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DrGUI

Extracting Matrix Parameters

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I want to be able to extract the left (l), right (r), top (t), bottom (b), znear (zn) and zfar (zf) parameters out of a projection matrix. I borrowed this from the DirectX docs for your convenience: 2*zn/(r-l) 0 0 0 0 2*zn/(t-b) 0 0 (l+r)/(l-r) (t+b)/(b-t) zf/(zf-zn) 1 0 0 zn*zf/(zn-zf) 0 EDIT: sorry about GameDev taking out my tabs, then collapsing my spaces I imagine doing this would entail solving a few simultaneous equations but it looks daunting to me and I am revising for my GCSEs (progress on my game has stopped completely :-( ). Someone must have done this before, but my Google search with 8 keywords kept on giving me thousands of irrelevant results. I will be grateful for your help and many ratings will come your way...

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There's a PDF on extracting the planes of the view frustum from the projection matrix here. You should be able to work out what you need from the planes.

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Thanks for your reply! I've already got the code to extract planes, and I worked out from that how to find the corners (to draw my view frustum). Here's the code to help solve this problem:

/// <summary>
/// Extracts planes from an arbitrary matrix.</summary>
/// <param name="matrixIn">The matrix from which to extract the planes.</param>
/// <param name="normalizePlanes">
/// Whether to normalize the planes. Non-normalized planes are only suitable for half-space testing.
/// No distance values calculated using these planes will be accurate other than to show whether positions lie in the positive or
/// negative half-space of the plane.</param>
/// <param name="outPlanes">The array of 6 planes to receive the extracted planes.</param>
/// <exception cref="System.ArgumentNullException">
/// outPlanes is a null reference (Nothing in Visual Basic).</exception>
/// <exception cref="System.ArgumentException">
/// The length of outPlanes is not 6.</exception>
public static void ExtractPlanes(Matrix matrixIn, bool normalizePlanes, ref Plane[] outPlanes)
{
if (outPlanes == null) throw new System.ArgumentNullException("outPlanes");
if (outPlanes.Length != 6) throw new System.ArgumentException("The length of outPlanes is not 6.");

//Left
outPlanes[(int)FrustumPlane.Left].A = matrixIn.M14 + matrixIn.M11;
outPlanes[(int)FrustumPlane.Left].B = matrixIn.M24 + matrixIn.M21;
outPlanes[(int)FrustumPlane.Left].C = matrixIn.M34 + matrixIn.M31;
outPlanes[(int)FrustumPlane.Left].D = matrixIn.M44 + matrixIn.M41;
//Right
outPlanes[(int)FrustumPlane.Right].A = matrixIn.M14 - matrixIn.M11;
outPlanes[(int)FrustumPlane.Right].B = matrixIn.M24 - matrixIn.M21;
outPlanes[(int)FrustumPlane.Right].C = matrixIn.M34 - matrixIn.M31;
outPlanes[(int)FrustumPlane.Right].D = matrixIn.M44 - matrixIn.M41;
//Top
outPlanes[(int)FrustumPlane.Top].A = matrixIn.M14 - matrixIn.M12;
outPlanes[(int)FrustumPlane.Top].B = matrixIn.M24 - matrixIn.M22;
outPlanes[(int)FrustumPlane.Top].C = matrixIn.M34 - matrixIn.M32;
outPlanes[(int)FrustumPlane.Top].D = matrixIn.M44 - matrixIn.M42;
//Bottom
outPlanes[(int)FrustumPlane.Bottom].A = matrixIn.M14 + matrixIn.M12;
outPlanes[(int)FrustumPlane.Bottom].B = matrixIn.M24 + matrixIn.M22;
outPlanes[(int)FrustumPlane.Bottom].C = matrixIn.M34 + matrixIn.M32;
outPlanes[(int)FrustumPlane.Bottom].D = matrixIn.M44 + matrixIn.M42;
//Near
outPlanes[(int)FrustumPlane.Near].A = matrixIn.M13;
outPlanes[(int)FrustumPlane.Near].B = matrixIn.M23;
outPlanes[(int)FrustumPlane.Near].C = matrixIn.M33;
outPlanes[(int)FrustumPlane.Near].D = matrixIn.M43;
//Far
outPlanes[(int)FrustumPlane.Far].A = matrixIn.M14 - matrixIn.M13;
outPlanes[(int)FrustumPlane.Far].B = matrixIn.M24 - matrixIn.M23;
outPlanes[(int)FrustumPlane.Far].C = matrixIn.M34 - matrixIn.M33;
outPlanes[(int)FrustumPlane.Far].D = matrixIn.M44 - matrixIn.M43;

//Non-normalized planes are only suitable for half-space testing.
//No distance values calculated using these planes will be accurate
//other than to show whether positions lie in the positive or negative
//half-space of the plane.
if (normalizePlanes) {
outPlanes[(int)FrustumPlane.Left].Normalize();
outPlanes[(int)FrustumPlane.Right].Normalize();
outPlanes[(int)FrustumPlane.Top].Normalize();
outPlanes[(int)FrustumPlane.Bottom].Normalize();
outPlanes[(int)FrustumPlane.Near].Normalize();
outPlanes[(int)FrustumPlane.Far].Normalize();
}
}

