关于SDL绘图

指定渲染区域：
http://sdl2.dxkite.cn/SDL2Teach/v3/08_set_viewport.html

  //将要显示的区域
    SDL_Rect poprect= {100,100,400,200};
    //将其加入设置
    Window::setViewport(&poprect);
    SDL_SetRenderDrawColor(m_render, 0xff, 0xff,0, 0xff);
    SDL_RenderFillRect(m_render,nullptr);

绘制像素点：
https://blog.csdn.net/subfate/article/details/8031994

static void SDL_PixelNolock(SDL_Surface* surface, int x, int y, Uint32 color)
{
    int bpp = surface->format->BytesPerPixel;
    Uint8* p = (Uint8 *)surface->pixels + y * surface->pitch + x * bpp;

    switch (bpp)
    {
    case 4:
        *(Uint32 *)p = color;
        break;
    case 1:
        *p = color;
        break;
    case 2:
        *(Uint16 *)p = color;
        break;
    case 3:
        if (SDL_BYTEORDER == SDL_BIG_ENDIAN)
        {
            p[0] = (color >> 16) & 0xff;
            p[1] = (color >> 8) & 0xff;
            p[2] = color & 0xff;
        }
        else
        {
            p[0] = color & 0xff;
            p[1] = (color >> 8) & 0xff;
            p[2] = (color >> 16) & 0xff;
        }
        break;
    }
}

static void SDL_PixelColor(SDL_Surface* surface, int x, int y, Uint32 color)
{
    if (SDL_MUSTLOCK(surface))
    {
        if (SDL_LockSurface(surface) < 0)
        {
            return;
        }
    }

    SDL_PixelNolock(surface, x, y, color);

    if (SDL_MUSTLOCK(surface))
    {
        SDL_UnlockSurface(surface);
    }
}

void SDL_Pixel(SDL_Surface* surface, int x, int y, Uint32 color)
{
    SDL_PixelColor(surface, x, y, color);
}

图片旋转与翻转绘制
https://jingyan.baidu.com/article/ed15cb1bb70a431be36981d4.html

表面缩放与获取点颜色 设置点颜色
https://www.cnblogs.com/klobohyz/archive/2012/07/13/2589854.html

Uint32 ReadPixel(SDL_Surface *surface, int x, int y)
{
    int bpp = surface->format->BytesPerPixel;
    /* Here p is the address to the pixel we want to retrieve */
    Uint8 *p = (Uint8 *)surface->pixels + y * surface->pitch + x * bpp;

    switch(bpp) {
    case 1:
        return *p;
        break;

    case 2:
        return *(Uint16 *)p;
        break;

    case 3:
        if(SDL_BYTEORDER == SDL_BIG_ENDIAN)
            return p[0] << 16 | p[1] << 8 | p[2];
        else
            return p[0] | p[1] << 8 | p[2] << 16;
        break;

    case 4:
        return *(Uint32 *)p;
        break;

    default:
        return 0;       /* shouldn't happen, but avoids warnings */
    }
}

void DrawPixel(SDL_Surface *surface, int x, int y, Uint32 pixel)
{
    int bpp = surface->format->BytesPerPixel;
    /* Here p is the address to the pixel we want to set */
    Uint8 *p = (Uint8 *)surface->pixels + y * surface->pitch + x * bpp;

    switch(bpp) {
    case 1:
        *p = pixel;
        break;

    case 2:
        *(Uint16 *)p = pixel;
        break;

    case 3:
        if(SDL_BYTEORDER == SDL_BIG_ENDIAN) {
            p[0] = (pixel >> 16) & 0xff;
            p[1] = (pixel >> 8) & 0xff;
            p[2] = pixel & 0xff;
        } else {
            p[0] = pixel & 0xff;
            p[1] = (pixel >> 8) & 0xff;
            p[2] = (pixel >> 16) & 0xff;
        }
        break;

    case 4:
        *(Uint32 *)p = pixel;
        break;
    }
}

SDL_Surface* SDL_ScaleSurface(SDL_Surface *Surface, Uint16 Width, Uint16 Height)
{
      SDL_Surface *_ret = SDL_CreateRGBSurface(Surface->flags, Width, Height, Surface->format->BitsPerPixel,
        Surface->format->Rmask, Surface->format->Gmask, Surface->format->Bmask, Surface->format->Amask);

     double    _stretch_factor_x = (static_cast<double>(Width)  / static_cast<double>(Surface->w)),
        _stretch_factor_y = (static_cast<double>(Height) / static_cast<double>(Surface->h));

    for(Sint32 y = 0; y < Surface->h; y++) //Run across all Y pixels.
        for(Sint32 x = 0; x < Surface->w; x++) //Run across all X pixels.
            for(Sint32 o_y = 0; o_y < _stretch_factor_y; ++o_y) //Draw _stretch_factor_y pixels for each Y pixel.
                for(Sint32 o_x = 0; o_x < _stretch_factor_x; ++o_x) //Draw _stretch_factor_x pixels for each X pixel.
                    DrawPixel(_ret, static_cast<Sint32>(_stretch_factor_x * x) + o_x, 
                        static_cast<Sint32>(_stretch_factor_y * y) + o_y, ReadPixel(Surface, x, y));

    return _ret;
}