PNG Drop Shadow Generator

How to use PNG Drop Shadow Generator

Create a real expanded transparent PNG with a declared offset and finite box-blurred alpha shadow beneath the source. Generate a reusable image artifact rather than a CSS declaration. Source placement and the expanded canvas bounds remain explicit, while opacity and blur follow a small reproducible pixel model that can be checked independently.

1. Provide static source image, shadow horizontal offset in pixels, shadow vertical offset in pixels, uniform square blur radius in pixels, shadow #rrggbbaa, including opacity.
2. Choose the supported mode and run the tool. Review its assumptions and any reported errors.
3. Copy the result or download a separate output; reset to clear the current form.

Good to know

• One static PNG, baseline JPEG or simple VP8/VP8L WebP: at most 8 MiB, 1,000,000 decoded pixels and 2,048 per side. Existing structure/CRC/dimension preflight precedes decoding; APNG, progressive JPEG, extended/animated WebP, compressed PNG metadata, SVG/GIF and native/header orientation disagreement are refused.
• Offsets are exact signed plain integer pixels from −128 through 128; radius is an exact plain integer 0–32. The color uses #RRGGBBAA, with its last byte controlling shadow opacity. Fractional/exponent and lossy near-integer controls are refused.
• The model averages source alpha over one uniform square of side 2 × radius + 1. Outside the source contributes zero and the full-window denominator stays constant. This is a finite box blur, not Gaussian or native CSS drop-shadow parity.
• Shadow alpha and source alpha are composed with the original source above the shadow using encoded-RGB straight-alpha Porter–Duff source-over. The arithmetic is bounded exact integer work before nearest eight-bit rounding; it is not linear-light colorimetry.
• Canvas bounds contain the full finite source/shadow support and may add space on any side. The report gives the original source offset. Bounds remain expanded even when shadow opacity is zero; output is capped at 2,048 per side and 1,100,000 pixels before core allocation.
• A bounded alpha prefix table avoids rescanning each blur neighborhood. The shadow uses the actual decoded source alpha, not a semantic object mask; an opaque rectangular image creates a rectangular shadow. No automatic background removal or lighting/occlusion inference occurs.
• Native decode waits at most 10 seconds and PNG encoding at most 5 seconds, but native codec work cannot be forcibly aborted. Late bitmaps are closed and all bitmap/canvas resources are released on success/failure. The PNG is at most 8 MiB; browser color conversion, premultiplication and re-encoding can change source metadata or hidden transparent samples.
• A same-origin pixel module prepares within 10 seconds before ready; a separate 2-second computation deadline applies. The input buffer is copied before transfer so the original source is not detached. Processing is local, with no user code execution, external URL loading, upload or automatic persistence.
• Structured text output is limited to 500,000 characters. Reduce the supplied source when that separate output budget is exceeded.
• Inputs are processed on this device and are not automatically saved or uploaded.