SVG SourceAlpha Filter Input
SourceAlpha provides a black, alpha-only version of the source graphic, commonly used as the basis for drop shadows and glows.
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Supports PNG, JPG, BMP, WEBP up to 5MB
What SourceAlpha Actually Contains
SourceAlpha is a built-in filter input keyword representing the source graphic's silhouette rendered entirely in black, using only its original alpha channel values for opacity — all color information is discarded, leaving just the shape's outline and transparency, which is exactly what's needed for shadow and glow effects.
- SourceAlpha renders the source graphic's silhouette entirely in black
- Only the original alpha channel opacity values are preserved
- All color information is discarded, leaving just shape and transparency
Using SourceAlpha for Shadows Instead of SourceGraphic
Feeding SourceAlpha into feGaussianBlur produces a soft black shadow shape, whereas feeding SourceGraphic would blur the original colors along with the shape, producing a colored smear instead of a clean shadow — this distinction is why most drop shadow filter recipes start from SourceAlpha rather than SourceGraphic.
- Blurring SourceAlpha produces a clean, soft black shadow shape
- Blurring SourceGraphic instead would smear the original colors together
- This distinction is why shadow recipes start from SourceAlpha, not SourceGraphic
Frequently Asked Questions
What's the difference between SourceAlpha and SourceGraphic?
SourceGraphic includes the full original colors and transparency, while SourceAlpha discards all color and keeps only a black silhouette based on the original alpha channel.
Why do drop shadow filters use SourceAlpha instead of SourceGraphic?
Using SourceAlpha ensures the blurred shadow stays a clean black shape rather than a blurred smear of the original artwork's colors.
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