Why work in millimetres?

A mat is judged after printing, where physical width matters. Millimetres resolve to pixels through the DPI field, so both figures remain visible.

Why is the colour not pure white?

A restrained warm white separates the mat from a white web page and resembles common uncoated mount board without pretending to reproduce its texture.

Can I use equal edges?

Yes. Set the lower-edge weight to one. The slightly larger opening value compensates for the way a perfectly centred picture can look visually low inside a frame.

Add a gallery mat to a photograph

The construction this opening uses

A gallery mat is negative space with a physical dimension. It separates the picture from the frame, gives the eye somewhere to settle, and prevents a dark edge from feeling trapped. This opening uses 18 mm on three sides and a small optical weight below rather than treating the surround as an arbitrary pixel count.

At 300 DPI, 18 mm resolves to about 213 pixels and the 1.12 lower weight to about 239 pixels. At 150 DPI those pixel counts halve while the intended printed width stays the same. The interface prints both units so a missing or incorrect DPI declaration cannot quietly change the job.

Where the treatment stops working

This produces the digital equivalent of the visible mat area only. It cannot predict how much board a physical frame rebate hides, simulate bevel depth or guarantee a printer's trimming tolerance. Leave additional bleed in the print workflow when a lab asks for it.

The opening values are a starting construction, not a filter baked into the page. Every width, colour, unit and mode remains editable after the photograph opens, and the output readout states the dimensions before export. That makes the result reproducible instead of leaving its proportions hidden inside a decorative preset.

Why work in millimetres?: A mat is judged after printing, where physical width matters. Millimetres resolve to pixels through the DPI field, so both figures remain visible.
Why is the colour not pure white?: A restrained warm white separates the mat from a white web page and resembles common uncoated mount board without pretending to reproduce its texture.
Can I use equal edges?: Yes. Set the lower-edge weight to one. The slightly larger opening value compensates for the way a perfectly centred picture can look visually low inside a frame.