找到两个精灵的碰撞点

Question

我正在使用 C# 和 XNA 开发一款 2D 游戏，并且我已经成功地通过 sprite 工作的逐像素碰撞检测。现在让我困惑的挑战是如何计算两个精灵碰撞的位置。

我需要这些信息的原因是，这款游戏的本质要求对象具有旋转性，并且根据物理原理做出一定程度的准确反应。我目前的测试环境涉及两个方块。任何关于如何找到联络点的建议都会很受欢迎。

另外，如果有人对如何计算碰撞时移动精灵的距离有任何建议，这样它们就不会重叠，那也很好。

我用于碰撞检测的代码来自这里：

http://xbox.create.msdn.com/en-US/education/catalog/tutorial/collision_2d_perpixel_transformed

谢谢大家！

Answer 1

关于获取接触点的第一部分，您链接到的代码已经完成了艰苦的工作，而不是返回 true，而是通过转换回世界空间来返回坐标。

这是来自http://xbox.create.msdn.com/en-US/education/catalog/tutorial/collision_2d_perpixel_transformed的稍作修改的代码

public static IEnumerable<Vector2> IntersectPixels(
                    Matrix transformA, int widthA, int heightA, Color[] dataA,
                    Matrix transformB, int widthB, int heightB, Color[] dataB)
{
    // Calculate a matrix which transforms from A's local space into
    // world space and then into B's local space
    Matrix transformAToB = transformA * Matrix.Invert(transformB);

    // When a point moves in A's local space, it moves in B's local space with a
    // fixed direction and distance proportional to the movement in A.
    // This algorithm steps through A one pixel at a time along A's X and Y axes
    // Calculate the analogous steps in B:
    Vector2 stepX = Vector2.TransformNormal(Vector2.UnitX, transformAToB);
    Vector2 stepY = Vector2.TransformNormal(Vector2.UnitY, transformAToB);

    // Calculate the top left corner of A in B's local space
    // This variable will be reused to keep track of the start of each row
    Vector2 yPosInB = Vector2.Transform(Vector2.Zero, transformAToB);

    // For each row of pixels in A
    for(int yA = 0; yA < heightA; yA++)
    {
        // Start at the beginning of the row
        Vector2 posInB = yPosInB;

        // For each pixel in this row
        for(int xA = 0; xA < widthA; xA++)
        {
            // Round to the nearest pixel
            int xB = (int)Math.Round(posInB.X);
            int yB = (int)Math.Round(posInB.Y);

            // If the pixel lies within the bounds of B
            if(0 <= xB && xB < widthB &&
                0 <= yB && yB < heightB)
            {
                // Get the colors of the overlapping pixels
                Color colorA = dataA[xA + yA * widthA];
                Color colorB = dataB[xB + yB * widthB];

                // If both pixels are not completely transparent,
                if(colorA.A != 0 && colorB.A != 0)
                {
                    // then an intersection has been found
                    yield return Vector2.Transform(new Vector2(xA, yA),transformA);
                }
            }

            // Move to the next pixel in the row
            posInB += stepX;
        }

        // Move to the next row
        yPosInB += stepY;
    }

    // No intersection found
}

至于第二部分，常用的方法是加一个与碰撞方向相反的小力将它们击退。这个关于游戏物理的article 是一本很好的入门书，很少有现成的物理引擎像Farseer 这样强大。

代码示例用于转换后的精灵，如果您不需要此功能，您可以简化代码。如果您在移动一个时不使用物理引擎来防止它们重叠，则可能需要移动另一个，依此类推，物理引擎会为您解决这个问题。

编辑： 这是对 MSDN 示例的一些小改动，因此每个接触点都用绿色像素绘制。

添加这些字段

//Contact points are cleared and re-added each update
List<Vector2> contactPoints = new List<Vector2>();
//Texture for contact display
Texture2D pixelTex;

在某处添加到LoadContent()

pixelTex = new Texture2D(GraphicsDevice, 1, 1);
pixelTex.SetData<Color>(new[] { Color.White });

用这个替换Update()的结尾

// Update each block
personHit = false;
contactPoints.Clear();
for(int i = 0; i < blocks.Count; i++)
{
    // Animate this block falling
    blocks[i].Position += new Vector2(0.0f, BlockFallSpeed);
    blocks[i].Rotation += BlockRotateSpeed;

    // Build the block's transform
    Matrix blockTransform =
        Matrix.CreateTranslation(new Vector3(-blockOrigin, 0.0f)) *
        // Matrix.CreateScale(block.Scale) *  would go here
        Matrix.CreateRotationZ(blocks[i].Rotation) *
        Matrix.CreateTranslation(new Vector3(blocks[i].Position, 0.0f));

    // Calculate the bounding rectangle of this block in world space
    Rectangle blockRectangle = CalculateBoundingRectangle(
                new Rectangle(0, 0, blockTexture.Width, blockTexture.Height),
                blockTransform);

    // The per-pixel check is expensive, so check the bounding rectangles
    // first to prevent testing pixels when collisions are impossible.
    if(personRectangle.Intersects(blockRectangle))
    {
        contactPoints.AddRange(IntersectPixels(personTransform, personTexture.Width,
                            personTexture.Height, personTextureData,
                            blockTransform, blockTexture.Width,
                            blockTexture.Height, blockTextureData));
        // Check collision with person
        if(contactPoints.Count != 0)
        {
            personHit = true;
        }
    }

    // Remove this block if it have fallen off the screen
    if(blocks[i].Position.Y >
        Window.ClientBounds.Height + blockOrigin.Length())
    {
        blocks.RemoveAt(i);

        // When removing a block, the next block will have the same index
        // as the current block. Decrement i to prevent skipping a block.
        i--;
    }
}

base.Update(gameTime);

在 spriteBatch.End() 之前添加到Draw()

foreach(Vector2 p in contactPoints)
{
    spriteBatch.Draw(pixelTex, new Rectangle((int)p.X, (int)p.Y, 1, 1), Color.FromNonPremultiplied(120, 255, 100, 255));
}