OmniDocPDF rotation at any angle: processing of page and image skew
Explains how to determine which layer the problem is when a PDF page or picture is skewed, distinguishes between right-angle rotation and small-angle correction, and provides the respective correction steps for scanned and digital PDFs, as well as the preparations that should be made before OCR.
Key points of this article
- The page rotation tool usually only rotates 90 degrees and 180 degrees. Small-angle distortion needs to be corrected back to the image itself, and page rotation cannot be expected to solve it.
- First use the method of selecting text to determine whether it is a digital PDF or a scanned copy. The two methods of editing are completely different. If you choose the wrong method, the editing will become more messy.
- The scanned document should be straightened in the image layer and the white edges should be cropped before generating PDF or performing OCR recognition.
- For skewed images in digital PDF, you need to return to the original Word, PPT or image file, correct it and re-export it, rather than hard-transfer it in the PDF.
Many people have encountered this situation: the scanned contract is high on one side and low on the other, the diagram inserted into the document through the screenshot is crooked a few degrees, and the handout taken with the mobile phone looks like it has been blown by the wind. I was looking for a way to "rotate PDF to any angle", but found that most page rotation buttons can only rotate 90 degrees or 180 degrees, and cannot rotate at small angles at all.
This article is for those who need to correct PDF skew. After reading, you can first determine which layer the skew is: the entire page is in the wrong direction, the paper was not placed straight when scanning, or a certain picture in the document itself is skewed. Each level corresponds to different methods of practice. After a clear judgment, most problems can be solved using common methods.
How to fix skewed PDF? One sentence conclusion
Errors in right-angle directions need to be solved by page rotation; small-angle distortions need to be corrected at the image level, or returned to the source file for modification. The processing paths of scanned documents and digital PDFs are different, so distinguish them first before proceeding.
| your situation | Recommended practices | reason |
|---|---|---|
| The entire page is in the wrong horizontal or vertical orientation. | Use the PDF editor to rotate the page and adjust it at right angles. | Right-angle rotation is a page property and can be modified directly |
| The whole page of the scanned document is crooked several degrees | Rotate and crop at a small angle in the image editing software, and then generate PDF | Skew exists in the pixels and page rotation cannot change it |
| A picture in the digital PDF is crooked | Go back to the original Word, PPT or picture file to make corrections and re-export the PDF | The image is saved together with the page content, and the text will be rotated when the entire page is rotated. |
| Scanned documents require OCR | Arrange the page first, then recognize the text | Skewing will cause lines of text to be misaligned, affecting recognition results and subsequent searches. |
First determine what kind of problem it is
Before you start, do a few simple checks. Most of the skew problems can be clearly seen at this step.
| symptoms | Common reasons | How to confirm |
|---|---|---|
| The text line of the whole page has a significant slope, but the page is in the right direction. | The paper was not placed correctly when scanning or taking pictures. | Stick a line of text with a ruler or screen reference line to measure the deviation |
| The whole page is in the wrong direction, like lying down. | The page property is horizontal, or previously rotated | View the page direction in the reader and compare the reading direction of the text |
| Only a certain picture or table is crooked. | Tilt when the original picture is taken or screenshot | Open the picture in the source file to see if the picture itself is crooked |
| Copy the text garbled, or the content cannot be found | This is a scanned piece. The page has only images and no text layer. | If you select a paragraph of text with the mouse, you can select and copy the description as numeric text. |
| The text is still crooked after rotation | The page is rotated, but it is askew in the scanned image. | Confirm whether it is a scanned piece. The scanned part needs to go back to the image layer for correction. |
# item-by-item troubleshooting and repair
It is recommended to operate according to the following order, first make an easy judgment, and then make corrections that need to change the image.
- confirm the file type. use the mouse to select a text in the reader. Can be selected and copied, that is, the digital PDF; is not selected, mostly scanned pieces or photo generated PDF.
