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glupickmatrix(3g) [osx man page]


gluPickMatrix - define a picking region C SPECIFICATION
void gluPickMatrix( GLdouble x, GLdouble y, GLdouble delX, GLdouble delY, GLint *viewport ) PARAMETERS
x, y Specify the center of a picking region in window coordinates. delX, delY Specify the width and height, respectively, of the picking region in window coordinates. viewport Specifies the current viewport (as from a glGetIntegerv call). DESCRIPTION
gluPickMatrix creates a projection matrix that can be used to restrict drawing to a small region of the viewport. This is typically useful to determine what objects are being drawn near the cursor. Use gluPickMatrix to restrict drawing to a small region around the cursor. Then, enter selection mode (with glRenderMode) and rerender the scene. All primitives that would have been drawn near the cursor are iden- tified and stored in the selection buffer. The matrix created by gluPickMatrix is multiplied by the current matrix just as if glMultMatrix is called with the generated matrix. To effectively use the generated pick matrix for picking, first call glLoadIdentity to load an identity matrix onto the perspective matrix stack. Then call gluPickMatrix, and finally, call a command (such as gluPerspective) to multiply the perspective matrix by the pick matrix. When using gluPickMatrix to pick NURBS, be careful to turn off the NURBS property GLU_AUTO_LOAD_MATRIX. If GLU_AUTO_LOAD_MATRIX is not turned off, then any NURBS surface rendered is subdivided differently with the pick matrix than the way it was subdivided without the pick matrix. EXAMPLE
When rendering a scene as follows: glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluPerspective(...); glMatrixMode(GL_MODELVIEW); /* Draw the scene */ a portion of the viewport can be selected as a pick region like this: glMatrixMode(GL_PROJECTION); glLoadIdentity(); gluPickMatrix(x, y, width, height, viewport); gluPerspective(...); glMatrixMode(GL_MODELVIEW); /* Draw the scene */ SEE ALSO
glGet, glLoadIndentity, glMultMatrix, glRenderMode, gluPerspective GLUPICKMATRIX(3G)

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gluPerspective - set up a perspective projection matrix C SPECIFICATION
void gluPerspective( GLdouble fovy, GLdouble aspect, GLdouble zNear, GLdouble zFar ) PARAMETERS
fovy Specifies the field of view angle, in degrees, in the y direction. aspect Specifies the aspect ratio that determines the field of view in the x direction. The aspect ratio is the ratio of x (width) to y (height). zNear Specifies the distance from the viewer to the near clipping plane (always positive). zFar Specifies the distance from the viewer to the far clipping plane (always positive). DESCRIPTION
gluPerspective specifies a viewing frustum into the world coordinate system. In general, the aspect ratio in gluPerspective should match the aspect ratio of the associated viewport. For example, aspect = 2.0 means the viewer's angle of view is twice as wide in x as it is in y. If the viewport is twice as wide as it is tall, it displays the image without distortion. The matrix generated by gluPerspective is multipled by the current matrix, just as if glMultMatrix were called with the generated matrix. To load the perspective matrix onto the current matrix stack instead, precede the call to gluPerspective with a call to glLoadIdentity. Given f defined as follows: f = cotangent(fovy/2) The generated matrix is f ------------ 0 0 0 aspect 0 f 0 0 zFar+zNear 2*zFar*zNear 0 0 ---------- ------------ zNear-zFar zNear-zFar 0 0 -1 0 NOTES
Depth buffer precision is affected by the values specified for zNear and zFar. The greater the ratio of zFar to zNear is, the less effec- tive the depth buffer will be at distinguishing between surfaces that are near each other. If r = zFar/zNear roughly log2(r) bits of depth buffer precision are lost. Because r approaches infinity as zNear approaches 0, zNear must never be set to 0. SEE ALSO
glFrustum, glLoadIdentity, glMultMatrix, gluOrtho2D GLUPERSPECTIVE(3G)
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