A sublimation PNG: transparent, mirrored and nothing white to hide

Sublimation is printing with a dye that turns to gas under heat and sets into polyester, which is why a sublimation print does not crack or peel and does not feel like a layer on the fabric. It is also why the file matters more than it does for most printing: whatever the printer is given, it prints, and there is no white ink to put a background back to neutral. A photo with its background still in it prints the background. A cutout, with nothing round the subject, prints the subject.

getPNG makes that cutout. It takes the background out of a photo and gives you a transparent PNG at the full resolution of the original. It does not mirror a picture, place it on a page, print it or press it, and none of those steps is hidden in a setting. They happen in your design program, your print dialog and your heat press, and the rest of this page is the order to do them in. For the file itself (pixels, edges, how to check transparency) it leans on the PNG for shirts guide, and for the mirror step and the trim it shares ground with the iron-on transfer guide.

Why a sublimation file has to be transparent

A transparent pixel has no colour to print. A pixel that merely looks white does, and a printer cannot tell the two apart unless the file says so.

That is the whole problem with the fake transparent PNG, the picture that shows a grey and white chequerboard on screen because somebody saved a screenshot of one. On a white screen a cream or white background is invisible. On a blank, a printed pale rectangle can show as a faint tint, a soft edge at the border of the photo, or a patch where the dye has gone into the fabric a shade darker than the cloth around it. A PNG that only looks transparent shows how to tell, and the quickest test is to open the file on a black background: a real alpha channel lets the black through, and a fake one keeps its white box.

There is a smaller reason as well. Sublimation papers and blanks are sold with print areas, and a design with a transparent edge can be placed right up to the line without a rectangle of picture crossing it. A rectangle is a thing you have to line up. An outline is a thing that simply sits where you put it.

What white and pale pixels do on polyester

Sublimation works on a light surface. The dye does not cover the garment, it stains into it, so the garment’s own colour is part of every colour you print. Three rules follow, and none of them needs a number.

White prints as the garment. The printer has no white ink. A white area in your design is a gap in the ink, and on a white blank the gap is white and looks right. On a pale-yellow blank it is pale yellow.

Pale pixels fade. A soft hair edge that fades to nothing looks natural on a white blank. A very light colour, such as a pale cream sleeve, can almost disappear, because there is so little dye to show.

A dark blank hides the dye. The dye adds colour to the cloth and does not lighten it, so a dark fabric swallows most of a design. The blank’s maker and the paper’s instructions give the rule for each product, and a test on a scrap of the real cloth beats a guess.

The practical effect for a photo: on a white polyester blank, a good cutout reproduces the subject as it appears on a white screen, and everything you see against a white preview is what you will get. That is why the check below uses a light preview colour rather than a dark one.

The mirror step

A sublimation print is made face down. The paper is printed, laid ink side to the blank and pressed, so the picture on the paper is reversed and the picture on the garment is the right way round. For most set-ups that means you print a mirror image, and forgetting it shows up as backwards lettering on the shirt.

A mirror is a flip of the picture from left to right. It moves and resizes nothing, and it is lossless on a PNG, so there is nothing to lose by doing it last. Three places to do it:

• The printer’s own dialog, which on many printers has a mirror or reverse option.
• An editor, such as Photopea or GIMP, with a horizontal flip on the layer before you export a copy.
• The design program’s print settings, if you placed the cutout on a page there. Some sublimation programs mirror automatically, which is why a double mirror is the commonest mistake.

Whichever you choose, keep the unmirrored cutout as your master. Mirrored copies are for printing, and a master that is already reversed gets mirrored twice next month.

If a design has no lettering and no obvious left or right, the mirror matters less, but a watch on one wrist or a parting in the hair is enough to show it. Test before you press.

Size and sharpness

The usual rule for print is about 300 pixels for every inch of the finished size, and it is arithmetic. A design 8 inches across needs about 2,400 pixels, and one that fills a 12 inch print area needs about 3,600. DPI explained is the page to read if the number stored in a file confuses you: it is a note, and the pixel count is the thing.

With a cutout you count the pixels of the subject after the crop, not those of the photo. If the subject fills a third of the frame, the cutout is a third as wide as the picture you started with. getPNG returns the cutout at the full size of the photo with nothing downscaled, so you begin with every pixel the camera recorded. If you still fall short, print smaller. Enlarging makes new pixels rather than recovering lost ones, and a photograph enlarges less well than lettering does.

Edges

The pale-rim problem is as real here as on any other garment, and it is sharper because dye shows every pixel. If the photo was taken against a light wall, the soft edge may carry a pale rim that you will not see on a white screen, and that on a pale blank blends in rather than shows.

Preview the cutout on the colour of the blank before you print. In getPNG, set the background to that colour, zoom in, and use Edge shift to pull the outline in by a pixel or two, then Clean edge colour to recolour the edge pixels. A white halo or jagged edges goes through the cause.

When sublimation is not the right job

A design that needs white on a coloured garment is not a sublimation job, because there is no white ink to put it down. A picture that has to sit on a dark garment is the same. For those, the PNG for shirts guide covers a print shop’s approach, and the iron-on guide covers transfer paper for dark fabric. A mug or a hard blank with a coating is another product again, and its maker’s instructions come first.

A checklist before the press

1. The subject is cut out and the file is a real PNG with transparency, checked on black.
2. A preview on the colour of the blank shows no pale rim.
3. The cutout has about 300 pixels for every inch of the width you want.
4. You know from your paper and press whether the print is mirrored, and a plain-paper test confirmed the direction.
5. Nothing is mirrored twice by a program that does it for you.
6. The unmirrored, full-size cutout is saved as the master.
7. You have read the blank’s own instructions for heat, time and pressure, which are not stated here because they differ by blank and press.

The short version

Sublimation prints whatever the file contains and has no white ink to hide a background, so take the subject out first and make sure the PNG is really transparent. White and pale pixels print as little or nothing on a light polyester blank, a dark blank hides the dye, and the print is usually mirrored, which your paper and press will say. Test on plain paper, keep the full-size, unmirrored cutout as the master, and leave heat, time and pressure to the blank’s instructions.

The steps, in short

1. Cut the subject out of the photo. Drop the photo on getPNG and download the transparent PNG at the full size of the original. Preview it on a light colour, because that is the kind of surface sublimation works on, and fix any rim with Edge shift and Clean edge colour.
2. Check the file is really transparent. Open it on a dark colour. If a white or grey box appears, the transparency is fake and the box will print. Go back to the cutout.
3. Work out the pixels for the print size. Multiply the width in inches you want by 300 to get the pixels the cutout needs, then place it on the page in your design program.
4. Mirror the print if your set-up needs it. Read the instructions for your paper and press. Many sublimation set-ups print a mirror image so that the garment reads the right way round. Mirror in the print dialog or an editor, because getPNG has no mirror control.
5. Print a test on plain paper. Hold it up to a mirror or against the garment, check any lettering, and only then print on the sublimation paper.

Questions

Why does a sublimation design need a transparent PNG?

Because the printer puts ink on every pixel it is given, including the ones that look like background. A picture with a white or off-white rectangle round the subject prints that rectangle as a faint tint or an edge on the garment. A transparent PNG gives the printer nothing to print outside the subject, so the fabric shows through on its own.

What does white do in a sublimation print?

On most set-ups it prints as nothing: the printer has no white ink, so white areas are simply paper, and the garment's own colour shows through. That is fine on a white polyester shirt and the reason pale pixels vanish on a pale one. It also means you cannot print a white design on a coloured garment this way. For that, other methods of printing a picture onto fabric exist, and the paper or shop you use will say which.

Can I use sublimation on a dark shirt?

Not in the way a light shirt works. Sublimation ink is a dye that bonds into polyester and does not cover what is underneath, so a dark fabric swallows it. The paper and blank maker give the rule for each product, and a test on a scrap of the real fabric is cheaper than a ruined blank.

Do I have to mirror a sublimation print?

Usually, because the printed side is pressed face down onto the blank, which reverses the picture on the paper and puts it the right way round on the garment. Some set-ups handle the mirror for you in their own software. Read the instructions for your press and test with lettering on plain paper first. getPNG does not mirror, so use the print dialog or an editor.

What size should a sublimation PNG be?

About 300 pixels for every inch of the finished print, so a design 8 inches wide is about 2,400 pixels across. Count the pixels of the subject after cropping, not those of the whole photo. getPNG returns the cutout at the full size of the photo, so you start with every pixel the camera recorded.

Will a cutout from getPNG work with my cutting machine?

The transparent PNG is the file those machines ask for when the job is to print a picture and cut round it. getPNG makes the cutout and does not draw cut lines or talk to a cutter, so the cut line is made in the cutter's own software.