Colors and Alpha Transparency

Understanding how Skija handles colors and transparency is essential for achieving correct visual results, especially when blending multiple layers or images.

Color Representation

In Skija, colors are most commonly represented as 32-bit integers in ARGB format.

• A (Alpha): bits 24-31
• R (Red): bits 16-23
• G (Green): bits 8-15
• B (Blue): bits 0-7

You can use the Color utility class to manipulate these values safely:

int myColor = Color.makeARGB(255, 66, 133, 244); // Opaque Blue
int transparentRed = Color.withA(0xFFFF0000, 128); // 50% Transparent Red

Alpha Type: Premultiplied vs. Straight

One of the most important concepts in Skia is the Alpha Type (ColorAlphaType).

Premultiplied (PREMUL)

This is the default and recommended format for rendering. In this format, the RGB components are already multiplied by the alpha value.

• Why? It makes blending much faster and prevents "dark edges" when filtering or scaling images.
• Example: A 50% transparent white (Alpha=128, R=255, G=255, B=255) becomes (128, 128, 128, 128) in premultiplied space.

Unpremultiplied (UNPREMUL)

Also known as "Straight Alpha". RGB components are independent of alpha. This is how most image files (like PNG) store data.

• Example: The same 50% transparent white remains (128, 255, 255, 255).

Color Spaces

Skija is color-space aware. While you can work with raw "naive" RGB, for professional results, you should specify a ColorSpace.

• ColorSpace.getSRGB(): The standard color space for the web and most monitors.
• ColorSpace.getDisplayP3(): For wide-gamut displays (like modern Macs and iPhones).

When creating a Surface or loading an Image, always consider the color space to ensure consistent appearance across different devices.

Best Practices

1. Always use Premultiplied Alpha for active rendering and composition.
2. Use Color4f when you need high-precision colors (floating point) or are working with wide-gamut color spaces.
3. Be mindful of the Alpha Mode when capturing snapshots or reading pixels; you may need to convert from PREMUL to UNPREMUL if you intend to save the data to a standard PNG.