Drawing 3D cubes with xna

Question

I managed to draw a 3d cube using triangles like this:

    protected override void LoadContent()
    {
        basicEffect = new BasicEffect(GraphicsDevice);   
        VertexPositionColor[] vertices = new VertexPositionColor[8];
        short[] indices = new short[36];
        vertexBuffer = new VertexBuffer(GraphicsDevice, typeof(VertexPositionColor), vertices.Length, BufferUsage.WriteOnly);
        indexBuffer = new IndexBuffer(graphics.GraphicsDevice, typeof(short), indices.Length, BufferUsage.WriteOnly);
        Color Color1 = Color.DarkRed;
        Color Color2 = Color.Pink;
        float width = 2;
        float height = 1;
        float depth = 0.5f;

        vertices[0] = new VertexPositionColor(new Vector3(0, 0, 0), Color1);
        vertices[1] = new VertexPositionColor(new Vector3(width, 0, 0), Color1);
        vertices[2] = new VertexPositionColor(new Vector3(width, -height, 0), Color1);
        vertices[3] = new VertexPositionColor(new Vector3(0, -height, 0), Color1);
        vertices[4] = new VertexPositionColor(new Vector3(0, 0, depth), Color1);
        vertices[5] = new VertexPositionColor(new Vector3(width, 0, depth), Color1);
        vertices[6] = new VertexPositionColor(new Vector3(width, -height, depth), Color1);
        vertices[7] = new VertexPositionColor(new Vector3(0, -height, depth), Color1);

        indices[0] = 0; indices[1] = 1; indices[2] = 2;
        indices[3] = 0; indices[4] = 3; indices[5] = 2;
        indices[6] = 4; indices[7] = 0; indices[8] = 3;
        indices[9] = 4; indices[10] = 7; indices[11] = 3;
        indices[12] = 3; indices[13] = 7; indices[14] = 6;
        indices[15] = 3; indices[16] = 6; indices[17] = 2;
        indices[18] = 1; indices[19] = 5; indices[20] = 6;
        indices[21] = 1; indices[22] = 5; indices[23] = 2;
        indices[24] = 4; indices[25] = 5; indices[26] = 6;
        indices[27] = 4; indices[28] = 7; indices[29] = 6;
        indices[30] = 0; indices[31] = 1; indices[32] = 5;
        indices[33] = 0; indices[34] = 4; indices[35] = 5;

        vertexBuffer.SetData<VertexPositionColor>(vertices);
        indexBuffer.SetData(indices);
    }

I have two questions:

• As you can see i am using float width = 2; float height = 1; float depth = 0.5f; but why "width = 2" looks like 80 pixels instead of 2? I want the cubes to be 72x24x12, i can't figure out how to calculate the pixels from the field of view / aspect ratio. To draw the cube on different positions i am using world = Matrix.CreateTranslation(10.0f, 10.0f, 0.0f);, is that the correct way to do this ?
• How can i draw each face of the cube a different color? (Right now i know how to give color to each vertice, see Color1 variable). Right now i am using 2 different BasicEffects, is there any better way perhaps ?

Answer 1

So , as already was sad there are no pixels in 3D spcae at all. There are usints that dont have any relations with 2D pixels. Pixel are also units, but they have fixed size couse screen have have fixed resolution. For example if your monitor is 12 inches in width and 9 inches in height and resolutins is set to 800x600 one pixel will have real size 0.015x0.015 inches^2.

But 3D's units is not pixels. Theare are no relations with hardvare and screen resolution. If you want create rectahgle with size 72x24x12 than do it. Next you need to create camera with distance, aspect rations and angle of view that allows you see all you scene. If scene if too big than move camera farther, or scale down the scene.

You can do tree types of transformation with matrices: translation, rotation, scaling. Most important thing you should now that if you apply few transformatins that every next will be applyind for all that was applyid before. For example if you do

transform = translate(10,8,10) * scale(2,3,5) * translate(4, 5, 10)

it means that you translate your first model at(10,8,10) units, but then scale ALL distances to (2,3,5). So, your model's size will be scaled, as far as distance of applyed translation and translate(10,8,10) will means translate(20,24,50). Then you translate model again at translate(4, 5, 10), and this translations will NOT be scaled. So, you can create all scene withoud scaling and then just scale it before drawing.

This article was very helpfull for me. If you don't understand russian click at "часть 1, часть 2, часть 3 и часть 4" links int top of article.

Answer 2

Like dustin said, the size of a 3d object isnt measured in pixels. But if you wish to scale it, change "world = Matrix.CreateTranslation(10.0f, 10.0f, 0.0f);" to "world = Matrix.CreateScale(2) * Matrix.CreateTranslation(10.0f, 10.0f, 0.0f);". Notice that I put the scale factor matrix BEFORE the translation. Otherwise, the translation would also get scaled. Matrix multiplying works from left to right.

And for your second question: if you want all faces to have a different color, you will have to place differently colored vertices at every point of the cube/block. This is because as you said, you can assign one color per vertex.

Answer 3

For: why "width = 2" looks like 80 pixels instead of 2?

Pixels are gone in 3d space, but you still use them in your UI canvas.

Your distances are in "units". How large something looks depends on if you put the camera 1 unit away from the object or 20, 100, etc.