Preview and opacity

The Preview row chooses what lies over the sheet while you adjust: the photo itself, the gray image the split is made from, the split, or the outlines Trace would make. Opacity sets how strongly it shows. Each view answers a different question, and the art is knowing which one to ask.

What it does

• Photo (the default): the photo as it is.
• Adjusted: the score image after tone, levels and blur, exactly what the Threshold, or Balance, cuts. In Brightness mode dark is front. In Color mode it shows the color score: the lighter a pixel, the nearer it is to your front colors.
• Two-tone: the split itself, after Detail, painted in your design's own colors: front in the sheet's color, back in the background's. This is what will be traced.
• Outlines: the photo stays underneath, and the outlines Trace would make are drawn over it in magenta, with a small square at every corner found. The note under the controls counts the shapes to trace, alongside any small shapes dropped and holes filled.

Opacity runs from 5% to 100%, starting at 50%. It sets how strongly the photo, or the Adjusted or Two-tone picture, shows over the sheet. The outlines are always drawn at full strength. The eye beside the panel heading hides and shows the lot.

The previews are made from a 1,024-pixel copy of the photo, about a tenth of a second after you stop moving a control; an answer that has been overtaken by a newer change is dropped. Trace itself works at the full 2,048 pixels, so the very finest detail can differ slightly from what the preview showed. After a trace, the preview goes back to Photo. While you crop, you see the plain photo, whole.

Why it works this way

Each view is a question. Photo: where are things? Adjusted: are the tones I care about pulled apart, and is there a clear gap between them? Two-tone: what will actually be cut? Outlines: what will the vector shapes look like, and where will their corners fall?

Two-tone uses your design's colors, not black and white, because you are judging a notan, and a split that looks fine in black and white can look wrong in a pale blue and a deep orange. It is the notan before it exists.

Outlines leaves the photo underneath because an outline only means something against what it is tracing. Magenta stands out against most photographs, and a line drawn at a fixed screen width stays readable at any zoom. The squares at the corners show what Corner angle and Corner span decided, which is otherwise invisible until you trace.

Opacity doesn't apply to the outlines so that you can fade the photo right down and still see the lines clearly. And the preview's lag of a tenth of a second is deliberate: it waits for you to pause, rather than working on every twitch of a slider.

The metaphor

The light box and the stack of overlays. On an old paste-up, a printed job was prepared in layers: the artwork on the board, a tissue overlay with notes and color breaks, a keyline showing where pictures went. You lifted and lowered them to check one thing at a time. Preview is that stack, with the layers switched instead of lifted.

It also borrows the printer's progressive proofs, a set showing each color on its own and then in combination, so that faults could be traced to the plate that caused them. Adjusted is the plate; Two-tone is the print.

The metaphor breaks on cost. A proof took hours and paper. Here every view is live and free, which changes how you work: instead of committing and checking, you can move a slider with Two-tone showing and watch the notan change under your hand.

A history

In art

Painters have always worked in stages that show different truths. A fresco painter drew the composition on the rough plaster in a red-brown earth called sinopia, then covered it patch by patch with the fine plaster that took the color. When frescoes have been lifted from walls, the sinopie underneath have sometimes survived, showing the stage no one was meant to see.

Many painters make value studies before color, small sketches in a few grays that settle where dark and light will fall. The grisaille, a painting in grays alone, is the same idea carried to a finished work. The Adjusted view is a value study made for you.

In printing

Before a job went to press it passed through proofs, and each proof answered one question. Progressive proofs checked the colors plate by plate. A keyline, an outline drawn on the artwork, showed where a halftone would sit and what shape it would take, without the halftone itself. The Outlines view is a keyline for your cuts.

In technology

Early versions of Adobe Illustrator could not be drawn in with the fills showing: you worked on the outlines and kept a second window open to see a preview. Editing in preview came with version 4, and on the Macintosh only with version 5.0 in 1993. The split between drawing on outlines and judging the finished look is still there in the Outline view every vector program keeps.

Opacity on screen rests on the alpha channel, a fourth number stored with each pixel's red, green and blue to say how opaque it is. Alvy Ray Smith and Ed Catmull came up with it at the New York Institute of Technology in 1977, and Thomas Porter and Tom Duff set out the rules for combining images with it in “Compositing Digital Images” (1984). Animators working on paper knew the idea long before as onion-skinning: thin sheets on a light box, the previous drawing showing faintly through the next.

In professional software

What Studio Notan calls Preview is, elsewhere, a view mode, a dimmed reference layer or a layer’s opacity. The idea that transfers: keep the reference faint and apart from the work, and switch views to ask one question at a time.

• Adobe Illustrator. Tick ‘Template’ in the Place dialog and the photo goes onto its own locked layer, dimmed to 50% and left out of printing; double-click the layer to change ‘Dim Images to’. ⌘Y (Ctrl+Y) switches the document between Outline and Preview. The Image Trace panel’s ‘View’ menu asks the same questions as the views here, down to ‘Outlines with Source Image’.
• Adobe Photoshop. Each layer has an Opacity in the Layers panel; with the Move tool active, typing a number sets it, 5 for 50% and 0 for 100%. A drawing layer set to the Multiply blend mode lets the photo beneath show through its white.
• Autodesk Fusion (formerly Fusion 360). A canvas has its own Opacity setting, and ‘Display Through’ lets it show through the model in front of it, so the reference stays visible while you build over it.
• Autodesk AutoCAD. IMAGEADJUST sets an attached image’s Fade, from 0 to 100, blending it toward the background color. It changes the display and the plot, never the image file.

What carries over. Fade the reference, not the work, and look at the result with the reference hidden before you call it done. One pitfall: Studio Notan never exports the photo, but elsewhere you must see to it. Illustrator’s template layer is left out of printing, while a faded photo layer in Photoshop is still part of the picture until you hide or delete it.

Try this

• With Two-tone showing, drag the Threshold slowly from one end to the other and watch the notan grow and shrink.
• Set Opacity to 100% with Two-tone, and judge the notan on its own, without the photo.
• Switch to Outlines and move Smoothness, then Corner angle, and watch the corner squares appear and vanish.

Going further

See Histogram and Threshold, Auto and Invert. Alvy Ray Smith, A Biography of the Pixel.