Per-pixel collision problem in C#

Question

I am writing a small 2d game engine in C# for my own purposes, and it works fine except for the sprite collision detection. I've decided to make it a per-pixel detection (easiest for me to implement), but it is not working the way it's supposed to. The code detects a collision long before it happens. I've examined every component of the detection, but I can't find the problem.

The collision detection method:

public static bool CheckForCollision(Sprite s1, Sprite s2, bool perpixel) {
 if(!perpixel) {
  return s1.CollisionBox.IntersectsWith(s2.CollisionBox);
 }
 else {
  Rectangle rect;
  Image img1 = GraphicsHandler.ResizeImage(GraphicsHandler.RotateImagePoint(s1.Image, s1.Position, s1.Origin, s1.Rotation, out rect), s1.Scale);
  int posx1 = rect.X;
  int posy1 = rect.Y;

  Image img2 = GraphicsHandler.ResizeImage(GraphicsHandler.RotateImagePoint(s2.Image, s2.Position, s2.Origin, s2.Rotation, out rect), s2.Scale);
  int posx2 = rect.X;
  int posy2 = rect.Y;

  Rectangle abounds = new Rectangle(posx1, posy1, (int)img1.Width, (int)img1.Height);
  Rectangle bbounds = new Rectangle(posx2, posy2, (int)img2.Width, (int)img2.Height);

  if(Utilities.RectangleIntersects(abounds, bbounds)) {

   uint[] bitsA = s1.GetPixelData(false);

   uint[] bitsB = s2.GetPixelData(false);

   int x1 = Math.Max(abounds.X, bbounds.X);
   int x2 = Math.Min(abounds.X + abounds.Width, bbounds.X + bbounds.Width);

   int y1 = Math.Max(abounds.Y, bbounds.Y);
   int y2 = Math.Min(abounds.Y + abounds.Height, bbounds.Y + bbounds.Height);

   for(int y = y1; y < y2; ++y) {
    for(int x = x1; x < x2; ++x) {
     if(((bitsA[(x - abounds.X) + (y - abounds.Y) * abounds.Width] & 0xFF000000) >> 24) > 20 &&
      ((bitsB[(x - bbounds.X) + (y - bbounds.Y) * bbounds.Width] & 0xFF000000) >> 24) > 20)
      return true;
    }
   }
  }
  return false;
 }
}

The image rotation method:

internal static Image RotateImagePoint(Image img, Vector pos, Vector orig, double rotation, out Rectangle sz) {
 if(!(new Rectangle(new Point(0), img.Size).Contains(new Point((int)orig.X, (int)orig.Y))))
  Console.WriteLine("Origin point is not present in image bound; unwanted cropping might occur");
 rotation = (double)ra_de((double)rotation);
 sz = GetRotateDimensions((int)pos.X, (int)pos.Y, img.Width, img.Height, rotation, false);
 Bitmap bmp = new Bitmap(sz.Width, sz.Height);
 Graphics g = Graphics.FromImage(bmp);
 g.SmoothingMode = SmoothingMode.AntiAlias;
 g.InterpolationMode = InterpolationMode.HighQualityBicubic;
 g.PixelOffsetMode = PixelOffsetMode.HighQuality;
 g.RotateTransform((float)rotation);
 g.TranslateTransform(sz.Width / 2, sz.Height / 2, MatrixOrder.Append);
 g.DrawImage(img, (float)-orig.X, (float)-orig.Y);
 g.Dispose();
 return bmp;
}     
internal static Rectangle GetRotateDimensions(int imgx, int imgy, int imgwidth, int imgheight, double rotation, bool Crop) {
 Rectangle sz = new Rectangle();
 if (Crop == true) {
  // absolute trig values goes for all angles
  double dera = de_ra(rotation);
  double sin = Math.Abs(Math.Sin(dera));
  double cos = Math.Abs(Math.Cos(dera));
  // general trig rules:
  // length(adjacent) = cos(theta) * length(hypotenuse)
  // length(opposite) = sin(theta) * length(hypotenuse)
  // applied width = lo(img height) + la(img width)
  sz.Width = (int)(sin * imgheight + cos * imgwidth);
  // applied height = lo(img width) + la(img height)
  sz.Height = (int)(sin * imgwidth + cos * imgheight);
 }
 else {
  // get image diagonal to fit any rotation (w & h =diagonal)
  sz.X = imgx - (int)Math.Sqrt(Math.Pow(imgwidth, 2.0) + Math.Pow(imgheight, 2.0));
  sz.Y = imgy - (int)Math.Sqrt(Math.Pow(imgwidth, 2.0) + Math.Pow(imgheight, 2.0));
  sz.Width = (int)Math.Sqrt(Math.Pow(imgwidth, 2.0) + Math.Pow(imgheight, 2.0)) * 2;
  sz.Height = sz.Width;

