[SDL] SDL Box2D Collision Contact Point & Normal Vector - Box2D 충돌 지점과 법선 벡터

오브젝트가 충돌한 포인트를 확인해 보자.

 

#include <iostream>
#include "SDL.h"
#include "box2d/box2d.h"
 
class DebugDrawer : public b2Draw
{
public:
    DebugDrawer(SDL_Renderer* renderer) {
        this->renderer = renderer;
    }
private:
    SDL_Renderer* renderer;
    void DrawSolidPolygon(const b2Vec2* vertices, int32 vertexCount, const b2Color& color) {}
    void DrawPoint(const b2Vec2& p, float size, const b2Color& color) {}
    void DrawPolygon(const b2Vec2* vertices, int32 vertexCount, const b2Color& color) {
        SDL_SetRenderDrawColor(renderer, 255, 0, 0, SDL_ALPHA_OPAQUE);
        SDL_FRect* rect = new SDL_FRect{ vertices[0].x, vertices[0].y, vertices[1].x
            - vertices[0].x, vertices[3].y - vertices[0].y };
        SDL_RenderDrawRectF(renderer, rect);
    }
    void DrawCircle(const b2Vec2& center, float radius, const b2Color& color) {}
    void DrawSolidCircle(const b2Vec2& center, float radius, const b2Vec2& axis, const b2Color& color) {}
    void DrawSegment(const b2Vec2& p1, const b2Vec2& p2, const b2Color& color) {}
    void DrawTransform(const b2Transform& xf) {}
};
 
int main(int argc, char* argv[]) {
    SDL_Init(SDL_INIT_VIDEO);
    SDL_Window* window = SDL_CreateWindow("SDL Test", SDL_WINDOWPOS_UNDEFINED,
        SDL_WINDOWPOS_UNDEFINED, 640, 480, SDL_WINDOW_RESIZABLE);
    SDL_Renderer* renderer = SDL_CreateRenderer(window, -1, 0);
    SDL_Surface* playerSurface = SDL_LoadBMP("player.bmp");
    SDL_Texture* texture = SDL_CreateTextureFromSurface(renderer, playerSurface);
 
    // World 정의
    b2Vec2 gravity(0.0f, 9.8f);
    b2World world(gravity);
 
    // DebugDraw 정의
    DebugDrawer* debugDrawer = new DebugDrawer(renderer);
    debugDrawer->SetFlags(b2Draw::e_shapeBit | b2Draw::e_jointBit |
        b2Draw::e_centerOfMassBit | b2Draw::e_aabbBit | b2Draw::e_pairBit);
    world.SetDebugDraw(debugDrawer);
 
    // Player 정의
    b2BodyDef playerBodyDef;
    playerBodyDef.type = b2_dynamicBody;
    playerBodyDef.position.Set(0.0f, 0.0f);
    playerBodyDef.linearVelocity = b2Vec2(50.0f, 0.0f);
    playerBodyDef.angularVelocity = 0.2f;
    b2Body* playerBody = world.CreateBody(&playerBodyDef);
    b2PolygonShape dynamicBox;
    dynamicBox.SetAsBox((float)playerSurface->w / 2, (float)playerSurface->h / 2);
    b2FixtureDef fixtureDef;
    fixtureDef.shape = &dynamicBox;
    fixtureDef.density = 1.0f;
    fixtureDef.friction = 0.3f;
    fixtureDef.restitution = 0.5f;
    playerBody->CreateFixture(&fixtureDef);
 
    // Ground 정의
    b2BodyDef groundBodyDef;
    groundBodyDef.position.Set(0.0f, 400.0f);
    b2Body* groundBody = world.CreateBody(&groundBodyDef);
    b2PolygonShape groundBox;
    groundBox.SetAsBox(500.0f, 0.0f);
    SDL_Rect groundRect = { 0, 400, 500, 10 };
    groundBody->CreateFixture(&groundBox, 0.0f);
 
    // Wall 정의
    b2BodyDef wallBodyDef;
    wallBodyDef.position.Set(300.0f, 0.0f);
    b2Body* wallBody = world.CreateBody(&wallBodyDef);
    b2PolygonShape wallBox;
    wallBox.SetAsBox(0.0f, 480.0f);
    SDL_Rect wallRect = { 300, 0, 10, 480 };
    wallBody->CreateFixture(&wallBox, 0.0f);
 
    float timeStep = 1.0f / 500.0f;
    int velocityIterations = 6;
    int positionIterations = 2;
 
    SDL_Event event;
    bool quit = false;
 
    while (!quit) {
        while (SDL_PollEvent(&event)) {
            switch (event.type) {
            case SDL_QUIT:
                quit = true;
                break;
            case SDL_KEYDOWN:
                printf("Key pressed: %s\n", SDL_GetKeyName(event.key.keysym.sym));
                if (event.key.keysym.sym == SDLK_SPACE) {
                    playerBody->SetTransform(b2Vec2(0.0f, 0.0f), 0.0f);
                    playerBody->SetLinearVelocity(b2Vec2(50.0f, 0.0f));
                    playerBody->SetAngularVelocity(0.2f);
                }
                if (event.key.keysym.sym == SDLK_ESCAPE)
                    quit = true;
                break;
 
            default:
                break;
            }
        }
 
        world.Step(timeStep, velocityIterations, positionIterations);
 
