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# File Documents/Positioning/Tutorial 04.xml

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`             </figure>`
`             <para>What we have are two similar right triangles; the triangle formed by E, R and`
`                     E<subscript>z</subscript>the origin; and the triangle formed by E, P, and`
`-                    P<subscript>z</subscript>+E<subscript>z</subscript>. We have the eye position`
`-                and the position of the unprojected point. To find the location of R, we simply do`
`-                this:</para>`
`+                    P<subscript>z</subscript>. We have the eye position and the position of the`
`+                unprojected point. To find the location of R, we simply do this:</para>`
`             <equation>`
`                 <title>Perspective Computation</title>`
`                 <mediaobject>`
`                 zFar are positive but refer to negative values).</para>`
`             <para>The location of the prism has also changed. In the original tutorial, it was`
`                 located on the 0.75 range in Z. Because camera space has a very different Z from`
`-                clip space, this had to change. Now, the Z location of the prims is between -1.25`
`+                clip space, this had to change. Now, the Z location of the prism is between -1.25`
`                 and -2.75.</para>`
`             <para>All of this leaves us with this result:</para>`
`             <figure>`