gluPerspective(few more)

thanks i think these basic i need to know now i have question

Q1 if camera is used then will this(gluPerspective) updated.if yes how

Q2 the code fraction
glMatrixMode(GL_PROJECTION);<- i know projetion(farther smaller nearer bigger(perspective,frustum))
gluPerspective(let it filled);
glMatrixMode(GL_MODELVIEW);<- how it fnctioning
these glMatrixMode()glLoadIdentity() used two times in codes
at DisplayFunc and ReshapeFunc why? Without them theres only screen of glClearColor() or black?

Q3. if object either side of zNear,zFar,two lines make angle at y then is object invisible(i think must be)?

(Please Help me cause i programmed many code with them but don’t know how that work like know how to drive but don’t know how to repair(I am only 16.9 year old and know vector,Matrix(all of summ…,Multi…),Little bit plane)
so please help me regard these which is not out of that maths
thanks in advance
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thanks in advance

q1. gluPerspective is just the perspective of your virtual camera. if you dont use zoom or something you can set it once and forget.

q2. used 2 times because it affects different matrix stack, namely GL_PROJECTION and GL_MODELVIEW.

q3. if object is inside zNear AND zFar its visible only if its inside other planes too (left,right,top,bottom) and not otherwise.
this is called a frustum if perspective projection is used (like you do).

than-q(thank you)