- measure the skew angle. * attach a line of text with a ruler or screen guide to estimate the deviation. If the deviation is close to 90 degrees, 180 degrees, or 270 degrees, it is a right-angle problem, proceed to step 3. If it's only a few degrees, and it's a small angle problem, skip to step 4.
- right-angle deviation: rotate with the page. * in the PDF editor of the OmniDoc desktop client (download address: download.omnidoc.top), use the rotation feature in the page operation, select the direction and save the changes. Turn it page by page and make sure it is in the right direction.
- small angle deviation: prefix the image. Page rotation is only performed at right angles, and small angles need to be processed at the image layer. For details, see the two sections below.
- check the edges after rotation. after turning at a small angle, the four corners of the page will show a white edge and need to be cut. Check to see if the page number, header, or body edge has been mistakenly cut.
- OCR comes last. straighten the page first, and then do OCR identification. After the completion of the recognition, a few paragraphs are selected, focusing on checking numbers, formulas and proper nouns, and the recognition results need to be checked manually.
scanned pieces: correct the image before generating PDF
-Save the scanned page as a picture, or use the original picture file directly. -use the rotation function of the general picture editing software to enter a small angle in the opposite direction (for example, 2 degrees to the right, turn back to 2 degrees) to make the text lines horizontal. -cut off the white edges exposed after rotation, leaving only the text area to avoid the page number being cut. -synthesize all the page images into a new PDF in the original order. -for text search, OCR the new document in the PDF editor and spot-check the recognition results.
Digital PDF: go back to the source file for correction
-if the inserted picture or table is crooked, open the original Word, PPT, or Excel file, rotate the picture in the source file, and then re-export PDF. -if you can't find the source file, you can cut the crooked page into a picture, turn it into a full member in the image editing software, and regenerate it into a new page of PDF. Then use the PDF editor to merge, split and other page operations, reassemble it with other pages. -it is not recommended to make small-angle changes to the whole page directly in the digital PDF, because the text and the picture will rotate together, and the layout is easy to be misplaced.
# how to avoid the next occurrence
-align the paper against the feed guide when scanning, and take pictures with the edge of the page when the phone takes a picture. After shooting, you will first become a regular member in the picture, and then generate a PDF. Before the screenshot is inserted into the document, straighten it out with the picture tool, and don't wait for the final version to fix it. Many picture editing software have the function of straightening or rotating, which is easy to operate. -unify page orientation in the source file. Horizontal tables and diagrams should be placed in the same section as far as possible to reduce the number of page rotations in the later stage. -if the same batch of scanned pieces are skewed at the same angle, first process one page, write down the angle value used, and then use the same parameters to deal with other pages to avoid re-estimation of each page. -keep the original file before modification, save another copy before operation, and go back to the starting point if you make an error.
# frequently asked questions (FAQ)
1. Can PDF be rotated at any angle directly?
Most page rotation functions are only in 90-degree steps. If you want to reduce the angle, you need to first turn the corresponding image into a positive, and then regenerate the PDF, which is a more general approach.
2. Why is the text still crooked after rotation?
It may be the properties of the rotated page, while the skew itself is branded in the scanned image. First confirm whether it is a scanned piece, the scanned part needs to go back to the image layer to correct, and then regenerate the PDF.
3. Can I do OCR directly after rotation?
Yes, but it is recommended to correct the page before identifying it. The more correct the text line, the less misplaced the identification will be. Spot checks are still needed after the recognition is completed, especially numbers and proper nouns.
4. Why is there a white edge on the edge of the page after rotating at a small angle?
After turning an angle, the original rectangular page will leave a blank in the four corners. Cut off the white edge, and keep the page number and text edge when cutting.
# Summary
PDF skew does not always have to be solved by a button called "how to rotate pdf at any angle". First distinguish whether the right angle direction is wrong, the scanning piece is skewed at a small angle, or the picture in the source file is crooked, and then choose the corresponding method, which can usually be repaired quickly. The important contents after revision, especially the OCR results, should be checked item by item.
