How do I perform image clipping efficiently within a parent RECT?

Question

So,
I'm playing around with the DIRECTX API in C++ and designing a sprite interface that does clipping within parent RECT's, like a UI window or another sprite or what have you.

This is because, later on, I'm going to make a scrolling function for a UI window.

I think an image would be appropriate to demonstrate what I am trying to achieve.

In order to do clipping calculations, at least four variables/structs are required (from what I can discern):

• Parent RECT: the "clipping" bounds
• D3DXIMAGE_INFO: containing the image information, such as width and height
• Sprite position
• Draw RECT: for the LPD3DXSPRITE->Draw() function. Basically, what part of the image should be drawn. This is where the "clipping" is done.

Now, I think it would be confusing if I showed you my entire interface and its inner workings and how it handles the variables.

So, instead,
this piece of code demonstrates how I'm currently calculating the clipping.

void ClippingCalculation(){
    RECT parent_bounds, draw_rect;
    D3DXVECTOR2 sprite_position; //Relative to parent rect
    D3DXIMAGE_INFO img_info;

    //Fill image info
    D3DXGetImageInfoFromFile("image.png", &img_info); //Image is 100x100px

    //Define the rects
    SetRect(&parent_bounds, 0, 0, 200, 200);
    SetRect(&draw_rect, 0, 0, img_info.Width, img_info.Height); //Draw the entire image by default

    //Give it a position that makes a portion go out of parent bounds
    sprite_position = D3DXVECTOR2(150, 150); // 3/4 of the entire image is out of bounds

    //Check if completely within bounds
    bool min_x = (sprite_position.x > parent_bounds.left),
         min_y = (sprite_position.y > parent_bounds.top),
         max_x = (sprite_position.x+img_info.Width < parent_bounds.right),
         max_y = (sprite_position.y+img_info.Height < parent_bounds.bottom);

    if(min_x && min_y && max_x && max_y) 
         return; //No clipping needs to be done

    float delta_top = parent_bounds.top - sprite_position.y,
          delta_left = parent_bounds.left - sprite_position.x,    
          delta_bottom = parent_bounds.bottom - sprite_position.y,
          delta_right = parent_bounds.right - sprite_position.x;

    //Check if completely outside bounds
    bool out_left = (delta_left >= img_info.Width),
         out_top = (delta_top >= img_info.Height),
         out_bottom = (delta_bottom >= img_info.Height),
         out_right = (delta_right >= img_info.Width);

    if(out_left || out_top || out_bottom || out_right){ 
         //No drawing needs to be done, it's not even visible
         return; //No clipping
    }

    if(!min_x) draw_rect.left = delta_left;

    if(!min_x) draw_rect.top = delta_top;

    if(!max_x) draw_rect.right = delta_right;

    if(!max_y) draw_rect.bottom = delta_bottom;

    return;
}

My questions are:

1. Do you see anything immediately wrong with the code?
2. Is it efficient or inefficient?
   The clipping is done every time the sprite is repositioned, loaded or when a parent is added.
3. Is the math solid? Should I approach it differently?
4. Could it be done in a better way and could you provide examples?

Answer 1

Clipping a RECT is a lot easier than this:

clipped_rect.left= MAX(rect_a.left, rect_b.left);
clipped_rect.right= MIN(rect_a.right, rect_b.right);
clipped_rect.top= MIN(rect_a.top, rect_b.top);
clipped_rect.bottom= MAX(rect_a.bottom, rect_b.bottom);

bool completely_clipped= 
    clipped_rect.left>=clipped_rect.right || 
    clipped_rect.bottom>=clipped_rect.top

This assumes Y increases as you go up. It should be pretty easy to find a platform specific version of MIN and MAX that are performant.