Image Watermark

A watermark composites text or a logo on top of the original pixels with Canvas. Opacity uses globalAlpha (0-1) so the photo shows through. Rotation does translate to the draw point, then rotate (degrees→radians) to tilt the canvas before drawing. Position is a 9-cell grid (corners, edges, center); each cell is computed about 20px inside the edge plus half the watermark size, so nothing clips off-frame. Repeat (tile) walks across and down in steps of the watermark size plus 80px gap, covering the whole image. Text scales by font size; a logo scales its width as a percentage of the photo's width.

There are two broad uses. Copyright / attribution puts a nickname, URL or logo somewhere visible but non-obscuring (bottom-right, 50-70% opacity) to mark whose work it is. Leak deterrence / tracing uses a repeating tile crossing the whole image + rotation + low opacity (30-40%), making it hard to crop or erase, often stamping the recipient and date so a leak can be traced back. A watermark doesn't make copying impossible — it raises the cost and effort. For valuable assets, also consider low-resolution publishing or copyright registration (Korea Copyright Commission, copyright.or.kr).

Position is chosen from a 9-cell grid (top-left, top-center, top-right, mid-left, center, mid-right, bottom-left, bottom-center, bottom-right), and each cell is offset inward by half the watermark size + 20px so the mark never clips at the edge (e.g. bottom-right uses x = w − (watermarkWidth/2 + 20)). With tile (repeat) on, it sweeps the whole image at a stride of the watermark size plus 80px of spacing. Rotation (−45° to +45°) is converted from degrees to radians, and opacity (10-100%) is applied via globalAlpha. Text size is an absolute px value (12-120), while logo size is expressed as a percentage of the photo width, so the same proportion applies consistently across photos of different widths. You position it on the first-photo preview, then apply to all uploaded photos with the same settings at once, with a progress bar showing the count. Duplicate filenames get a suffix like _watermark-2 to avoid overwriting, and if one image fails to encode mid-batch the already-succeeded images are preserved and shown. Output is JPG (quality 0.95), PNG or WebP; since JPG has no transparency it is flattened onto a white background to avoid a black backdrop — choose PNG to keep a transparent logo on a transparent background.