Overview
Introduction
This tool overlays one PNG (a logo, signature, or badge) on top of another as a watermark, entirely client-side, using real alpha compositing.
You control exactly where the watermark lands and how opaque it appears.
What Is PNG Watermark Adder?
A client-side PNG compositor that takes two uploaded images — a base image and a watermark image — and blends the watermark onto the base at a chosen position and opacity.
It uses the standard 'over' alpha-compositing formula, the same math browsers use internally when layering semi-transparent elements.
How PNG Watermark Adder Works
For every pixel the watermark covers (after clipping to the base image's bounds), the tool computes the watermark pixel's effective alpha (its own alpha times the opacity slider), then blends its color with the base pixel's existing color and alpha using the standard source-over formula.
The base image's own dimensions are always preserved in the output; only the watermark is positioned and blended on top.
When To Use PNG Watermark Adder
Use it to add a logo, signature, or copyright badge onto a photo or graphic before sharing it.
It's also useful for previewing how a badge or sticker overlay looks at different positions and opacities before finalizing a design.
Often used alongside PNG Text Adder and PNG Background Adder.
Features
Advantages
- Runs entirely client-side; neither uploaded image is ever sent to a server.
- Uses real alpha compositing, so semi-transparent watermark PNGs blend correctly rather than just being pasted opaquely.
- Independent opacity control lets you dial in a subtle or bold watermark.
Limitations
- The watermark can only be placed at a single fixed position per run; tiling a repeated watermark pattern requires running the tool multiple times.
- Very large base images combined with a large watermark can take a moment to process since every covered pixel is blended individually.
Examples
Best Practices & Notes
Best Practices
- Use a watermark PNG with real transparency (not a solid rectangle) so it blends naturally into the base image.
- Lower the opacity for a subtler branding effect that doesn't overpower the underlying content.
Developer Notes
addWatermarkToPng is a pure function operating on two plain Uint8ClampedArray PixelBuffers: it clips the watermark to the base canvas bounds and applies the standard over-compositing formula (outAlpha = srcAlpha + dstAlpha*(1-srcAlpha)) per pixel; the component's job is only decoding both uploads and encoding the result.
PNG Watermark Adder Use Cases
- Branding photos with a company logo before publishing
- Adding a signature or copyright mark to digital artwork
- Previewing sticker or badge overlay placement on a design mockup
Common Mistakes
- Uploading an opaque, solid-background watermark image and expecting it to blend transparently — only areas that are actually transparent in the watermark PNG let the base show through.
- Setting the position so the watermark falls entirely outside the base image's bounds, resulting in no visible change.
Tips
- Export your watermark/logo PNG with a transparent background for the most natural-looking overlay.
- Try a few different opacity values (e.g. 0.3, 0.5, 0.8) to find the right balance between visibility and subtlety.