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I've a color image and want to print on my black and white printer. So, How do I convert it to black & white using GIMP?

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    Well, there is a big, proeminent "Colors" top level menu in there. There are at least three ways of doing it in that menu. – jsbueno Apr 3 '16 at 4:56
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There are several ways of doing it in GIMP, and it depends on the end-result you want. From your question, one can't know if your desired result is really a "black and white" (only) image, or a desaturated image, with up to 256 shades of gray.

Also, you have options to either desaturate only the active layer or selected region, and continue editing the image in the RGB color model, or convert the image grayscale colorspace - which will convert all layers, regardless of selection, and automatically cast any colors to gray on painting. To do that, go to Image->Mode->Grayscale - just as suggested on the other answer.

To simply reduce saturation of the current layer to gray levels, and continue editing the image in the RGB color space, there are several options:

  1. Colors->Hue saturation... and pull the Saturation control full to the left
  2. Colors->colorize...and again, pull the `Saturation control to the leftmost position
  3. Colors->Desaturate which offers 3 different algorithms to generate a gray value from given colors; (the options above allow a lot of control on the grey levels, since you can slide the Luminosity control as well. This is GIMP 2.8 - Unstable GIMP 2.9 has even more algorithms to desaturate.
  4. Colors->Components->Channel Mixer... allow you to compose channel values on the resulting from a percentage of each component on the source image. If you check the "monochrome" box, the target image is grayscale - this allows a lot of control on the resulting image
  5. Components->Decompose... Separate each of the color channels on the image - with an optional interpretation of the color image as "CMYK" or "HSL" besides "RGB" into a separate, grayscale, image (or layer).
  6. Colors->Map->Gradient maps an image to the active gradient. If the current gradient is grayscale (or even pure black and white), you get the grayscale image - again with a different way to control the outcome
  7. The same for Colors->Map->Palette
  8. Colors->Colorify, picking white for the target color
  9. Colors->Filter Pack - select the "saturation" on the "windows" column, and just click repeatedly on Less sat before applying
  10. As mentioned above, just go to Image->Mode->Grayscale (which changes the working colorspace for all layers)
  11. The most beautiful grayscale effect is hidden on GIMP 2.8: Use Tools->Gegl operation... and pick C2G on the drop down filter list. This is just amazing - it calculates pixel values based on local grayscale levels. (This filter has moved to Colors->Desaturate on GIMP 2.9)

So, these are the ways to reduce an image to "Grayscale". To reduce it to Black and white, there are a few other possible tricks:

  1. Image->Mode->Indexed mode and pick "1 bit palette" - you can opt to enable dithering bellow, on the same dialog before applying the effect. This will change the image (all layers), and confine the color model to use to the 1bit palette - modify it back to RGB color model (Image->Mode->RGB) if you plan to continue editing the image.
  2. Colors->Threshold Changes all the colors on the active layer (or selection) to B&W only - allowing you to choose which lightness level represents the threshold from black to white (and even with a further option to "wrap" lighter colors back to black). If you want Black and White (not grayscale) , without dithering, this is what you want.
  3. For an artistic dithering, that will use large spots (nice to use on artistic printing process), desaturate the image first, converting it to shades of gray, using one of the methods above, and use Filters-Distorts->Newsprint, pick the CMYK option, and change the spot function using the Black tab. (Raise the Oversample for a nicer output if you don mind having gray that does not "look" like gray. Otherwise, set the Oversample to 0 to pure B&W)

    • So, these are the ways that either jump to my mind + the ones I verified sequentially by exploring the Colors menu. As you can see, there are many ways to do it..
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    This is a very exhaustive answer! – Paolo Gibellini Apr 3 '16 at 6:39
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Goto Image > Mode > Grayscale :

enter image description here

Which converts color images to black & white

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    Grayscale is not properly black and white: there are 254 tones of gray other than black and white... Please update your question or your answer. – Paolo Gibellini Apr 2 '16 at 14:03

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