Custom drawing in 2D

Why?

Godot has nodes to draw sprites, polygons, particles, and all sorts of stuff. For most cases, this is enough; but not always. Before crying in fear, angst, and rage because a node to draw that specific something does not exist… it would be good to know that it is possible to easily make any 2D node (be it Control or Node2D based) draw custom commands. It is really easy to do it, too.

But…

Custom drawing manually in a node is really useful. Here are some examples why:

  • Drawing shapes or logic that is not handled by nodes (example: making a node that draws a circle, an image with trails, a special kind of animated polygon, etc).
  • Visualizations that are not that compatible with nodes: (example: a tetris board). The tetris example uses a custom draw function to draw the blocks.
  • Drawing a large number of simple objects. Custom drawing avoids the overhead of using nodes which makes it less memory intensive and potentially faster.
  • Making a custom UI control. There are plenty of controls available, but it’s easy to run into the need to make a new, custom one.

OK, how?

Add a script to any CanvasItem derived node, like Control or Node2D. Then override the _draw() function.

GDScript
C#
extends Node2D

func _draw():
    # Your draw commands here
    pass

public override void _Draw()
{
    // Your draw commands here
}

Draw commands are described in the CanvasItem class reference. There are plenty of them.

Updating

The _draw() function is only called once, and then the draw commands are cached and remembered, so further calls are unnecessary.

If re-drawing is required because a state or something else changed, simply call CanvasItem.update() in that same node and a new _draw() call will happen.

Here is a little more complex example, a texture variable that will be redrawn if modified:

GDScript
C#
extends Node2D

export (Texture) var texture setget _set_texture

func _set_texture(value):
    # If the texture variable is modified externally,
    # this callback is called.
    texture = value #texture was changed
    update() # update the node

func _draw():
    draw_texture(texture, Vector2())

public class CustomNode2D : Node2D
{
    private Texture _texture;
    public Texture Texture
    {
        get
        {
            return _texture;
        }

        set
        {
            _texture = value;
            Update();
        }
    }

    public override void _Draw()
    {
        DrawTexture(_texture, new Vector2());
    }
}

In some cases, it may be desired to draw every frame. For this, just call update() from the _process() callback, like this:

GDScript
C#
extends Node2D

func _draw():
    # Your draw commands here
    pass

func _process(delta):
    update()

public class CustomNode2D : Node2D
{
    public override void _Draw()
    {
        // Your draw commands here
    }

    public override void _Process(float delta)
    {
        Update();
    }
}

An example: drawing circular arcs

We will now use the custom drawing functionality of the Godot Engine to draw something that Godot doesn’t provide functions for. As an example, Godot provides a draw_circle() function that draws a whole circle. However, what about drawing a portion of a circle? You will have to code a function to perform this and draw it yourself.

Arc function

An arc is defined by its support circle parameters, that is, the center position and the radius. The arc itself is then defined by the angle it starts from and the angle at which it stops. These are the 4 arguments that we have to provide to our drawing function. We’ll also provide the color value, so we can draw the arc in different colors if we wish.

Basically, drawing a shape on the screen requires it to be decomposed into a certain number of points linked from one to the next. As you can imagine, the more points your shape is made of, the smoother it will appear, but the heavier it will also be in terms of processing cost. In general, if your shape is huge (or in 3D, close to the camera), it will require more points to be drawn without it being angular-looking. On the contrary, if your shape is small (or in 3D, far from the camera), you may decrease its number of points to save processing costs; this is known as Level of Detail (LOD). In our example, we will simply use a fixed number of points, no matter the radius.

GDScript
C#
func draw_circle_arc(center, radius, angle_from, angle_to, color):
    var nb_points = 32
    var points_arc = PoolVector2Array()

    for i in range(nb_points + 1):
        var angle_point = deg2rad(angle_from + i * (angle_to-angle_from) / nb_points - 90)
        points_arc.push_back(center + Vector2(cos(angle_point), sin(angle_point)) * radius)

    for index_point in range(nb_points):
        draw_line(points_arc[index_point], points_arc[index_point + 1], color)

public void DrawCircleArc(Vector2 center, float radius, float angleFrom, float angleTo, Color color)
{
    int nbPoints = 32;
    var pointsArc = new Vector2[nbPoints];

    for (int i = 0; i < nbPoints; ++i)
    {
        float anglePoint = Mathf.Deg2Rad(angleFrom + i * (angleTo - angleFrom) / nbPoints - 90f);
        pointsArc[i] = center + new Vector2(Mathf.Cos(anglePoint), Mathf.Sin(anglePoint)) * radius;
    }

    for (int i = 0; i < nbPoints - 1; ++i)
        DrawLine(pointsArc[i], pointsArc[i + 1], color);
}