 }
 return sz;
}

Pixel getting method:

public uint[] GetPixelData(bool useBaseImage) {
 Rectangle rect;
 Image image;
 if (useBaseImage)
  image = Image;
 else
  image = GraphicsHandler.ResizeImage(GraphicsHandler.RotateImagePoint(Image, Position, Origin, Rotation, out rect), Scale);

 BitmapData data;
 try {
  data = ((Bitmap)image).LockBits(new Rectangle(0, 0, image.Width, image.Height), ImageLockMode.ReadOnly, PixelFormat.Format32bppArgb);
 }
 catch (ArgumentException) {
  data = ((Bitmap)image).LockBits(new Rectangle(0, 0, image.Width, image.Height), ImageLockMode.ReadOnly, image.PixelFormat);
 }

 byte[] rawdata = new byte[data.Stride * image.Height];
 Marshal.Copy(data.Scan0, rawdata, 0, data.Stride * image.Height);
 ((Bitmap)image).UnlockBits(data);
 int pixelsize = 4;
 if (data.PixelFormat == PixelFormat.Format24bppRgb)
  pixelsize = 3;
 else if (data.PixelFormat == PixelFormat.Format32bppArgb || data.PixelFormat == PixelFormat.Format32bppRgb)
  pixelsize = 4;

 double intdatasize = Math.Ceiling((double)rawdata.Length / pixelsize);
 uint[] intdata = new uint[(int)intdatasize];

 Buffer.BlockCopy(rawdata, 0, intdata, 0, rawdata.Length);

 return intdata;
}

The pixel retrieval method works, and the rotation method works as well, so the only place that the code might be wrong is the collision detection code, but I really have no idea where the problem might be.

Answer 1

I don't think many people here will bother to scrutinize your code to figure out what exactly is wrong. But I can come with some hints to how you can find the problem.

If collision happens long before it is supposed to I suggest your bounding box check isn't working properly.

I would change the code to dump out all the data about rectangles at collision. So you can create some code that will display the situation at collision. That might be easier than looking over the numbers.

Apart from that I doubt that per pixel collision detection easier for you to implement. When you allow for rotation and scaling that quickly becomes difficult to get right. I would do polygon based collision detection instead.

I have made my own 2D engine like you but I used polygon based collision detection and that worked fine.

Answer 2

I doubt this is the actual problem, but LockBits doesn't guarantee that the bits data is aligned to the image's Width.

I.e., there may be some padding. You need to access the image using data[x + y * stride] and not data[x + y * width]. The Stride is also part of the BitmapData.

Answer 3

I believe that there might be a problem with GetPixelData for Format24bppRgb, because in that case size of each pixel is 3 bytes, and you are still using BlockCopy to copy it to 32-bit Integer.

Answer 4

I think I've found your problem.

internal static Rectangle GetRotateDimensions(int imgx, int imgy, int imgwidth, int imgheight, double rotation, bool Crop) {
    Rectangle sz = new Rectangle(); // <-- Default constructed rect here.
    if (Crop == true) {
            // absolute trig values goes for all angles
            double dera = de_ra(rotation);
            double sin = Math.Abs(Math.Sin(dera));
            double cos = Math.Abs(Math.Cos(dera));
            // general trig rules:
            // length(adjacent) = cos(theta) * length(hypotenuse)
            // length(opposite) = sin(theta) * length(hypotenuse)
            // applied width = lo(img height) + la(img width)
            sz.Width = (int)(sin * imgheight + cos * imgwidth);
            // applied height = lo(img width) + la(img height)
            sz.Height = (int)(sin * imgwidth + cos * imgheight);

            // <-- Never gets the X & Y assigned !!!
    }

Since you never assigned imgx and imgy to the X and Y coordinates of the Rectangle, every call of GetRotateDimensions will produce a Rectangle with the same location. They may be of differing sizes, but they will always be in the default X,Y position. This would cause the really early collisions that you are seeing because any time you tried to detect collisions on two sprites, GetRotateDimensions would put their bounds in the same position regardless of where they actually are.

Once you have corrected that problem, you may run into another error:

Rectangle rect;
Image img1 = GraphicsHandler.ResizeImage(GraphicsHandler.RotateImagePoint(s1.Image, s1.Position, s1.Origin, s1.Rotation, out rect), s1.Scale);
// <-- Should resize rect here.
int posx1 = rect.X;
int posy1 = rect.Y;

You get your boundary rect from the RotateImagePoint function, but you then resize the image. The X and Y from the rect are probably not exactly the same as that of the resized boundaries of the image. I'm guessing that you mean for the center of the image to remain in place while all points contract toward or expand from the center in the resize. If this is the case, then you need to resize rect as well as the image in order to get the correct position.