发黄和脏背景

纸放久了会发黄，扫描仪也会把白纸扫成灰底。字看起来都脏，但处理时要看浅色字有没有一起被抹掉。

你看到的是什么

整页偏黄或偏灰，四角往往更重，字仍然是深色。如果只有装订处一条由深到浅的暗带，那是阴影，请看去阴影，不要先用去背景。四周是一条纯黑框时，先看黑边。

在体验页怎么试

打开去污体验。先选「一键去污」，用示例或上传这一页，点运行，对比左右。底色仍脏、字还清楚时，再选「去背景」跑一次。去背景比一键去污更狠，浅灰铅笔字和浅色表格线更容易消失。

跑完要核对

放大看页眉页码、浅色批注，以及红色以外的印章边缘。底色变白但笔画断了，就改回一键去污，或保留原图。发黄本身不是必须去掉的东西。字已经能读时，留着原扫描往往更稳妥。

不适合的情况

字和污渍已经是同一种深浅时，软件分不清。大块手指阴影或订口暗带用去阴影。细麻点用降噪或去斑点，降噪不会把黄底变白。

Yellowing and dirty backgrounds

Old paper turns yellow, and a scanner can turn white paper gray. Both make text look dirty. The risk is that light text disappears with the background.

What you are looking at

The whole page is yellow or gray, often worse in the corners, while the text is still dark. A dark band that fades away from the binding is a shadow. Read deshadow first, not remove background. A solid black frame around the page is a border problem.

How to try it

Open the cleanup demo. Select One-click, open a sample or upload the page, and run. Compare the two sides. If the background is still dirty and the text is still clear, run Remove BG once. Remove background is stronger than one-click. Light pencil and pale rules are easier to lose.

What to check afterward

Zoom in on headers, page numbers, light notes, and the edge of any seal that is not red. If the background is white but strokes are broken, go back to one-click or keep the original. Yellowing does not have to be removed. If the words are already readable, the original scan is often the safer copy.

When this is the wrong tool

If the ink and the stain are the same darkness, the software cannot separate them. A finger shadow or a gutter band needs deshadow. Fine grain needs denoise or despeckle. Denoise will not turn a yellow page white.