Remember the number of points our shape has to be decomposed into? We fixed this number in the nb_points variable to a value of 32. Then, we initialize an empty PoolVector2Array, which is simply an array of Vector2s.

The next step consists of computing the actual positions of these 32 points that compose an arc. This is done in the first for-loop: we iterate over the number of points for which we want to compute the positions, plus one to include the last point. We first determine the angle of each point, between the starting and ending angles.

The reason why each angle is decreased by 90° is that we will compute 2D positions out of each angle using trigonometry (you know, cosine and sine stuff…). However, to be simple, cos() and sin() use radians, not degrees. The angle of 0° (0 radian) starts at 3 o’clock, although we want to start counting at 12 o’clock. So we decrease each angle by 90° in order to start counting from 12 o’clock.

The actual position of a point located on a circle at angle angle (in radians) is given by Vector2(cos(angle), sin(angle)). Since cos() and sin() return values between -1 and 1, the position is located on a circle of radius 1. To have this position on our support circle, which has a radius of radius, we simply need to multiply the position by radius. Finally, we need to position our support circle at the center position, which is performed by adding it to our Vector2 value. Finally, we insert the point in the PoolVector2Array which was previously defined.

Now, we need to actually draw our points. As you can imagine, we will not simply draw our 32 points: we need to draw everything that is between each of them. We could have computed every point ourselves using the previous method, and drew it one by one. But this is too complicated and inefficient (except if explicitly needed), so we simply draw lines between each pair of points. Unless the radius of our support circle is big, the length of each line between a pair of points will never be long enough to see them. If that were to happen, we would simply need to increase the number of points.

Draw the arc on the screen

We now have a function that draws stuff on the screen; it is time to call it inside the _draw() function:

GDScript
C#
func _draw():
    var center = Vector2(200, 200)
    var radius = 80
    var angle_from = 75
    var angle_to = 195
    var color = Color(1.0, 0.0, 0.0)
    draw_circle_arc(center, radius, angle_from, angle_to, color)

public override void _Draw()
{
    var center = new Vector2(200, 200);
    float radius = 80;
    float angleFrom = 75;
    float angleTo = 195;
    var color = new Color(1, 0, 0);
    DrawCircleArc(center, radius, angleFrom, angleTo, color);
}

Result:

../../_images/result_drawarc.png

Arc polygon function

We can take this a step further and not only write a function that draws the plain portion of the disc defined by the arc, but also its shape. The method is exactly the same as before, except that we draw a polygon instead of lines:

GDScript
C#
func draw_circle_arc_poly(center, radius, angle_from, angle_to, color):
    var nb_points = 32
    var points_arc = PoolVector2Array()
    points_arc.push_back(center)
    var colors = PoolColorArray([color])

    for i in range(nb_points + 1):
        var angle_point = deg2rad(angle_from + i * (angle_to - angle_from) / nb_points - 90)
        points_arc.push_back(center + Vector2(cos(angle_point), sin(angle_point)) * radius)
    draw_polygon(points_arc, colors)

public void DrawCircleArcPoly(Vector2 center, float radius, float angleFrom, float angleTo, Color color)
{
    int nbPoints = 32;
    var pointsArc = new Vector2[nbPoints + 1];
    pointsArc[0] = center;
    var colors = new Color[] { color };

    for (int i = 0; i < nbPoints; ++i)
    {
        float anglePoint = Mathf.Deg2Rad(angleFrom + i * (angleTo - angleFrom) / nbPoints - 90);
        pointsArc[i + 1] = center + new Vector2(Mathf.Cos(anglePoint), Mathf.Sin(anglePoint)) * radius;
    }

    DrawPolygon(pointsArc, colors);
}
../../_images/result_drawarc_poly.png

Dynamic custom drawing

All right, we are now able to draw custom stuff on the screen. However, it is static; let’s make this shape turn around the center. The solution to do this is simply to change the angle_from and angle_to values over time. For our example, we will simply increment them by 50. This increment value has to remain constant or else the rotation speed will change accordingly.

