Custom drawing in 2D¶
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.
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.
Draw commands are described in the CanvasItem class reference. There are plenty of them.
_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:
In some cases, it may be desired to draw every frame. For this, just
update() from the
_process() callback, like this:
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
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.
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.
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
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,
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
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
center position, which is performed by adding it to our
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
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:
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
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
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
_draw(). This way, you can control when you want to refresh the frame.
Also, don’t forget to modify the
_draw() function to make use of these variables:
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
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
control your drawing ensures that your program runs at the same speed on
In our case, we simply need to multiply our
rotation_angle variable by
_process() function. This way, our 2 angles will be increased by a much
smaller value, which directly depends on the rendering speed.
Let’s run again! This time, the rotation displays fine!