[Solved] Invisible triangles in orthographic projection? (Gimbal Lock)

@mgulde wrote:

Dear all, I have come encountered this problem a few times now without ever solving it. So maybe you can help me understand where the actual issue lies:

I have a very simple 3D project and would like to just display a single triangle with the BasicEffect class and a custom VertexDeclaration. However, when using an orthographic projection, the triangle becomes invisible as soon as I am looking from above. A tiny offset solves the problem.

The VertexDeclaration is as follows:

public readonly static VertexDeclaration VertexDeclaration
        = new VertexDeclaration(
            new VertexElement(0, VertexElementFormat.Vector3, VertexElementUsage.Position, 0),
            new VertexElement(sizeof(float) * 3, VertexElementFormat.Vector3, VertexElementUsage.Normal, 0),
            new VertexElement(sizeof(float) * 6, VertexElementFormat.Color, VertexElementUsage.Color, 0)
            );

My display vectors are:

View = Matrix.CreateLookAt(Position, Target, Vector3.Up);
Projection = Matrix.CreateOrthographic(Settings.ScreenWidth, Settings.ScreenHeight, Settings.NearPlane, Settings.FarPlane);

with Position = (0, 10, 0) and Target = (0, 0, 0).

The "Model" / triangle is defined as

VX.Add(new VertexPositionNormalColor(new Vector3(0, 0, 0), new Vector3(0, 1, 0), Color.DarkOrange));
VX.Add(new VertexPositionNormalColor(new Vector3(100, 0, 0), new Vector3(0, 1, 0), Color.DarkOrange));
VX.Add(new VertexPositionNormalColor(new Vector3(0, 0, 100), new Vector3(0, 1, 0), Color.DarkOrange));

The drawing routine is

// effect
Eff.World = Matrix.Identity;
Eff.Projection = Cam.Projection;
Eff.View = Cam.View;
Eff.VertexColorEnabled = true;

// draw
foreach (EffectPass pass in Eff.CurrentTechnique.Passes)
{
pass.Apply();
GraphicsDevice.SetVertexBuffer(VB);
GraphicsDevice.DrawPrimitives(PrimitiveType.TriangleList, 0, VX.Count);
}

The result is invisible. Only by adding a tiny tilt in the camera angle I can see the triangle as planned: Position = (0, 10, 0.1f).

Any ideas?

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