Question

我正在使用SDL2在C中编写一个游戏引擎，我正在尝试使用bit twiddling来存储我当前的移动方向。
我不确定如何重现它，但这是一个显示错误的视频：
https://youtu.be/RnPZaUDPElU

这是有问题的代码：

#define BMXSPEED 0x01
#define BMYSPEED 0x04

void BEvent (btree_t *t) {
    SDL_Event event;
    while (SDL_PollEvent(&event)) {
        switch (event.type) {
        case SDL_QUIT:
            t->run = 0;
            break;
        case SDL_KEYDOWN:
            switch (event.key.keysym.scancode) {
            case SDL_SCANCODE_ESCAPE:
                t->run = 0;
                break;
            case SDL_SCANCODE_D:
                if (!event.key.repeat) {
                    t->player->speed |= BMXSPEED;
                }
                break;
            case SDL_SCANCODE_A:
                if (!event.key.repeat) {
                    t->player->speed |= (BMXSPEED << 1);
                }
                break;
            case SDL_SCANCODE_S:
                if (!event.key.repeat) {
                    t->player->speed |= BMYSPEED;
                }
                break;
            case SDL_SCANCODE_W:
                if (!event.key.repeat) {
                    t->player->speed |= (BMYSPEED << 1);
                }
                break;
            }
            break;
        case SDL_KEYUP:
            switch (event.key.keysym.scancode) {
            case SDL_SCANCODE_D:
                t->player->speed &= ~BMXSPEED;
                break;
            case SDL_SCANCODE_A:
                t->player->speed &= ~(BMXSPEED << 1);
                break;
            case SDL_SCANCODE_S:
                t->player->speed &= ~BMYSPEED;
                break;
            case SDL_SCANCODE_W:
                t->player->speed &= ~(BMYSPEED << 1);
                break;
            }
            break;
        }
    }

    /* X movement */
    if (t->player->speed & BMXSPEED && (t->player->speed & (BMXSPEED << 1)) != (BMXSPEED << 1))
        t->player->xspeed += (t->player->xspeedMax - abs(t->player->xspeed)) / t->player->xspeedMax;
    else
    if (t->player->speed & (BMXSPEED << 1) && (t->player->speed & BMXSPEED) != BMXSPEED)
        t->player->xspeed -= (t->player->xspeedMax - abs(t->player->xspeed)) / t->player->xspeedMax;
    else
        t->player->xspeed += (t->player->xspeedMin - t->player->xspeed) / t->player->xspeedMax;

    /* Y movement */
    if (t->player->speed & BMYSPEED && (t->player->speed & (BMYSPEED << 1)) != (BMYSPEED << 1))
        t->player->yspeed += (t->player->yspeedMax - abs(t->player->yspeed)) / t->player->yspeedMax;
    else
    if (t->player->speed & (BMYSPEED << 1) && (t->player->speed & BMYSPEED) != BMYSPEED)
        t->player->yspeed -= (t->player->yspeedMax - abs(t->player->yspeed)) / t->player->yspeedMax;
    else
        t->player->yspeed += (t->player->yspeedMin - t->player->yspeed) / t->player->yspeedMax;

    t->player->rect.x += (int)t->player->xspeed;
    t->player->rect.y += (int)t->player->yspeed;
}

编辑：改变存储方向的方法后，我确定问题与bit twiddling无关。我怀疑它是加速方法。

Answer 1

你的加速方法有点模糊（双关语），这里有2条建议：

可以将位测试简化为单个测试，用2位屏蔽播放器speed并检查单个位。
减速情况也应该使用abs功能，并调整减速情况。
如果fabs()和abs()成员是浮点值，xspeed函数似乎比yspeed更合适。
为播放器使用中间指针可提高可读性：

以下是生成的代码：

    Player *p = t->player;

    /* X movement */
    if (p->speed & (BMXSPEED * 3) == BMXSPEED)
        p->xspeed += (p->xspeedMax - fabs(p->xspeed)) / p->xspeedMax;
    else
    if (p->speed & (BMXSPEED * 3) == BMXSPEED << 1)
        p->xspeed -= (p->xspeedMax - fabs(p->xspeed)) / p->xspeedMax;
    else
        p->xspeed -= copysign((p->xspeedMin - fabs(p->xspeed)) / p->xspeedMax, p->xspeed);

    /* Y movement */
    if (p->speed & (BMYSPEED * 3) == BMYSPEED)
        p->yspeed += (p->yspeedMax - fabs(p->yspeed)) / p->yspeedMax;
    else
    if (p->speed & (BMYSPEED * 3) == BMYSPEED << 1)
        p->yspeed -= (p->yspeedMax - fabs(p->yspeed)) / p->yspeedMax;
    else
        p->yspeed -= copysign((p->yspeedMin - fabs(p->yspeed)) / p->yspeedMax, p->yspeed);

为什么玩家在切换方向时有时会向错误的方向移动

1 个答案: