how to translate matrix in .net.android?

Question

I want to use canvas in .net.android and want to draw cube on it.

I create custom view and rewrite ondraw

how to draw one cube using canvas?

if there is x,y,z and width height how to draw it?

Answer 1

Hello,

Firstly, if you want to draw cube in .NET for android, Use GLSurfaceView, it a better way to do it. I can be draw 2D or 3D cube and implement Dynamic Transformation simplely.

However, if you want create custom view and rewrite ondraw.

You can refer to the following code to draw the cube. I use Use 8 three-dimensional coordinate points to represent the vertices of the cube, each point contains x/y/z coordinates; Define 6 faces, each face consists of 4 vertex indices; Assign a different ARGB color value to each face.

Normally, use Canvas to draw 2D cube, if want to draw 3D, we need to do following things: ‌Perspective projection formula‌: Calculate the scaling factor by _depth/(_depth - point‌:ml-citation{ref="5" data="citationList"}) to achieve a simple perspective effect. ‌Coordinate system conversion‌: Convert 3D coordinates to 2D screen coordinates, considering the center point of the View (centerX, century) as the projection reference .

public class MyCanvasView : View
{
     private readonly Paint _paint;
     private readonly float[][] _cubeVertices;
     private readonly int[][] _faces;
     private readonly int[] _faceColors;
     private readonly float _depth = 400f;
 
     public MyCanvasView(Context context) : base(context)
     {
         _paint = new Paint { AntiAlias = true };
         _paint.SetStyle(Paint.Style.Fill);
         // Define cube vertices (x, y, z)
         _cubeVertices = new float[][]
         {
             new float[] { -100, -100, -100 }, // 0: Front top-left
             new float[] {  100, -100, -100 }, // 1: Front top-right
             new float[] {  100,  100, -100 }, // 2: Front bottom-right
             new float[] { -100,  100, -100 }, // 3: Front bottom-left
             new float[] { -100, -100,  100 }, // 4: Back top-left
             new float[] {  100, -100,  100 }, // 5: Back top-right
             new float[] {  100,  100,  100 }, // 6: Back bottom-right
             new float[] { -100,  100,  100 }  // 7: Back bottom-left
         };
 
         // Define faces using 4 vertices each
         _faces = new int[][]
         {
             new int[] { 0, 1, 2, 3 }, // Front
             new int[] { 4, 5, 6, 7 }, // Back
             new int[] { 0, 1, 5, 4 }, // Top
             new int[] { 3, 2, 6, 7 }, // Bottom
             new int[] { 0, 3, 7, 4 }, // Left
             new int[] { 1, 2, 6, 5 }  // Right
         };
 
         // Assign unique colors to each face
         _faceColors = new int[]
         {
             Color.Red, Color.Blue, Color.Green, Color.Yellow, Color.Cyan, Color.Magenta
         };
     }
 
     protected override void OnDraw(Canvas canvas)
     {
         base.OnDraw(canvas);
         canvas.DrawColor(Color.Black); // Background color
 
         int centerX = Width / 2;
         int centerY = Height / 2;
 
         // Project 3D points to 2D
         float[][] projected = new float[8][];
         for (int i = 0; i < 8; i++)
         {
             projected[i] = ProjectPoint(_cubeVertices[i], centerX, centerY);
         }
 
         // Sort faces by depth for simple visibility handling
         List<Tuple<float, int>> faceDepths = new();
         for (int i = 0; i < _faces.Length; i++)
         {
             float avgDepth = _faces[i].Select(idx => _cubeVertices[idx][2]).Average();
             faceDepths.Add(new Tuple<float, int>(avgDepth, i));
         }
         faceDepths = faceDepths.OrderByDescending(f => f.Item1).ToList();
 
         // Draw faces back-to-front
         foreach (var (depth, faceIndex) in faceDepths)
         {
             DrawFace(canvas, projected, _faces[faceIndex], _faceColors[faceIndex]);
         }
     }
 
     private void DrawFace(Canvas canvas, float[][] projected, int[] face, int color)
     {
         Android.Graphics.Path path = new Android.Graphics.Path();
         path.MoveTo(projected[face[0]][0], projected[face[0]][1]);
         for (int i = 1; i < face.Length; i++)
         {
             path.LineTo(projected[face[i]][0], projected[face[i]][1]);
         }
         path.Close();
 
         _paint.Color = new Color(color);
         canvas.DrawPath(path, _paint);
     }
 
     private float[] ProjectPoint(float[] point, int centerX, int centerY)
     {
         float scale = _depth / (_depth - point[2]);
         float x = point[0] * scale + centerX;
         float y = point[1] * scale + centerY;
         return new float[] { x, y };
     }
}

Best Regards,

Leon Lu

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