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Gl.Frustum (gb.opengl)
`STATIC SUB Frustum ( Left AS Float, Right AS Float, Bottom AS Float, Top AS Float, Near AS Float, Far AS Float )`

Multiply the current matrix by a perspective matrix.

Parameters

left, right

Specify the coordinates for the left and right vertical clipping planes.

bottom, top

Specify the coordinates for the bottom and top horizontal clipping planes.

nearVal, farVal

Specify the distances to the near and far depth clipping planes. Both distances must be positive.

Description

Gl.Frustum describes a perspective matrix that produces a perspective projection. The current matrix (see Gl.MatrixMode) is multiplied by this matrix and the result replaces the current matrix, as if Gl.MultMatrix were called with the following matrix as its argument:

$\left[\begin{array}{cccc}\frac{2\mathit{nearVal}}{\mathit{right}-\mathit{left}}& 0& \mathit{A}& 0\\ 0& \frac{2\mathit{nearVal}}{\mathit{top}-\mathit{bottom}}& \mathit{B}& 0\\ 0& 0& \mathit{C}& \mathit{D}\\ 0& 0& -1& 0\end{array}\right]$

$\mathit{A}=\frac{\mathit{right}+\mathit{left}}{\mathit{right}-\mathit{left}}$

$\mathit{B}=\frac{\mathit{top}+\mathit{bottom}}{\mathit{top}-\mathit{bottom}}$

$\mathit{C}=-\frac{\mathit{farVal}+\mathit{nearVal}}{\mathit{farVal}-\mathit{nearVal}}$

$\mathit{D}=-\frac{2\mathit{farVal}\mathit{nearVal}}{\mathit{farVal}-\mathit{nearVal}}$

Typically, the matrix mode is Gl.PROJECTION, and $\left(\mathit{left},\mathit{bottom},-\mathit{nearVal}\right)$ and $\left(\mathit{right},\mathit{top},-\mathit{nearVal}\right)$ specify the points on the near clipping plane that are mapped to the lower left and upper right corners of the window, assuming that the eye is located at (0, 0, 0). $-\mathit{farVal}$ specifies the location of the far clipping plane. Both nearVal and farVal must be positive.

Use Gl.PushMatrix and Gl.PopMatrix to save and restore the current matrix stack.

Notes

Depth buffer precision is affected by the values specified for nearVal and farVal. The greater the ratio of farVal to nearVal is, the less effective the depth buffer will be at distinguishing between surfaces that are near each other. If

$\mathit{r}=\frac{\mathit{farVal}}{\mathit{nearVal}}$

roughly ${\mathit{log}}_{2}\left(\mathit{r}\right)$ bits of depth buffer precision are lost. Because $\mathit{r}$ approaches infinity as nearVal approaches 0, nearVal must never be set to 0.

Errors

Gl.INVALID_VALUE is generated if nearVal or farVal is not positive, or if left = right, or bottom = top, or near = far.

Gl.INVALID_OPERATION is generated if Gl.Frustum is executed between the execution of Gl.Begin and the corresponding execution of Gl.End.

Associated Gets

Gl.Get with argument Gl.MATRIX_MODE

Gl.Get with argument Gl.MODELVIEW_MATRIX

Gl.Get with argument Gl.PROJECTION_MATRIX

Gl.Get with argument Gl.TEXTURE_MATRIX

Gl.Get with argument Gl.COLOR_MATRIX

Vedi anche

Gl.Ortho, Gl.MatrixMode, Gl.MultMatrix, Gl.PushMatrix, Gl.Viewport