Can Tesseract OCR recognize subscripts and superscripts? - ocr

Question

I have problems with the general recognition of subscript and superscript in text fragments.
Example-image:
I used Tesseract 4.1.1 with the training data available under https://github.com/tesseract-ocr/tessdata_best. The numerous options had default values except:
tessedit_create_hocr = 1 (to get result as HOCR)
hocr_font_info = 1 (to get additional font infos like font size)
hocr_char_boxes = 1 (to get character-based result)
The language was set to eng. Neither with page segmentation mode 3 (PSM_AUTO_OSD) nor 11 (PSM_SPARSE_TEXT) nor 12 (PSM_SPARSE_TEXT_OSD) the subscript/superscript was recognized correctly.
In the output the sub/sup-fragments were all more or less wrong:
"SubtextSub" is recognized as "Subtextsu,"
"SuptextSub" is recognized as "Suptexts?"
"P0" is recognized as "Po"
"P100" is recognized as "P1go"
"a2+b2" is recognized as "a+b?"
Using Tesseract for OCR is there a way to ...?
optimize subscript/superscript handling
get infos about recognized subscript/superscript (in the hocr-output - ideally for each character)

Answer 1

Working on the quality of the image as suggested in other questions/answers to this topic didn't really change anything.
Following these 2 links from the tesseract-google-newsgroup at first it really seemed to be a question of training:
link1 and link2.
But after doing some experiments I found out, that the used OEM_DEFAULT-OCR engine mode just doesn't bring up the needed information. I found a partial solution to the problem. Partial, because I now get most infos about sub/sup and also the recognized characters are right in most cases, but not for all characters.
Using the OEM_TESSERACT_ONLY-OCR engine mode (=the legacy mode) and some API methods provided by Tess4J I came up with the following java test class:
public class SubSupEvaluator {
public void determineSubSupCharacters(BufferedImage image) {
//1. initialize Tesseract and set image infos
TessBaseAPI handle = TessAPI1.TessBaseAPICreate();
try {
int bpp = image.getColorModel().getPixelSize();
int bytespp = bpp / 8;
int bytespl = (int) Math.ceil(image.getWidth() * bpp / 8.0);
TessBaseAPIInit2(handle, new File("./tessdata/").getAbsolutePath(), "eng", TessOcrEngineMode.OEM_TESSERACT_ONLY);
TessBaseAPISetPageSegMode(handle, TessPageSegMode.PSM_AUTO_OSD);
TessBaseAPISetImage(handle, ImageIOHelper.convertImageData(image), image.getWidth(), image.getHeight(), bytespp, bytespl);
//2. start actual OCR run
TessBaseAPIRecognize(handle, null);
//3. iterate over the result character-wise
TessResultIterator ri = TessBaseAPIGetIterator(handle);
TessPageIterator pi = TessResultIteratorGetPageIterator(ri);
TessPageIteratorBegin(pi);
do {
//determine character
Pointer ptr = TessResultIteratorGetUTF8Text(ri, TessPageIteratorLevel.RIL_SYMBOL);
String character = ptr.getString(0);
TessDeleteText(ptr); //release memory
//determine position information
IntBuffer leftB = IntBuffer.allocate(1);
IntBuffer topB = IntBuffer.allocate(1);
IntBuffer rightB = IntBuffer.allocate(1);
IntBuffer bottomB = IntBuffer.allocate(1);
TessPageIteratorBoundingBox(pi, TessPageIteratorLevel.RIL_SYMBOL, leftB, topB, rightB, bottomB);
//write info to console
System.out.println(String.format("%s - position [%d %d %d %d], subscript: %b, superscript: %b", character, leftB.get(), topB.get(),
rightB.get(), bottomB.get(), TessAPI1.TessResultIteratorSymbolIsSubscript(ri) == TessAPI1.TRUE,
TessAPI1.TessResultIteratorSymbolIsSuperscript(ri) == TessAPI1.TRUE));
} while (TessPageIteratorNext(pi, TessPageIteratorLevel.RIL_SYMBOL) == TessAPI1.TRUE);
} finally {
TessBaseAPIDelete(handle); //release memory
}
}
}
The legacy mode only works with 'normal' training data. Using the '-best' training data is bringing an error.

Answer 2

There is very little information on this topic.
One option to enhance sub/superscript character recognition (even if not the position itself) is by preprocessing the image, with cv2 / pil (also pillow) e.g., and then tesseract it.
See
How to detect subscript numbers in an image using OCR?
Related (but otherwise not answering the question):
https://www.mail-archive.com/tesseract-ocr#googlegroups.com/msg19434.html
https://github.com/tesseract-ocr/tesseract/blob/master/src/ccmain/superscript.cpp

Answer 3

what do you guys think about getting tesseract to recognize single letters?
Tesseract does not recognize single characters
I tried it with the option --psm 10
tesseract imTstg.png out5 --psm 10
but it did not seem to work. I am thinking about just running yolo to detect the single letters.