Per-pixel collision detection - why does XNA transform matrix return NaN when adding scaling?
- by JasperS
I looked at the TransformCollision sample on MSDN and added the Matrix.CreateTranslation part to a property in my collision detection code but I wanted to add scaling.
The code works fine when I leave scaling commented out but when I add it and then do a Matrix.Invert() on the created translation matrix the result is NaN ({NaN,NaN,NaN},{NaN,NaN,NaN},...)
Can anyone tell me why this is happening please?
Here's the code from the sample:
// 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));
public static bool 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
return true;
}
}
// Move to the next pixel in the row
posInB += stepX;
}
// Move to the next row
yPosInB += stepY;
}
// No intersection found
return false;
}
