为什么每当玩家靠近墙壁时，光线投射器就会变慢？

Question

我正在编写一个 2.5d 游戏，我注意到每当我靠近并面对墙壁时，游戏速度就会大大减慢。当我添加纹理时，这种情况更加严重。我正在使用一种低效的渲染方法，并研究了如何改进它，所以这个问题不是关于如何使其更快。我最初的想法是，这只是因为屏幕上有更多内容。这是部分正确的，但是我确保渲染器会剪掉屏幕上没有的任何内容。我还添加了一个检查来查看每帧绘制了多少像素（矩形）。当我这样做时，我注意到随着我越来越近，即使矩形数量完全相同，我仍然会变得越来越慢。为什么即使绘制相同数量的矩形，此代码也会变慢？

int drawStart = y_offset > 0 ? y_offset : 0;
int drawEnd = (y_offset + bar_height) < 480 ? y_offset + bar_height : 480;
int texNum = map[cell_y][cell_x] - 49;

//calculate value of wallX
double wallX; //where exactly the wall was hit
if (side == 0) wallX = player->y + distance * ray_angle_y;
else           wallX = player->x + distance * ray_angle_x;
wallX -= floor(wallX);

//x coordinate on the texture
int texX = wallX * TEXTURE_WIDTH;
if (side == 0 && ray_angle_x > 0) texX = TEXTURE_WIDTH - texX - 1;
if (side == 1 && ray_angle_y < 0) texX = TEXTURE_WIDTH - texX - 1;

// How much to increase the texture coordinate per screen pixel
double step = 1.0 * TEXTURE_HEIGHT / bar_height;
double pixel_size = bar_height / TEXTURE_HEIGHT;
// Starting texture coordinate
double texPos = bar_height > 480 ? (-y_offset * step) : 0;
for(int y = drawStart; y < drawEnd; y++) {
    // Cast the texture coordinate to integer, and mask with (TEXTURE_HEIGHT - 1) in case of overflow
    int texY = (int)texPos & (TEXTURE_HEIGHT - 1);
    texPos += step;
    Uint32 color = textures[texNum].array[texY][texX];
    if(side == 1) color = (color >> 1) & 8355711;
    
    
    SDL_Rect rect = {i * rect_width, y, rect_width, pixel_size};
 /* even when this is called the same amount of times, it will be slower depending on how close to the wall the player is*/
    SDL_FillRect( screenSurface, &rect, color );
}

Answer 1

我按照@Ry-的建议尝试在程序上使用分析器。通过这样做，我能够追踪到有问题的代码：

double pixel_size = bar_height / TEXTURE_HEIGHT;
// ...    
SDL_Rect rect = {i * rect_width, y, rect_width, pixel_size};

我将 Pixel_size 计算为常量值，这是不正确的。它需要根据距墙壁的距离而增大或缩小。离墙越近，纹理的每个“像素”就应该越大，但这使它们超出了预期的长度。