/// <summary>
/// Extracts points from an arbitrary matrix.</summary>
/// <param name="matrixIn">The matrix from which to extract the points.</param>
/// <param name="outPoints">The array of 8 vectors to receive the extracted points.</param>
/// <exception cref="System.ArgumentNullException">
/// outPoints is a null reference (Nothing in Visual Basic).</exception>
/// <exception cref="System.ArgumentException">
/// The length of outPoints is not 8.</exception>
public static void ExtractPoints(Matrix matrixIn, ref Vector3[] outPoints)
{
if (outPoints == null) throw new System.ArgumentNullException("outPoints");
if (outPoints.Length != 8) throw new System.ArgumentException("The length of outPoints is not 8.");

//A projection matrix concat is likely to be used, so everything needs to be divided by w
float w;

//Near bottom left
w = 1 / (-matrixIn.M14 - matrixIn.M24 + matrixIn.M44);
outPoints[(int)FrustumPoint.NearBottomLeft].X = (-matrixIn.M11 - matrixIn.M21 + matrixIn.M41) * w;
outPoints[(int)FrustumPoint.NearBottomLeft].Y = (-matrixIn.M12 - matrixIn.M22 + matrixIn.M42) * w;
outPoints[(int)FrustumPoint.NearBottomLeft].Z = (-matrixIn.M13 - matrixIn.M23 + matrixIn.M43) * w;

//Near bottom right
w = 1 / (matrixIn.M14 - matrixIn.M24 + matrixIn.M44);
outPoints[(int)FrustumPoint.NearBottomRight].X = (matrixIn.M11 - matrixIn.M21 + matrixIn.M41) * w;
outPoints[(int)FrustumPoint.NearBottomRight].Y = (matrixIn.M12 - matrixIn.M22 + matrixIn.M42) * w;
outPoints[(int)FrustumPoint.NearBottomRight].Z = (matrixIn.M13 - matrixIn.M23 + matrixIn.M43) * w;

//Near top left
w = 1 / (-matrixIn.M14 + matrixIn.M24 + matrixIn.M44);
outPoints[(int)FrustumPoint.NearTopLeft].X = (-matrixIn.M11 + matrixIn.M21 + matrixIn.M41) * w;
outPoints[(int)FrustumPoint.NearTopLeft].Y = (-matrixIn.M12 + matrixIn.M22 + matrixIn.M42) * w;
outPoints[(int)FrustumPoint.NearTopLeft].Z = (-matrixIn.M13 + matrixIn.M23 + matrixIn.M43) * w;

//Near top right
w= 1 / (matrixIn.M14 + matrixIn.M24 + matrixIn.M44);
outPoints[(int)FrustumPoint.NearTopRight].X = (matrixIn.M11 + matrixIn.M21 + matrixIn.M41) * w;
outPoints[(int)FrustumPoint.NearTopRight].Y = (matrixIn.M12 + matrixIn.M22 + matrixIn.M42) * w;
outPoints[(int)FrustumPoint.NearTopRight].Z = (matrixIn.M13 + matrixIn.M23 + matrixIn.M43) * w;

//Far bottom left
w = 1 / (-matrixIn.M14 - matrixIn.M24 + matrixIn.M34 + matrixIn.M44);
outPoints[(int)FrustumPoint.FarBottomLeft].X = (-matrixIn.M11 - matrixIn.M21 + matrixIn.M31 + matrixIn.M41) * w;
outPoints[(int)FrustumPoint.FarBottomLeft].Y = (-matrixIn.M12 - matrixIn.M22 + matrixIn.M32 + matrixIn.M42) * w;
outPoints[(int)FrustumPoint.FarBottomLeft].Z = (-matrixIn.M13 - matrixIn.M23 + matrixIn.M33 + matrixIn.M43) * w;

//Far bottom right
w = 1 / (matrixIn.M14 - matrixIn.M24 + matrixIn.M34 + matrixIn.M44);
outPoints[(int)FrustumPoint.FarBottomRight].X = (matrixIn.M11 - matrixIn.M21 + matrixIn.M31 + matrixIn.M41) * w;
outPoints[(int)FrustumPoint.FarBottomRight].Y = (matrixIn.M12 - matrixIn.M22 + matrixIn.M32 + matrixIn.M42) * w;
outPoints[(int)FrustumPoint.FarBottomRight].Z = (matrixIn.M13 - matrixIn.M23 + matrixIn.M33 + matrixIn.M43) * w;

//Far top left
w = 1 / (-matrixIn.M14 + matrixIn.M24 + matrixIn.M34 + matrixIn.M44);
outPoints[(int)FrustumPoint.FarTopLeft].X = (-matrixIn.M11 + matrixIn.M21 + matrixIn.M31 + matrixIn.M41) * w;
outPoints[(int)FrustumPoint.FarTopLeft].Y = (-matrixIn.M12 + matrixIn.M22 + matrixIn.M32 + matrixIn.M42) * w;
outPoints[(int)FrustumPoint.FarTopLeft].Z = (-matrixIn.M13 + matrixIn.M23 + matrixIn.M33 + matrixIn.M43) * w;

//Far top right
w = 1 / (matrixIn.M14 + matrixIn.M24 + matrixIn.M34 + matrixIn.M44);
outPoints[(int)FrustumPoint.FarTopRight].X = (matrixIn.M11 + matrixIn.M21 + matrixIn.M31 + matrixIn.M41) * w;
outPoints[(int)FrustumPoint.FarTopRight].Y = (matrixIn.M12 + matrixIn.M22 + matrixIn.M32 + matrixIn.M42) * w;
outPoints[(int)FrustumPoint.FarTopRight].Z = (matrixIn.M13 + matrixIn.M23 + matrixIn.M33 + matrixIn.M43) * w;

}




[Edited by - DrGUI on April 3, 2005 12:47:30 PM]

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