First, we have to make both angle_from and angle_to variables global at the top of our script. Also note that you can store them in other nodes and access them using get_node().

GDScript
C#
extends Node2D

var rotation_angle = 50
var angle_from = 75
var angle_to = 195

public class CustomNode2D : Node2D
{
    private float _rotationAngle = 50;
    private float _angleFrom = 75;
    private float _angleTo = 195;
}

We make these values change in the _process(delta) function.

We also increment our angle_from and angle_to values here. However, we must not forget to wrap() the resulting values between 0 and 360°! That is, if the angle is 361°, then it is actually 1°. If you don’t wrap these values, the script will work correctly, but the angle values will grow bigger and bigger over time until they reach the maximum integer value Godot can manage (2^31 - 1). When this happens, Godot may crash or produce unexpected behavior.

Finally, we must not forget to call the update() function, which automatically calls _draw(). This way, you can control when you want to refresh the frame.

GDScript
C#
func _process(delta):
    angle_from += rotation_angle
    angle_to += rotation_angle

    # We only wrap angles when both of them are bigger than 360.
    if angle_from > 360 and angle_to > 360:
        angle_from = wrapf(angle_from, 0, 360)
        angle_to = wrapf(angle_to, 0, 360)
    update()

private float Wrap(float value, float minVal, float maxVal)
{
    float f1 = value - minVal;
    float f2 = maxVal - minVal;
    return (f1 % f2) + minVal;
}

public override void _Process(float delta)
{
    _angleFrom += _rotationAngle;
    _angleTo += _rotationAngle;

    // We only wrap angles when both of them are bigger than 360.
    if (_angleFrom > 360 && _angleTo > 360)
    {
        _angleFrom = Wrap(_angleFrom, 0, 360);
        _angleTo = Wrap(_angleTo, 0, 360);
    }
    Update();
}

Also, don’t forget to modify the _draw() function to make use of these variables:

GDScript
C#
func _draw():
   var center = Vector2(200, 200)
   var radius = 80
   var color = Color(1.0, 0.0, 0.0)

   draw_circle_arc( center, radius, angle_from, angle_to, color )

public override void _Draw()
{
    var center = new Vector2(200, 200);
    float radius = 80;
    var color = new Color(1, 0, 0);

    DrawCircleArc(center, radius, _angleFrom, _angleTo, color);
}

Let’s run! It works, but the arc is rotating insanely fast! What’s wrong?

The reason is that your GPU is actually displaying the frames as fast as it can. We need to “normalize” the drawing by this speed; to achieve that, we have to make use of the delta parameter of the _process() function. delta contains the time elapsed between the two last rendered frames. It is generally small (about 0.0003 seconds, but this depends on your hardware), so using delta to control your drawing ensures that your program runs at the same speed on everybody’s hardware.

In our case, we simply need to multiply our rotation_angle variable by delta in the _process() function. This way, our 2 angles will be increased by a much smaller value, which directly depends on the rendering speed.

GDScript
C#
func _process(delta):
    angle_from += rotation_angle * delta
    angle_to += rotation_angle * delta

    # We only wrap angles when both of them are bigger than 360.
    if angle_from > 360 and angle_to > 360:
        angle_from = wrapf(angle_from, 0, 360)
        angle_to = wrapf(angle_to, 0, 360)
    update()

public override void _Process(float delta)
{
    _angleFrom += _rotationAngle * delta;
    _angleTo += _rotationAngle * delta;

    // We only wrap angles when both of them are bigger than 360.
    if (_angleFrom > 360 && _angleTo > 360)
    {
        _angleFrom = Wrap(_angleFrom, 0, 360);
        _angleTo = Wrap(_angleTo, 0, 360);
    }
    Update();
}

Let’s run again! This time, the rotation displays fine!

Tools

Drawing your own nodes might also be desired while running them in the editor to use as a preview or visualization of some feature or behavior.

Remember to use the “tool” keyword at the top of the script (check the GDScript basics reference if you forgot what this does).