--black-normalize=<-1, 0-100>

The black-normalize option increases the number of black pixels in a grayscale image by a specified percentage in order to sharpen the contrast between its black and white colors. Use this option in conjunction with the white-normalize option when encoding grayscale images that contain text and backgrounds of similar hues. For example, dark gray text on a light gray background can be converted to black text on a white background, thus improving its readability.

Assign values to the black-normalize option as described in the table below.

 

Supporting command:

documenttodjvu

 

Value

Description

-1

Default. Attempts to auto-normalize the separation between an image's black and white colors.

0

Does not change the separation between an image’s black and white colors.

1-100

Increases the number of black pixels in the image by this percentage. For example:

documenttodjvu --black-normalize=50 survey.jpeg survey.djvu

This use of --black-normalize increases the number of black pixels in the output image, survey.djvu, by 50 percent.