SVG study notes (coordinate system and coordinate transformation)

1. The concept of SVG world, field of view, window
1. width, height-control window
2. SVG code-define the world
3. viewBox, preserveAspectRatio-control field of view

<svg xmlns="..."
       width="800" height="600"
       viewBox="0 0 400 300"
       preserveAspectRatio="xMidYMid meet">
</svg>