The smallest unit of digital image is pixel. Therefore, there is no any other description except for pixel.
Let’s take this small image (20×20 pixel) for example. Then, try to draw a filled circle. Seems like the circle shown below, right?
![図:ジャギジャギした円の画像](https://hub.firealpaca.net/ja/lectures/fa_tips_0035_01.png)
See!! The circle is not as pretty as imagined right. However, there is no other description other than using 1 pixel. You would like to draw a circle like this (on the left side of examples below) at first, but you are not able to make the units more smaller.
![](https://hub.firealpaca.net/wp-content/uploads/2021/07/en_388_02.gif)
As the former image, you might think that is difference between “fill” and “unfill”.
For example, take this “covered in half” pixel, set in 50% opacity pixel. Or take this “covered in one quarter” pixel, set in 25% opacity opacity pixel. What if you adjust the opacity based on the area covered while you are drawing like this?
![](https://hub.firealpaca.net/wp-content/uploads/2021/07/en_388_01.gif)
![図:アンチエイリアシング処理した小さな円](https://hub.firealpaca.net/ja/lectures/fa_tips_0035_04.png)