My friend has an colored image with Chinese handwriting (basically by taking photo of or scanning what he wrote on a piece of white paper), and he would like me to convert it into a black and white binary image. Are there applications under Ubuntu that can accomplish that?
Here is an example image:
What you want is referred to as "threshold" in image processing. Basically, it takes an image as an input and outputs an image that has all pixels with a value below a given threshold set to black, and all pixels the value of which is above the threshold set to white. This results in a black-and-white image from an arbitrary input image.
Generally, you want to convert to grayscale first for more predictable results, but it is possible to threshold a full-color image as well.
You can use a graphical tool such as GIMP to do this interactively (you'll find the tool through the main menu -> Colors -> Threshold), or you can use ImageMagick something like this:
convert colored.png -threshold 75% thres_colored.png Running the above command on the example image produces the result shown below.
Since thresholding is often somewhat of a trial-and-error process to get a result you're happy with, particularly if the source image is not very close to black-and-white already, I recommend the GUI approach if possible, but if that is not an option for whatever reason you can do it through the command line as well. For finer control of the output, you can use tools like color curves, levels and contrast first to isolate the light and dark portions of the image better before thresholding. (Actually, threshold can be seen as an extreme case of using the color curves tool.)
-channel option) to be applied to some sort of "value" of the pixel, which is computed from all channels. That's the reason why I said you might want to convert to grayscale first for more predictable results. (Also see my edit.) -lat which performs a local adaptive threshold, taking into account the surrounding pixels. You can use Imagemagick:
convert test.png -colorspace Gray gray_colorspace.png From here.
Here is what I got after applying to your image:
ImageMagick convert -monochrome
The -monochrome option uses some smart dithering, and makes the output much more visible than -threshold if you intend it for human consumption:
convert -monochrome signature.png out.png 
Does not make much difference for such a simple image, but for larger ones, it is striking:
convert has -lat (experimentally the parameters like convert -grayscale Rec709Luma -lat 20x20-5% seems to work). Demo maybe later after I prepare image (this image is too simple, any method would work) You can also do this easily with the netpbm toolkit:
anytopnm inputfile | ppmtopgm | pgmtopbm > outputfile ppmtopgm converts to a grayscale image, pgmtopbm converts to a black-or-white image, and we then redirect the output to a file. It will be in the pbm format; if you want something more common, you'll have to add an output convertor (e.g., pnmtopng or some such)
anytopnm inputfile | ppmtopgm | pgmtopbm -thershold > outputfile works so much faster than convert -threshold 50% inputfile outputfile! (And avoids the latter’s resource constraints and “security” policies.) Otsu's method will perform better when dealing with texts over a background with slightly gradient colors:
According to @fmw42 's comparison on multiple threshold methods: the Local Adaptive(its equivalent in imagemagick is -lat) algorithm might work best on texts over background:
Also try to combine with connected components processing that was introduced in another answer made by @fmw42 to remove unnecessary edges/dots which might get confused by any further OCR process.
