Using graphic filters you can adjust color, remove distortion and resize captcha image for better recognition.

What you should get

Ideally, after applying filters, you should get the following results:

1) Black&white images, without shades of gray. Black letters, white background.
2) Minimum distortion such as unwanted lines, circles, dots, spots, etc.
3) Medium size for simple captchas and 1.5-2 times enlarge for complex captchas.

1) Filters for captcha images
2) Filters for symbols

Both, captcha images and symbols should look similar after filtering, i.e. you should use the same filters for images and for symbols. But, there may be some exceptions, for example, CropBorders filter should be used only for symbols, not for images, or the filter, which removes letters from house numbers in digits recaptcha, is not required for symbols. That is why we divided filters into two groups.

Using filter Thresold you can reset colors and make your captcha image black&white. Every pixel will be converted to black or white if its color value exceeds certain threshold, which you can set in filter settings.

The filter Resize allows to enlarge/reduce captcha image. Note that if you increase size of captcha image, symbols size will be increased as well as size of frame for recignition (mass centers settings). All this will lead to higher module core complicity and increase of captcha recognition time.

NOTE
We don't recommend to apply many filters in order to get rid of distortion.