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class image {
public:
    virtual gfx::gfx_result initialize()=0;
    virtual bool initialized() const=0;
    virtual void deinitialize()=0;
    virtual gfx::size16 dimensions() const=0;
    virtual gfx::rect16 bounds() const;
    virtual gfx::gfx_result draw(const gfx::rect16& bounds, image_draw_callback callback, void* callback_state=nullptr) const=0;
};

// we're within the destination
if((x2>=st.bounds->x1 && x1<=st.bounds->x2) && 
    (y2>=st.bounds->y1 && y1<=st.bounds->y2)) {
    const int offsx = -st.bounds->x1;
    const int offsy = -st.bounds->y1;
    const bool left_edge = x2>=st.bounds->x1 && x1<st.bounds->x1;
    const bool top_edge = y2>=st.bounds->y1 && y1<st.bounds->y1;
    int xs=0,xpe=w,xc=w;
    if(left_edge) {
        xs=st.bounds->x1-x1;
        xc = w-xs;
    }
    int ys=0,ype=h,yc=h;
    if(top_edge) {
        ys=st.bounds->y1-y1;
        yc = h-ys;
    }
    const uint8_t* pbs = (const uint8_t*)bmp;
    uint8_t* pbd = ( uint8_t*)st.bmp;
    for(int y=0;y<yc;++y) {
        const uint8_t* ps = pbs+(((y+ys)*w*3)+(xs*3));
        uint8_t* pd = pbd+(y*xc*4);
        for(int x=0;x<xc;++x) {
            *pd++=*ps++;
            *pd++=*ps++;
            *pd++=*ps++;
            *pd++=0xFF;
        }
    }
    image_data data;
    data.is_fill = false;
    const const_bitmap<rgba_pixel<32>> csrc(size16(xc,yc),st.bmp);
    data.bitmap.region = &csrc;
    data.bitmap.location = point16(x1+offsx,y1+offsy);
    gfx_result r =st.cb(data,st.cb_state);
    if(gfx_result::success!=r) {
        st.error = r;
        return 0;
    }
        
} else {
    if(y1>st.bounds->y2) {
        st.error = gfx_result::success;
        // don't need to display any more...
        return 0;
    } 
}