Using Negative Coordinates in SDL2

Setting negative values for x and y in the destination rectangle in SDL2 can lead to interesting and sometimes unexpected results. Let's explore what happens and how we can use this knowledge:

1#include <SDL.h>
2
3#include <iostream>
4
5class Image {
6public:
7  Image(const char* file) : surface{
8    SDL_LoadBMP(file)} {
9    if (!surface) {
10      std::cerr << "Failed to load image: " <<
11        SDL_GetError() << '\n';
12    }
13  }
14
15  void Render(SDL_Surface* destSurface, int x,
16              int y) {
17    if (!surface || !destSurface) return;
18
19    SDL_Rect srcRect{
20      0, 0, surface->w, surface->h};
21    SDL_Rect destRect{
22      x, y, surface->w, surface->h};
23
24    SDL_BlitSurface(surface, &srcRect,
25                    destSurface, &destRect);
26
27    std::cout << "Rendered area: x=" << destRect
28      .x << ", y=" << destRect.y
29      << ", w=" << destRect.w << ", h=" <<
30      destRect.h << '\n';
31  }
32
33private:
34  SDL_Surface* surface;
35};
36
37int main() {
38  SDL_Init(SDL_INIT_VIDEO);
39
40  SDL_Window* window =
41    SDL_CreateWindow(
42      "Negative Coordinates Test",
43      SDL_WINDOWPOS_UNDEFINED,
44      SDL_WINDOWPOS_UNDEFINED, 640, 480, 0);
45  SDL_Surface* screenSurface =
46    SDL_GetWindowSurface(window);
47
48  Image img{"example.bmp"};
49
50  // Test different coordinates
51  img.Render(screenSurface, 0, 0);
52  // Normal rendering
53  img.Render(screenSurface, -50, -50);
54  // Partially off-screen
55  img.Render(screenSurface, -200, -200);
56  // Mostly off-screen
57
58  SDL_UpdateWindowSurface(window);
59
60  SDL_Delay(3000); // Display for 3 seconds
61
62  SDL_DestroyWindow(window);
63  SDL_Quit();
64
65  return 0;
66}

When you set negative values for x and y in the destination rectangle, SDL2 will attempt to render the image at those coordinates. This means part of the image may be rendered off-screen. Here's what happens in different scenarios:

1. Partially off-screen: If you set x or y to a small negative value (like -50), part of the image will be rendered off-screen, and only the visible portion will appear on the window.
2. Mostly or entirely off-screen: If you set x or y to a large negative value (like -200 for a 100x100 image), the entire image might be rendered off-screen, resulting in nothing visible on the window.
3. Clipping: SDL2 automatically clips the rendering to the boundaries of the destination surface. This means you won't cause any errors or crashes by using negative coordinates - SDL2 will simply not render the portions that fall outside the destination surface.

The Render() method in our example prints out the actual rendered area after blitting. This can help you understand what's happening:

1// Fully visible
2Rendered area: x=0, y=0, w=100, h=100
3
4// Partially visible
5Rendered area: x=0, y=0, w=50, h=50          
6
7// Not visible at all
8Rendered area: x=0, y=0, w=0, h=0

Using negative coordinates can be useful in several scenarios:

• Creating sliding or scrolling effects where images gradually enter the screen.
• Implementing "overflow" rendering for UI elements that extend beyond their container.
• Creating special visual effects where only part of an image is visible.

Remember that while SDL2 handles negative coordinates gracefully, it's generally a good practice to check your rendering coordinates to ensure they produce the desired visual effect and don't waste resources rendering invisible content.