Technical Issues

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Resolution

Image resolution needed for OCR return is a tricky subject. The general rule that one commonly hears is that the preferred resolution of 300 dpi (dots per inch) is needed for good OCR results. However, this generally applies to images of text obtained from a scanner and dpi values can be misleading when a camera is used to image text. DPI is only relevant if the document size is 1:1 and with a camera, this ratio can vary depending on camera placement and distance between camera and text document. Font size is another factor that can lead to poor OCR output. For instance, a 16pt font at 200 dpi will return better OCR results than an 8pt font at 300 dpi. Therefore, a better measure of image resolution for OCR purposes is obtained by counting the x-height of the text. X-height is the pixel height of a lower case x within the document. According to Tesseract, an x-height of 20 pixels or better is preferred. See Tesseract's FAQ for more information concerning this issue.