Image Watermarking in JavaScript
Add text and image watermarks to photos using the Canvas API in the browser — with code examples.
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Tags: image watermark JavaScript Canvas, Canvas API watermark, JavaScript photo watermark
Image Watermarking in JavaScript The Canvas API makes client-side watermarking straightforward: draw the source image, composite the watermark text or logo on top, then export the result. Everything runs in the browser without a server. --- What is step 1: set up the canvas? Using and ensures you work at the image's actual resolution, not any scaled display size. What is step 2: add a text watermark? What is step 3: add a logo/image watermark? What is step 4: tiled watermark? A tiled watermark covers the entire image and is the hardest to crop out: What is step 5: export the result? How can I measuring text for centering and sizing? Use to get the rendered width of text before placing it: What is responsive font size? Auto-scale font size to a percentage of the image width: What is batch…
Frequently Asked Questions
How do I add a watermark with JavaScript Canvas?
Draw your source image on a Canvas, then set ctx.globalAlpha for transparency and use ctx.fillText() for text or ctx.drawImage() for a logo. ctx.save() and ctx.restore() isolate the transform so only the watermark is affected.
How do I draw text on an image in JavaScript?
Set ctx.font to your desired font string (e.g., '24px Arial'), ctx.fillStyle to a color, then call ctx.fillText(text, x, y). For centered text, set ctx.textAlign = 'center' and ctx.textBaseline = 'middle' before drawing.
How do I overlay a logo on an image with Canvas?
Load the logo as an Image object, draw the source photo onto the canvas first, then draw the logo with ctx.drawImage(logo, x, y, width, height). Set ctx.globalAlpha before the logo draw call to control transparency.
How do I control transparency with Canvas?
Set ctx.globalAlpha to a value between 0 (fully transparent) and 1 (fully opaque) before any draw call. All subsequent draws use that alpha until you reset it. Use ctx.save()/ctx.restore() to scope the alpha change.
How do I export a Canvas as a PNG?
Call canvas.toBlob(callback, 'image/png') for an async Blob you can download, or canvas.toDataURL('image/png') for a synchronous base64 data URL. For JPEG with quality control, use canvas.toBlob(callback, 'image/jpeg', 0.92).
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