresent(ren);
                        a=b;
                    }
                    break;
            }
        }


        SDL_Delay(100); //speedbump
    }

    return 0;
}