Seams, gutters and the colour behind them
Step three of the joiner asks three questions in a row and they are all really the same question: what do you want to see where the pictures don’t?
Zero gap, and why it is the default
The gap between pictures starts at zero because that is what this site is for. Two photographs that touch read as one picture with a division in it. Two photographs with eight pixels of white between them read as two photographs that have been put on a page. Both are legitimate; only one of them is what somebody means when they say they want two pictures merged into one.
A hard join has one requirement that is easy to miss: the edge has to land on a whole pixel. Everything in the layout is rounded to integers before anything is drawn, because a picture placed at x = 412.5 gets sampled across two columns of pixels and a hairline of the ground colour appears down the seam. If you have ever joined two images somewhere else and found a faint pale line between them, that is what it was.
When a gutter is the right answer
- The pictures are similar in tone. Two photographs of the same beach on the same afternoon touch and become one confusing photograph. A few pixels of separation tell the eye there are two.
- You want it to read as a set. A grid of nine product shots with a consistent gutter and a matching outer margin looks arranged. The same nine touching look like a contact sheet.
- Something has to go between them later. If a caption or a rule is going to be added afterwards, leave it room now.
The gutter is the space between the pictures; the margin is the space around the whole thing. They are separate numbers on purpose — a grid with a generous gutter and no margin is a common and deliberate look, and so is the reverse.
Choosing a ground colour
The ground shows in three places: in the gutter, in the margin, and beside any picture that is shorter than its neighbours because you chose not to scale. If none of those exist, the ground colour is drawn and then completely covered, and it does not matter what you set it to.
Making it disappear
If you want the ground unnoticed, match it to the pictures rather than to the page. Screenshots of a light-themed app are almost never pure white — most sit around a very pale grey, and white between them reads as a bright stripe. The eyedropper is there for exactly this: pick the colour out of the picture itself and the gutter stops being visible.
Making it deliberate
The other approach is to stop trying. A black ground behind a set of photographs, or a warm off-white behind a set of scans, is a frame — it declares that the spacing was a decision. Anything that is obviously chosen looks better than something that was nearly the right colour.
See-through, and what JPEG does to it
Transparent is one of the ground options, and it survives into PNG and WebP exactly as you would expect: the gaps come out with nothing in them and whatever the picture is placed on afterwards shows through.
JPEG cannot do it. There is no channel in the format for it, so every transparent pixel has to become some actual colour on the way out, and the colour browsers use is white. That is not a failure and it is not reversible — it is simply what the format is. The step-four panel says so before you save rather than leaving you to discover it in the file, and the fix is to pick PNG or WebP instead.
One thing worth being straight about: a seam can be invisible in the file and still visible on screen. Browsers and image viewers scale pictures to fit, and a 6,000-pixel-wide join shown at 900 pixels is being resampled by whatever is displaying it — which can put a faint line at a join that is perfectly clean at full size. Before concluding a seam is wrong, look at it at 100%.
Colour at the seam
The other thing that shows up at a join is a colour shift, and it has nothing to do with the gap. A photograph taken on a modern phone can carry a wide-gamut colour profile; a screenshot almost never does. Placed side by side untouched, the two would meet along a line where the greens visibly change. Everything opened here is converted to ordinary sRGB as it is read, so the shift is dealt with before the pictures ever meet. It is the least visible thing this tool does and one of the more important ones.