a rectangular pattern of lines that an electron beam follows on a television or computer screen

The characters can be exposed line by line using a raster scan. 使用光栅扫描可以逐行暴露字符。

a type of computer image that is based on a rectangular grid (= pattern of lines and columns) of pixels (= small dots)

PNG is a graphics file format for raster images. PNG是光栅图像的一种图形文件格式。