Alpha Channel
An 8-bit (or 16-bit) channel stored alongside the RGB channels of an image that defines per-pixel transparency.
The alpha channel is the part of an image file that encodes transparency. In a 32-bit PNG, each pixel has four channels: red, green, blue, and alpha. The alpha value (0–255) controls how much of that pixel is opaque — 0 means fully see-through, 255 means fully solid.
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The alpha channel is the part of an image file that encodes transparency. In a 32-bit PNG, each pixel has four channels: red, green, blue, and alpha. The alpha value (0–255) controls how much of that pixel is opaque — 0 means fully see-through, 255 means fully solid. Without the alpha channel, every pixel is fully opaque and there is no way to make parts of the image transparent.
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8-bit alpha gives 256 levels of transparency (0–255), which is the standard for web and most design work. 16-bit alpha gives 65,536 levels, which matters for high-end print work where banding in gradients is visible at 8-bit.
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Some image formats (including the internal storage used by some software) store 'pre-multiplied' alpha — the RGB values are multiplied by the alpha value, so a 50% transparent red pixel is stored as (128, 0, 0, 128) rather than (255, 0, 0, 128). Pre-multiplied alpha is faster to composite but can confuse older image editors.
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In CSS, alpha is part of the rgba() and hsla() colour syntax: rgba(255, 0, 0, 0.5) is a 50% opaque red. The alpha value is between 0 and 1, not 0 and 255.
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Saving a transparent PNG as a 24-bit PNG (the alpha is discarded). Exporting as JPG (the alpha is replaced with white). Uploading to a platform that does not support alpha (the file is flattened to the platform's background colour).
Saving a transparent PNG as a 24-bit PNG discards the alpha channel permanently. The file looks the same in the editor (because the editor falls back to a checkered background), but the alpha is gone. Always verify the file is 32-bit before sharing it as a transparent asset.
Continue learning
What is a Transparent PNG?
The full glossary entry on PNG with alpha channel.
What is RGBA?
The four-channel colour model that includes the alpha channel.
What is an Image Mask?
The relationship between masks and alpha channels in modern pipelines.
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