Why does the preview look coarser than the exported file?

Dot spacing is measured in pixels of the output, so a preview drawn smaller has to shrink its spacing to show the same screen. Halftone draws the preview no larger than 900 pixels on its long edge and refuses to go below 2 pixels of spacing, because below that a dot is no longer a dot on screen. When it has to draw coarser than the export will, it says so. The exported file always uses the spacing you set, at full resolution.

What angles does CMYK use?

The four screens printers have used for a century, 15° apart: cyan at 15°, magenta at 75°, yellow at 0° and black at 45°. Yellow — the ink the eye notices least — is left on the axis and black is put on the diagonal where a pattern hides best. Halftone draws each separation on its own transparent layer and multiplies it down onto the paper, so the inks overprint the way they do on a press instead of dots of the same ink darkening each other where they overlap.

Is a fine screen slow?

It can be. A 3-pixel screen over a twelve-megapixel photograph is more than a million cells, and CMYK draws four screens rather than one. Halftone warns once the estimate passes 400,000 dots and tells you roughly how many the largest file holds. It batches up to 2048 dots into a single fill to keep that manageable, and files are processed one at a time so a batch does not hold several full-size canvases at once.

Halftone

Halftone throws the pixels away and draws a grid of dots in their place, the way a photograph is printed: each cell takes the average tone of the area it stands for, and that tone sets the dot's area — the radius is the square root of the coverage, which is what stops the midtones going too dark. Dot spacing runs from 3 to 40 pixels of the exported image and the screen can be turned to any angle from 0° to 90°, with 45° the traditional choice because a diagonal grid is the least visible to the eye. Dots can be circles, squares, diamonds or lines, in one of three color modes: one ink on one paper, CMYK at the classic press angles, or dots that each keep the color of the area they cover.

How it works

Drop your images Drag one or more files onto the drop area. The preview redraws the screen live as you change it.
Set the spacing and the angle Dot spacing is the distance between cell centres, from 3 to 40 pixels of the exported image. The screen angle slider runs 0° to 90° and starts at 45°. The readout estimates how many dots the largest file will hold.
Pick a dot shape Circle is the round dot of a printed photograph. Square and diamond are the same four corners a quarter turn apart. Line turns the screen into a line screen, where tone becomes line width.
Choose the color mode Monochrome uses the ink and paper swatches. CMYK replaces the angle slider with its own four fixed screens. Keep color gives every dot the color of the area it covers. "Invert" puts the big dots in the highlights instead of the shadows.
Process and download Pick Keep, PNG, WebP or JPEG, press Process, then Download. A batch comes back as file4edit-halftone.zip.

Questions

Why does the preview look coarser than the exported file?: Dot spacing is measured in pixels of the output, so a preview drawn smaller has to shrink its spacing to show the same screen. Halftone draws the preview no larger than 900 pixels on its long edge and refuses to go below 2 pixels of spacing, because below that a dot is no longer a dot on screen. When it has to draw coarser than the export will, it says so. The exported file always uses the spacing you set, at full resolution.
What angles does CMYK use?: The four screens printers have used for a century, 15° apart: cyan at 15°, magenta at 75°, yellow at 0° and black at 45°. Yellow — the ink the eye notices least — is left on the axis and black is put on the diagonal where a pattern hides best. Halftone draws each separation on its own transparent layer and multiplies it down onto the paper, so the inks overprint the way they do on a press instead of dots of the same ink darkening each other where they overlap.
Is a fine screen slow?: It can be. A 3-pixel screen over a twelve-megapixel photograph is more than a million cells, and CMYK draws four screens rather than one. Halftone warns once the estimate passes 400,000 dots and tells you roughly how many the largest file holds. It batches up to 2048 dots into a single fill to keep that manageable, and files are processed one at a time so a batch does not hold several full-size canvases at once.

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