Greyscale, indexed, bitmap and duotone modes in Paint by Zaweba

Some jobs need fewer colours rather than more. A two-colour flyer, a pixel-art sprite, a fax-style line drawing or a moody duotone poster all start with the same menu. In Paint by Zaweba, Image > Mode converts the pixels for real to Greyscale, Indexed, Bitmap or Duotone, rather than just dressing the picture up to look that way while it stays full colour underneath.

That distinction matters when the file goes somewhere with strict rules. A screen printer who wants a true one-bit file, or a game engine that expects an 8-bit palette, will get exactly that.

Greyscale

Greyscale keeps only lightness. It is the simplest of the modes and the right one for black and white photos destined for single-colour printing, scanned documents and line art you plan to colour later. Do your creative black and white conversion first with adjustments while the document is still in colour, so you can choose how reds and blues turn into grey, then switch the mode once the tones look right.

Indexed colour

Indexed mode builds a palette and maps every pixel to one of its entries. You choose how the palette is made: exact, web, uniform, median cut or octree, with between 2 and 256 colours, and you can dither so gradients survive the reduction. The colour table can be edited afterwards, which is useful when a brand colour must be one precise value, or when you want to swap a palette entry and recolour every pixel that uses it at once.

Exact works when the picture already has few colours. Median cut and octree are the ones to try for photographs, because they pick the palette from the image itself. Uniform and web give predictable fixed palettes. Try a couple and compare at full size before settling.

Bitmap

Bitmap mode is one bit per pixel, black or white and nothing else. Paint by Zaweba offers threshold, pattern, diffusion and halftone methods. Threshold gives hard graphic shapes, good for stencils and logos. Diffusion scatters dots to suggest tone. Pattern and halftone give the regular textures associated with print, and they make striking effects for posters even when nobody will print the file in one colour.

Start from a greyscale image, since bitmap conversion is about how grey tones become dots. Adjust contrast first, because the conversion is unforgiving to muddy midtones.

Duotone and multichannel

Duotone prints a greyscale image with between one and four inks, each with its own curve. A black plus a warm brown gives a classic sepia; a deep blue with a pale cyan gives a cool, modern look. Presets get you started, and the curves let you decide where in the tonal range each ink appears. It is the mode to reach for when a design is printed in two spot colours and the photograph has to live within them.

Multichannel is the specialist's option. It makes one ink layer for each of the C, M and Y channels, which is handy when you want to work on the plates individually.

Before you convert

Keep a full-colour master. Every one of these conversions throws information away, and that is the point of them, but you will want the original when a different version is requested. Save the master as .ptx and convert a copy.

Convert at the end of the job, not the start. Retouching and resizing are better done with full tone and colour available. And always inspect the result at 100 percent: dithering and halftone patterns can look quite different when the view is zoomed out.

These modes are the quiet workhorses of print and retro graphics, and Paint by Zaweba puts all four in one place with the options laid out plainly.