I am working in OCR android Application. Now I can take images and extract word easily using google vision API but the result is not 100% according to the angle of capturing the image. and illumination. So I tried to make some image processing techniques on image before extracting text. but I search a lot but I can't deiced what is the best image processing technique to use.(blurring,filtering) to smooth image and improve its quality. So if there is any libraries or guide line to follow up with it in this subject. How to improve image quality before extracting text
I testes this libraries for OCR operation
Without having your sample images, it is hard to say exactly what image processing functions would help you here. Every image can be different in its own way depending on the environment the images were captured in so there are not a set number of image processing functions that you can use for ALL images. If you tried Tesseract, they have a post on Github explaining the best ways to improve an image to get better recognition results. I can't really give you any specific image processing functions that would help you from the link since I don't know exactly what your images look like.
Another way to improve recognition results is to train the OCR engine itself to be able to better recognize the text you have in your images. Tesseract gives you the ability to do this as well:
https://github.com/tesseract-ocr/tesseract/wiki/TrainingTesseract-4.00