Grayscale Alpha Mask

[corey....@gmail.com]

What is the most efficient way in Skia to treat an opaque grayscale RGBA image as an alpha mask suitable for use with clipShader ? e.g. Where the whitest portion of the image implies my masked layer contents is most visible and the darkest the least visible.

e.g. Do I need to construct a new kAlpha_8 image from one of the image channels, or is there a convenience method or automatic mechanism to do this for me?

[Brian Osman]

The first thing that comes to mind:

Turn your image into an SkShader (image->makeShader). Then use imageShader->makeWithColorFilter(SkColorFilters::Matrix(...)). That will give you a shader that is the result of running the original image through a matrix color filter. You can construct the color matrix to populate alpha from a single channel, or from the average of the three RGB channels, etc...

On Tue, Oct 25, 2022 at 8:20 AM corey....@gmail.com <corey....@gmail.com> wrote:

What is the most efficient way in Skia to treat an opaque grayscale RGBA image as an alpha mask suitable for use with clipShader ? e.g. Where the whitest portion of the image implies my masked layer contents is most visible and the darkest the least visible.

e.g. Do I need to construct a new kAlpha_8 image from one of the image channels, or is there a convenience method or automatic mechanism to do this for me?

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