05-27-2002, 09:30 AM

Can anyone provide me with a full definition as to what a normal is, why its so important in 3D Graphics etc? either an explantion or link would be fantastic. Thanks.

Martin.

Martin.

View Full Version : Definition of Normals

05-27-2002, 09:30 AM

Can anyone provide me with a full definition as to what a normal is, why its so important in 3D Graphics etc? either an explantion or link would be fantastic. Thanks.

Martin.

Martin.

Jeeeez

05-27-2002, 09:31 AM

First, a normal is a vector that is perpendicular to a surface.

It is important because lighting calculations use the surface normal to calculate the intensity of the light at each point on the surface.

It is important because lighting calculations use the surface normal to calculate the intensity of the light at each point on the surface.

nemesis

05-28-2002, 03:13 AM

In addition to what Jeeeez have said...

When calculating the effect that a light has on a surface, it is needed to know how the surface is oriented. If the surface is facing the light, it will receive more light than if it is sideways. OpenGL will know it from the normal information.

-nemesis-

When calculating the effect that a light has on a surface, it is needed to know how the surface is oriented. If the surface is facing the light, it will receive more light than if it is sideways. OpenGL will know it from the normal information.

-nemesis-

satan

05-28-2002, 04:29 AM

and another small addition

they are nomalized (meaning having unit length which in most cases and for sure in opengl is 1)

they are nomalized (meaning having unit length which in most cases and for sure in opengl is 1)

Julian

05-29-2002, 01:45 AM

I also understand that opengl defines normals for each vertex. I think these are obtained by averaging the normals of all of the faces that that vertex is a part of.

satan

05-29-2002, 08:15 AM

Originally posted by Julian:

I also understand that opengl defines normals for each vertex. I think these are obtained by averaging the normals of all of the faces that that vertex is a part of.

i am not sure if i got i right, but it sounds like opengl shall be doing this

just to clear it up

opengl only knows vertex normals

meaning you have to specify one normal for each vertex

if you want averaged normals it is your task to do the averaging but you don't always want averaged normals

think of a simple cube

averaging the normals would give you undesired lighting effects since you want face normals for your cube

therefor you have to specify each corner vertex 3 times so that you can give it 3 different normals dependent on the face you are drawing

so instead of 8 vertices (which are enough for an unlit cube) you have to use 24 vertices for a lit one

I also understand that opengl defines normals for each vertex. I think these are obtained by averaging the normals of all of the faces that that vertex is a part of.

i am not sure if i got i right, but it sounds like opengl shall be doing this

just to clear it up

opengl only knows vertex normals

meaning you have to specify one normal for each vertex

if you want averaged normals it is your task to do the averaging but you don't always want averaged normals

think of a simple cube

averaging the normals would give you undesired lighting effects since you want face normals for your cube

therefor you have to specify each corner vertex 3 times so that you can give it 3 different normals dependent on the face you are drawing

so instead of 8 vertices (which are enough for an unlit cube) you have to use 24 vertices for a lit one

Phillfrog

05-29-2002, 09:06 PM

More on normals:

The normal is the gradient perpendicular to the line(or in this case face). If on the other hand, the line is a curve. you have to differentiate the equation of the curve to give you the equation of the tangent and from there (using m1.m2=-1) you can derive the equation of the normal. then (if you have co-ordiates, you can find out the gradient by using. if the equation of the line is in the order y=mx+c, m is the gradient. http://www.opengl.org/discussion_boards/ubb/confused.gif Well, this is how it basically works. but it gets a little more complicated when you add the third dimension. and really, the normal isn't that important to work out for you, because you dont need to know where to light is coming from. only OpenGL needs to know that, and it works it out for it's self. So your probably best to ignore all that.

Hav fun, and don't worry about normals, all you need to know, everyone has already explained to you.

GuruFrog(Phillfrog);

The normal is the gradient perpendicular to the line(or in this case face). If on the other hand, the line is a curve. you have to differentiate the equation of the curve to give you the equation of the tangent and from there (using m1.m2=-1) you can derive the equation of the normal. then (if you have co-ordiates, you can find out the gradient by using. if the equation of the line is in the order y=mx+c, m is the gradient. http://www.opengl.org/discussion_boards/ubb/confused.gif Well, this is how it basically works. but it gets a little more complicated when you add the third dimension. and really, the normal isn't that important to work out for you, because you dont need to know where to light is coming from. only OpenGL needs to know that, and it works it out for it's self. So your probably best to ignore all that.

Hav fun, and don't worry about normals, all you need to know, everyone has already explained to you.

GuruFrog(Phillfrog);

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