        SDL_SetRenderDrawColor(renderer, 255, 255, 255, SDL_ALPHA_OPAQUE);
        SDL_RenderClear(renderer);
 
        // Ground 그리기
        SDL_SetRenderDrawColor(renderer, 220, 140, 20, SDL_ALPHA_OPAQUE);
        SDL_RenderFillRect(renderer, &groundRect);
 
        // Wall 그리기
        SDL_SetRenderDrawColor(renderer, 64, 64, 64, SDL_ALPHA_OPAQUE);
        SDL_RenderFillRect(renderer, &wallRect);
 
        // Player 그리기        
        b2Vec2 playerPosition = playerBody->GetPosition();
        SDL_Rect destRect = { (int)playerPosition.x - playerSurface->w / 2,
            (int)playerPosition.y - playerSurface->h / 2, playerSurface->w, playerSurface->h };
        SDL_RendererFlip flip = SDL_FLIP_NONE;
        float angle = playerBody->GetAngle() * (180 / (float)M_PI);
        SDL_RenderCopyEx(renderer, texture, NULL, &destRect, angle, NULL, flip);
 
        // DebugDraw 그리기
        world.DebugDraw();
 
        // 가장 마지막에 contact point 그리기
        SDL_SetRenderDrawColor(renderer, 0, 255, 0, SDL_ALPHA_OPAQUE);
        for (b2Contact* contact = world.GetContactList(); contact; contact = contact->GetNext()) {
            for (int i = 0; i < contact->GetManifold()->pointCount; i++)
            {
                b2Vec2 point = contact->GetFixtureA()->GetBody()->GetWorldPoint(
                    contact->GetManifold()->points[i].localPoint);
                SDL_FRect rect = { point.x - 5, point.y - 5, 10, 10 };
                SDL_RenderFillRectF(renderer, &rect);
            }
        }
        // A very important point to note if you do this, is that the existence of a contact in these
        // lists does not mean that the two fixtures of the contact are actually touching - it only means
        // their AABBs are touching. If you want to know if the fixtures themselves are really touching
        // you can use IsTouching() to check.
 
        SDL_RenderPresent(renderer);
    }
 
    delete debugDrawer;
 
    SDL_DestroyTexture(texture);
    SDL_FreeSurface(playerSurface);
    SDL_DestroyRenderer(renderer);
    SDL_DestroyWindow(window);
    SDL_Quit();
 
    return 0;
}

 

코드를 입력하고 빌드한다.

 

충돌 포인트가 녹색 사각형으로 표시된다.

 

완전히 멈추면 두 개의 포인트가 표시된다.

 

 

이제 검은색으로 법선 벡터 그리는 코드를 추가하자.

 

// 가장 마지막에 contact point & 법선 벡터 그리기
for (b2Contact* contact = world.GetContactList(); contact; contact = contact->GetNext()) {
    for (int i = 0; i < contact->GetManifold()->pointCount; i++)
    {
        b2Vec2 point = contact->GetFixtureA()->GetBody()->GetWorldPoint(
            contact->GetManifold()->points[i].localPoint);
        SDL_FRect rect = { point.x - 5, point.y - 5, 10, 10 };
        SDL_SetRenderDrawColor(renderer, 0, 255, 0, SDL_ALPHA_OPAQUE);
        SDL_RenderFillRectF(renderer, &rect);
 
        SDL_SetRenderDrawColor(renderer, 0, 0, 0, SDL_ALPHA_OPAQUE);
        SDL_RenderDrawLineF(renderer, point.x, point.y,
            point.x + contact->GetManifold()->localNormal.x * 20, point.y + contact->GetManifold()->localNormal.y * 20);
    }
}
// A very important point to note if you do this, is that the existence of a contact in these
// lists does not mean that the two fixtures of the contact are actually touching - it only means
// their AABBs are touching. If you want to know if the fixtures themselves are really touching
// you can use IsTouching() to check.

 

코드를 수정하고 빌드한다.

 

녹색 충돌 포인트 위에 검은색 법선 벡터가 그려진다.

 

 

 

속도(velocity) 벡터는 아래와 같은 코드로 표시할 수 있다.

 

SDL_SetRenderDrawColor(renderer, 255, 0, 0, SDL_ALPHA_OPAQUE);
b2Vec2 position = playerBody->GetPosition();
SDL_RenderDrawLineF(renderer, position.x, position.y,
    position.x + playerBody->GetLinearVelocity().x * 10, position.y + playerBody->GetLinearVelocity().y * 10);

 

 

 

 

※ 참고

Box2D b2ContactListener