GL.DrawArrays

When this function is called, OpenGL iterates over each of the currently enabled arrays, rendering primitives as it goes. Lets take a look at the function to understand how it works:

void GL.DrawArrays(PrimitiveType type, int first, int count);

type serves the same basic function as the paramater of GL.Begin. It specifies what type of primitives the vertex data creates. Valid values are: Points, LineStrip, LineLoop, Lines, TriangleStrip, TriangleFan, Triangles, QuadStrip, Quads and Polygon.

first specifies the index at which we should start drawing. This means you can choose to only draw a part of the array data.

count is how many indices to draw. If your model has 92 vertices, this will be 92. it's the number of vertices, NOT the number of triangles. Even after 5 years, this argument still trips me up sometimes!

It should be noted that after calling GL.DrawArrays the state of the arrays being processed is undefined. Meaning you have to re-bind them or the behaviour of the next DrawArrays call is undefined.

For example, lets try to render two meshes from the same arrays. The following code is bad:

void GL.EnableClientState(ArrayCap.VertexArray);
void GL.EnableClientState(ArrayCap.NormalArray);

GL.VertexPointer(3, VertexPointerType.Float, 0, pverts);
GL.NormalPointer(NormalPointerType.Float, 0, pnorms);

GL.DrawArrays(BeginMode.Triangles, mesh1.offset, mesh1.Length); // Draw mesh 1
GL.DrawArrays(BeginMode.Triangles, mesh2.offset, mesh2.Length); // Draw mesh 2

void GL.DisableClientState(ArrayCap.NormalArray);
void GL.DisableClientState(ArrayCap.VertexArray);

The proper way to render them would be like this:

void GL.EnableClientState(ArrayCap.VertexArray);
void GL.EnableClientState(ArrayCap.NormalArray);

GL.VertexPointer(3, VertexPointerType.Float, 0, pverts);
GL.NormalPointer(NormalPointerType.Float, 0, pnorms);
GL.DrawArrays(BeginMode.Triangles, mesh1.offset, mesh1.Length); // Draw mesh 1

GL.VertexPointer(3, VertexPointerType.Float, 0, pverts);
GL.NormalPointer(NormalPointerType.Float, 0, pnorms);
GL.DrawArrays(BeginMode.Triangles, mesh2.offset, mesh2.Length); // Draw mesh 2

void GL.DisableClientState(ArrayCap.NormalArray);
void GL.DisableClientState(ArrayCap.VertexArray);

Take note of how the VertexPointer and Normal pointers are re-defined between calls to DrawArrays