Thanks, Steve, for all the valuable info. I've used bicubic interpolation in scaling the screenshots and been able to achieve acceptable results. The scale factor I used was 300 divided by the image's resolution. If sharpening and keeping the bit depth to 8 improve the recognition rates further, then I will definitely consider using them in future attempts.
Regards. On Sep 9, 6:14 pm, SteveP <[email protected]> wrote: > "Ideal" may be hard to define for image size. The wiki (I believe) > says the lower case letters (for English) should be at least 20 to 30 > pixels in height. By default, I scale everything by a factor of 3. > If your screen is set to 96 dpi resolution, 300 dpi would be about a > factor of 3. If your font size is large enough, then sometimes you > can get better results without scaling, since scaling often blurs the > image a little. > > What I said about leptonica is for software developers building a > front end to tesseract. If you are using ImageMagick, I suspect that > is fine. > > I think 8-bit per color is standard for tesseract if you are not doing > black and white. > > ClearType is an implementation of sub-pixel rendering, which is > designed for an LCD screen with the red, green and blue sub-pixels in > separate locations. Printers and scanners and OCR typically are not > oriented to sub-pixels. I think OCR accuracy is better with sub-pixel > rendering disabled. > > On Sep 8, 4:56 am, haratron <[email protected]> wrote: > > > I'm also interested in this topic. > > > I have a couple of questions: > > 1. How can I calculate the ideal image size (300dpi?) to feed to > > tesseract? I mean, how do I identify how much scaling the image needs, > > before the OCR procedure. > > 2. I'm currently using ImageMagick's convert program for scaling and > > converting to grayscale. Would it make a difference if I used > > leptonica instead? > > 3. Do the bits of color matter? Is there an optimal color depth? > > 4. Does the OCR work best when ClearType is enabled or disabled? > > > On Tue, Sep 7, 2010 at 11:30 PM, SteveP <[email protected]> wrote: > > > Hi Quan, > > > There is more than one way to scale as you may know. I have seen > > > OCR fail in some cases depending on how you scale. I have a front end > > > I use for my software that calls tesseract. I ended up providing > > > options for scaling and options for converting from 24-bit color to > > > gray or black and white. > > > > Let me start with some simple answers, though. Scaling with > > > interpolation seems to work best most of the time. Converting to gray- > > > scale seems to work most of the time. (I read that Ray Smith did not > > > design tesseract for color screen images, so I really have not > > > experimented with leaving things in color.) I do not think tesseract > > > pays attention to the Alpha channel since it does not pertain to when > > > a single image sits by itself. (Converting to gray-scale does not > > > work in general if the text is rendered with ClearType or sub-pixel > > > rendering. If anybody figures out a good approach for OCR of > > > ClearType, I would appreciate getting an email since I don't read a > > > lot of the posts. Post your answer too.) > > > > I think the scaling software at the leptonica web site is good. I > > > have had some trouble with the method in Windows that uses > > > createGraphics and drawImage. (Someone I worked with used the Windows > > > method on a blank image and got non-blank OCR results because the > > > Windows method seemed to me to introduce a row of black around a > > > couple of the edges. That's how it appeared to me, but it is possible > > > I did something wrong.) > > > > Relative to scaling, I made a post in August about using nearest- > > > neighbor scaling when the characters are close together. This is > > > because scaling with interpolation without sharpening tends to blur > > > the edges of text characters. Leptonica has code for sharpening, I > > > believe, but I have not used it yet. Scaling by a factor of 2 without > > > interpolation and then by a variable factor with interpolation to the > > > needed size is a simple way to get some sharpening and some separation > > > between characters. > > > > On Aug 31, 7:26 pm, Quan Nguyen <[email protected]> wrote: > > >> Hi Ian, > > > >> I'm implementing a feature in my program to enable OCR of screenshots. > > >> The results have been generally better after the captured images were > > >> rescaled from 96 DPI to 300 DPI. I was wondering if other simple > > >> manipulations could be done programmatically to the images to produce > > >> even better results. > > > >> The types of the screenshots are either 32bppArgb or 24bppRgb. Would > > >> changing to grayscale or stripping the Alpha help? > > > >> Quan > > > >> On Aug 31, 12:17 pm, "Ian Ozsvald (A.I. Cookbook)" > > > >> <[email protected]> wrote: > > >> > Hi Quan. > > > >> > I've used tesseract to OCR frames from 640x480 screencast videos, > > >> > generally it worked > > >> > fine:http://ianozsvald.com/2010/05/17/extracting-keyword-text-from-screenc... > > > >> > What problems are you seeing when you try tesseract? > > > >> > Ian. > > > >> > On 30 August 2010 23:46, Quan Nguyen <[email protected]> wrote: > > > >> > > I understand the resolutions of screenshots are typically inadequate > > >> > > for OCR, but besides rescaling to a higher resolution, say, 300 DPI, > > >> > > what other preprocessing operations may be needed on the images to > > >> > > yield optimal OCR results? > > > >> > > Thanks. > > > >> > > -- > > >> > > You received this message because you are subscribed to the Google > > >> > > Groups "tesseract-ocr" group. > > >> > > To post to this group, send email to [email protected]. > > >> > > To unsubscribe from this group, send email to > > >> > > [email protected]. > > >> > > For more options, visit this group > > >> > > athttp://groups.google.com/group/tesseract-ocr?hl=en. > > > >> > -- > > >> > Ian Ozsvald (A.I. researcher, screencaster) > > >> > [email protected] > > > >> >http://IanOzsvald.comhttp://MorConsulting.com/http://blog.AICookbook....quoted > > >> > text - > > > >> - Show quoted text - > > > > -- > > > You received this message because you are subscribed to the Google Groups > > > "tesseract-ocr" group. > > > To post to this group, send email to [email protected]. > > > To unsubscribe from this group, send email to > > > [email protected]. > > > For more options, visit this group > > > athttp://groups.google.com/group/tesseract-ocr?hl=en.-Hide quoted text - > > > - Show quoted text - -- You received this message because you are subscribed to the Google Groups "tesseract-ocr" group. To post to this group, send email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/tesseract-ocr?hl=en.
