Batch Watermark

Size by percentage, not by pixels, is what makes batch stamping work

A fixed 48 pixel watermark is huge on a 800 pixel phone shot and invisible on a 6000 pixel camera photo. Scaling by percentage fixes that: at 8% of width, the text is 64 pixels tall on an 800 pixel photo and 480 pixels tall on a 6000 pixel photo, the same relative size in both. The same rule applies to the margin: 3% of width is 24 pixels on the small photo and 180 on the large one, so the mark sits the same distance from the edge in relative terms. That is the "smart scaling" in the brief, and it is just one division, applied per photo.

Position is a nine-point grid, and the maths for each point is the same: start from the corner or centre anchor, add the margin, and subtract the mark's own width or height where the anchor is a right or bottom edge. Bottom right on a 4000 by 3000 photo with an 8% mark (320 pixels tall, roughly 1500 wide for a short brand name) and a 3% margin (120 pixels) puts the mark's right edge at 3880 and its bottom edge at 2880. Centre ignores the margin, because a centred mark with a margin is not centred. Opacity is applied as a canvas alpha value, 60% meaning the mark's pixels are blended 60% mark and 40% photo underneath.