What goes wrong with scans and screenshots?
Scans arrive with a margin of scanner lid around the page, a slight tilt, specks of dust and a gray cast where the paper was not quite white. Screenshots arrive with dead space, window edges and, too often, the smeared text of having been saved as a JPEG. Both usually carry more file size than they need.
Each problem has its own tool, and the order below loses the least: fix the geometry first, then the pixels, and choose the format last.
How do you trim and straighten?
- Trim the uniform border away. The tool finds the border color for itself, and the tolerance setting decides how close to it a pixel must be to count as border, which handles scanner edges that are almost, but not quite, one color. Trim whitespace and borders
- If the page is a photo taken at an angle, straighten it: drag four handles onto the corners of the page, pick Letter, A4 or an automatic size, and the perspective is undone. Straighten a document photo
Trim works on screenshots too, cutting a capture down to the content inside the empty space around it.
How do you fix specks, grayness and softness?
- Remove specks and dust with the median filter, which replaces each pixel with the middle value of its neighbors and takes isolated specks out without smearing edges. Reduce image noise
- For a page of black text, make it pure black and white. Otsu's method picks the cut-off from the image itself, and adaptive thresholding copes with a page that is lit unevenly. Black and white threshold
- Sharpen a little to recover the crispness a scanner or a phone camera loses. The preview is at full size, so halos show up before they reach the file. Sharpen image
Go gently with sharpening. An unsharp mask raises the contrast along every edge, and too much of it draws a bright outline around each letter.
Should you save a screenshot as PNG or JPG?
PNG. A screenshot is made of flat color and hard edges: text, icons, lines and blocks of one shade. PNG stores those exactly and compresses them well, because it describes runs of repetition. JPEG is built for photographs; on a screenshot it blurs every letter and scatters faint blocks around each edge, and the saving is often small.
The same goes for a scan of text or line art, especially after thresholding to black and white. A scan of a photograph is the exception, and belongs in JPEG or WebP. The Image converter writes any of them; pick PNG for text and diagrams.
Worth knowingNever re-save a screenshot as JPEG to make it smaller. If it is too big, crop away what is not needed or reduce its dimensions, and the text stays readable either way.
What if the result has to be a PDF?
Clean each page first, then put them together with Image to PDF, one page per image in the order you set. A scan that is already a PDF and simply too large to send can be shrunk with the Rasterize pages mode of Compress PDF, which is built for scans; How to make a PDF small enough to email walks through it.