rendering problems


I have a problem which I cannot seem to solve. When I render my image, it should be smooth. However, I am getting a messy rendering which looks like there is some sort of grid ontop of it - as seen in the screenshot below.

Basically it’s a fairly simple program. I am plotting a large list of vertex, calculating the normal vector for each polygon and adding ambient light.

Could it be my normals are not facing the right direction?


I tried using glNormal3f() before defining three glVertex3f().as opposed to the other way around.

The image improved slightly, but still seems wrong :frowning:

Could anyone suggest if this is a lighting problem? i.e. my normals are wrong, or are my vertex/polygons likely to be wrong


glNormal, as glColor etc, must be called before its glVertex.
Otherwise, yes, it looks like you do not specify normals correctly, for some parts.

As a check, try setting always the same normal, for all vertices : you should ge a fairly uniform color. In that case, you only have aproblem with normals. Otherwise, something is wrong with vertices too.

Hi, thanks for the help.

I tried just using glNormal3f(1.0f,1.0f,1.0f) and I’m getting my image in a plain grey colour.

Can I just ask something. I have polygons made up of three vertices. I am currently doing something like this:


Is it right that I have one normal for three vertex3f() calls?


Using only one normal for a triangle will give you flat shading.
But that does not explain the weird image you get…

To have smooth shading, a vertex shared by all is adjacent triangles should have a normal somewhat averaging the triangles normals.

When I change GL_POLYGON to GL_TRIANGLES, I get this.
I thought a polygon with three points was the same as a triangle.

Actually, it seems like my problems are coming from this line:

gluPerspective(45, 1, 0, 10);

If I comment out that line, I get a correct rendering!, but only with GL TRIANGLES, not GL POLYGON. I heard somewhere that polygon is not good for flat shading anyway.

zNear = 0 in your above call. This is not good and the doc explicitely states it :

Try any >0 value, like 0.001 instead.

Solved…Thanks mate!!

Sorry to keep asking questions, but I’m still having problems with gluPerspective. Even at a value of 0.01 when I’m changing the values of gluLookat() I’m getting various bits of my image chopped off. It’s fine if I look head on, but to the side it’s not correct

if I comment out both glulookat and gluPerspective I get the image looking correct, but zoomed out quite far. The same angle, but zoomed in using glulookat and perspective seems to chop off half my image. I still see a red colour from the ambient lighting, but the shading goes completely


edit - it seems as though the light is hitting the wrong side of my surface when using gluPerspective

Well you are doing wrong, but to help you more will be difficult without the complete code related to your transformations matrices, for both modelview and projection.

I found my problem, my light source was positioned on the wrong side of my image. Apologies