Cropping and Positioning Images

The fourth argument to SDL_BlitSurface() is an SDL_Rect representing where we want the image to be blitted on our destination surface. The x and y properties specify where we want the image positioned within the destination surface.

The w and h are used a little differently in the context of a destination rectangle. We'll set them to 0 for now, and talk about them in more detail later.

To replicate the default behavior of SDL_BlitSurface(), we'd set the x and y values to 0, too. This places the image at the top left edge of the surface:

1// Image.h
2class Image {
3public:
4  // ...
5
6  void Render(SDL_Surface* DestinationSurface) {
7    SDL_BlitSurface(
8      ImageSurface, &SourceRectangle,
9      DestinationSurface, &DestinationRectangle
10    );
11  }
12  
13  // ...
14
15private:
16  SDL_Surface* ImageSurface;
17  SDL_Rect SourceRectangle{
18    0, 0, ImageSurface->w, ImageSurface->h};
19  SDL_Rect DestinationRectangle{
20    0, 0, 0, 0};
21};

To move the image away from the left edge, we'd update the x and y components of the destination rectangle accordingly:

1// Image.h
2#pragma once
3#include <iostream>
4#include <SDL.h>
5#include <string>
6
7class Image {
8 //...
9 private:
10  SDL_Surface* ImageSurface;
11  SDL_Rect SourceRectangle{
12    0, 0, ImageSurface->w, ImageSurface->h};
13  SDL_Rect DestinationRectangle{
14    50, 100, 0, 0};
15};

As we can see in the previous examples, we sometimes copy fewer pixels than our source rectangle contains. Clipping occurs when SDL needs to adjust what's being rendered. This happens in two main scenarios:

Source clipping: When the source rectangle extends beyond the boundaries of the source image. In this case, SDL2 only renders the part of the rectangle that's within the image.

Destination clipping: When the image (or part of it) would be rendered outside the boundaries of the destination surface. This can happen if:

• The destination position would place the image partially or fully off-screen.
• The image is larger than the destination surface.

In both cases, SDL automatically adjusts the rendering to show only the visible portion of the image.

If we need to detect and react to clipping, we can examine our destination rectangle after the call to SDL_BlitSurface(). Whilst SDL_BlitSurface() receives the source rectangle as a const SDL_Rect*, the destination rectangle parameter is simply an SDL_Rect*.

This indicates that SDL_BlitSurface() is going to modify our destination rectangle, and that's indeed the case. Specifically, SDL_BlitSurface() updates the w and h properties of the destination rectangle with the dimensions of the area that was blitted, after clipping:

1// Image.h
2class Image {
3public:
4  // ...
5
6  void Render(SDL_Surface* DestinationSurface) {
7    SDL_BlitSurface(
8      ImageSurface, &SourceRectangle,
9      DestinationSurface, &DestinationRectangle
10    );
11
12    std::cout << "\nDestination Width: "
13      << DestinationRectangle.w
14      << ", Height:"
15      << DestinationRectangle.h;
16  }
17
18  // ...
19};

1Destination Width: 512, Height:200
2Destination Width: 512, Height:200
3Destination Width: 512, Height:200
4...

We can compare the sizes of our source and destination rectangles to understand if clipping is happening:

1// Image.h
2class Image {
3public:
4  // ...
5
6  void Render(SDL_Surface* DestinationSurface) {
7    SDL_BlitSurface(
8      ImageSurface, &SourceRectangle,
9      DestinationSurface, &DestinationRectangle
10    );
11
12    if (DestinationRectangle.w !=
13      SourceRectangle.w) {
14      std::cout << "Clipping Horizontally\n";
15    }
16
17    if (DestinationRectangle.h !=
18      SourceRectangle.h) {
19      std::cout << "Clipping Vertically\n";
20    }
21  }
22
23  // ...
24};

1Clipping Vertically
2Clipping Vertically
3Clipping Vertically
4...

Whilst SDL can handle source and destination rectangles being invalid in this way, it's generally not something we want to rely on. Proactively taking steps to keep our object's state accurate makes adding more powerful capabilities.

For example, adding more capabilities to this Image class may want to use the SourceRectangle, and that becomes more complex if we can't assume the SourceRectangle is accurate.

Later in this chapter, we'll extend our Image class to add this and additional capabilities.

In this lesson, we've explored several key concepts for manipulating images in SDL2:

• Source and Destination Rectangles: We learned how to use SDL_Rect structures to control which parts of an image are rendered (source) and where they appear on screen (destination).
• Cropping: We practiced selecting specific portions of an image by adjusting the source rectangle's dimensions and position.
• Positioning: We discovered how to place images at different locations on the screen by modifying the destination rectangle's x and y coordinates.
• Clipping: We understood how SDL2 handles situations where images don't fit entirely on the screen and learned to detect when clipping occurs.

These techniques form the foundation for more advanced image manipulation in SDL2. Later in this section, we'll use this knowledge to expand our Image class with new capabilities. First, though, we'll learn some more topics that will influence how we design that API.

In the next lesson, we'll see how we can control the size of our rendering, by blitting a larger or smaller copy of our image onto the destination surface.