Cropping and Positioning Images

Detecting Image Clipping in SDL2

How do I check if an image is being clipped when rendered?

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Detecting if an image is being clipped when rendered in SDL2 involves comparing the dimensions of the source rectangle (the part of the image we're trying to render) with the dimensions of the area that was actually rendered.

SDL2 helps us do this by modifying the destination rectangle after SDL_BlitSurface() is called. Let's see how we can implement this:

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  bool Render(SDL_Surface* destSurface, int x,
16              int y) {
17    if (!surface || !destSurface) return false;
18
19    SDL_Rect srcRect{
20      0, 0, surface->w, surface->h};
21    SDL_Rect destRect{x, y, 0, 0};
22    // w and h will be set by SDL_BlitSurface
23
24    SDL_BlitSurface(surface, &srcRect,
25                    destSurface, &destRect);
26
27    // Check for clipping
28    bool clipped = IsClipped(srcRect, destRect);
29
30    if (clipped) {
31      std::cout <<
32        "Image was clipped during rendering.\n";
33    } else {
34      std::cout <<
35        "Image was fully rendered without clipping.\n";
36    }
37
38    return clipped;
39  }
40
41private:
42  SDL_Surface* surface;
43
44  bool IsClipped(const SDL_Rect& src,
45                 const SDL_Rect& dest) {
46    return (src.w != dest.w) || (src.h != dest.
47      h);
48  }
49};
50
51int main() {
52  SDL_Init(SDL_INIT_VIDEO);
53
54  SDL_Window* window =
55    SDL_CreateWindow("Clipping Test",
56                     SDL_WINDOWPOS_UNDEFINED,
57                     SDL_WINDOWPOS_UNDEFINED,
58                     640, 480, 0);
59  SDL_Surface* screenSurface =
60    SDL_GetWindowSurface(window);
61
62  Image img{"example.bmp"};
63
64  // Try rendering at different positions
65  img.Render(screenSurface, 0, 0);
66  // Likely not clipped
67  img.Render(screenSurface, 500, 400);
68  // Likely clipped
69
70  SDL_UpdateWindowSurface(window);
71
72  SDL_Delay(3000); // Display for 3 seconds
73
74  SDL_DestroyWindow(window);
75  SDL_Quit();
76
77  return 0;
78}

In this example, we've added an IsClipped() method to our Image class. This method compares the width and height of the source rectangle (what we're trying to render) with the destination rectangle (what actually got rendered).

The key part is in the Render() method:

1SDL_BlitSurface(surface, &srcRect, destSurface,
2                &destRect);
3
4// Check for clipping
5bool clipped = IsClipped(srcRect, destRect);

After calling SDL_BlitSurface(), the destRect is updated with the actual dimensions of the rendered area. If these dimensions don't match the source rectangle's dimensions, we know that clipping occurred.

In the main() function, we test this by rendering the image twice: once at (0, 0), which is likely to render fully, and once at (500, 400), which is likely to be clipped if the image is larger than 140x80 pixels.

This technique allows you to detect clipping and potentially take action, such as repositioning the image or notifying the user that not all content is visible.

This Question is from the Lesson: