Overview
Introduction
This tool sets every pixel in a PNG to the exact same alpha value, entirely in your browser.
It's the simplest, most direct way to force a uniform transparency level across an entire image.
What Is PNG Alpha Channel Filler?
A client-side alpha-channel tool that reads an uploaded PNG's pixel data and replaces every pixel's alpha byte with a single fixed value from 0 (fully transparent) to 255 (fully opaque).
The red, green, and blue channels are left completely unchanged; only the fourth (alpha) byte of every pixel is overwritten.
How PNG Alpha Channel Filler Works
The tool walks the pixel buffer and, for every pixel, writes the chosen alpha value directly into the alpha byte, ignoring whatever value was there before.
Because this is a direct overwrite rather than a proportional scale, any existing variation in transparency across the image (like a soft drop shadow fading to transparent) is flattened into one uniform value.
When To Use PNG Alpha Channel Filler
Use it when you need a whole image to have exactly one transparency level, rather than a scaled or blended version of its existing alpha.
It's handy for quickly testing how a design looks at a specific fixed opacity level applied uniformly.
Often used alongside PNG Opacity Changer, PNG Alpha Channel Inverter and PNG Alpha Channel Remover.
Features
Advantages
- Runs entirely client-side; the uploaded image is never sent to a server.
- Simple, predictable, single-value control with no compositing math involved.
- Works at any target alpha value, not just fully opaque or fully transparent.
Limitations
- Discards any existing variation in the alpha channel; it can't preserve relative transparency differences the way the opacity changer's proportional scaling does.
- Filling with a partial alpha value doesn't remove any RGB color baked in under previously-transparent pixels, which can be visible if the image is later composited onto a background.
Examples
Best Practices & Notes
Best Practices
- Use the opacity changer instead if you want to preserve relative transparency differences rather than flattening them to one value.
- Preview the result before downloading, since a low fill value can make the whole image nearly invisible.
Developer Notes
The lib function is a simple, direct loop writing a constant byte to every fourth position (the alpha channel) of the Uint8ClampedArray buffer, with input validation ensuring the fill value is a valid 0-255 integer before touching any pixel data.
PNG Alpha Channel Filler Use Cases
- Creating a uniformly semi-transparent watermark layer
- Testing how a design looks at one specific fixed opacity
- Resetting an image's alpha channel to a known, consistent state
Common Mistakes
- Expecting existing transparency variation (like a soft shadow) to be preserved — this tool always overwrites to one flat value.
- Confusing this with the opacity changer, which scales proportionally instead of setting a fixed value.
Tips
- If you need to preserve relative transparency while scaling it down, use the opacity changer instead.
- Combine with the alpha channel remover afterward if you decide you actually want a fully opaque, cleanly composited result.