Cropping and Positioning Images

Image Rotation in SDL2 with Blitting

Is it possible to rotate an image using SDL2's blitting functions?

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Unfortunately, it's not possible to rotate an image using SDL2's basic blitting functions like SDL_BlitSurface(). These functions are designed for simple copying of rectangular areas from one surface to another, without any transformations like rotation.

However, SDL2 does provide other ways to achieve image rotation. The most common approach is to use SDL2's rendering API, which includes the SDL_RenderCopyEx() function. This function allows for rotation, scaling, and flipping of textures.

Here's an example of how you can rotate an image using SDL2's rendering API:

1#include <SDL.h>
2
3#include <iostream>
4
5class RotatableImage {
6public:
7  RotatableImage(SDL_Renderer* renderer,
8                 const char* file)
9    : renderer{renderer}, texture{nullptr},
10      angle{0.0} {
11    SDL_Surface* surface = SDL_LoadBMP(file);
12    if (!surface) {
13      std::cerr << "Failed to load image: " <<
14        SDL_GetError() << '\n';
15      return;
16    }
17
18    texture = SDL_CreateTextureFromSurface(
19      renderer, surface);
20    SDL_FreeSurface(surface);
21
22    if (!texture) {
23      std::cerr << "Failed to create texture: "
24        << SDL_GetError() << '\n';
25    }
26  }
27
28  void Render(int x, int y) {
29    if (!texture) return;
30
31    SDL_Rect destRect{x, y, 100, 100};
32    // Adjust size as needed
33    SDL_RenderCopyEx(renderer, texture, nullptr,
34                     &destRect, angle, nullptr,
35                     SDL_FLIP_NONE);
36  }
37
38  void Rotate(double degrees) {
39    angle += degrees;
40    if (angle >= 360.0) angle -= 360.0;
41    else if (angle < 0.0) angle += 360.0;
42  }
43
44  ~RotatableImage() {
45    if (texture) SDL_DestroyTexture(texture);
46  }
47
48private:
49  SDL_Renderer* renderer;
50  SDL_Texture* texture;
51  double angle;
52};
53
54int main() {
55  SDL_Init(SDL_INIT_VIDEO);
56
57  SDL_Window* window =
58    SDL_CreateWindow("Rotating Image",
59                     SDL_WINDOWPOS_UNDEFINED,
60                     SDL_WINDOWPOS_UNDEFINED,
61                     640, 480, 0);
62  SDL_Renderer* renderer =
63    SDL_CreateRenderer(window, -1,
64                       SDL_RENDERER_ACCELERATED);
65
66  RotatableImage img{renderer, "example.bmp"};
67
68  for (int i = 0; i < 360; ++i) {
69    SDL_SetRenderDrawColor(renderer, 255, 255,
70                           255, 255);
71    SDL_RenderClear(renderer);
72
73    img.Rotate(1.0);
74    // Rotate by 1 degree each frame
75    img.Render(320, 240);
76    // Render at center of 640x480 window
77
78    SDL_RenderPresent(renderer);
79    SDL_Delay(10);
80    // 10ms delay to slow down rotation
81  }
82
83  SDL_DestroyRenderer(renderer);
84  SDL_DestroyWindow(window);
85  SDL_Quit();
86
87  return 0;
88}

In this example, we've created a RotatableImage class that uses SDL2's rendering API. The key differences from our previous surface-based approach are:

We create a texture from our loaded surface.
We use SDL_RenderCopyEx() instead of SDL_BlitSurface().
We keep track of the rotation angle.

The SDL_RenderCopyEx() function allows us to specify a rotation angle, which is what enables the rotation effect.

Note that this approach requires using SDL2's renderer, which is generally more flexible and performant than surface blitting, especially for operations like rotation.

Remember, when working with rotation, you might need to adjust your positioning logic to account for the image's pivot point and potential changes in size due to rotation.

This Question is from the Lesson: