distributed development - Page 482

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; }

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