Adjustment layers are layers that consist of a filter and an optional selection. The filter effect is applied to the composite image of all layers under the adjustment layer in the current layer group. The big thing is, adjustment layers apply these effects non-destructively. The original image data is not modified.
Almost all chalk filters are suitable for use in adjustment layers -- even filters that would downgrade the image quality. For instance, the raindrops filter converts to 8-bit RGB before working its magic. If you would try to use this filter directly on a 16-bit L*a*b* layer, chalk would warn you about the conversion to RGB and back again this filter would cause. Not so with adjustment layers: the original data isn't touched, so applying the filter is safe.
What about the colorspace of an adjustment layer then? In order to examine this issue, you need to know what happens when chalk renders an adjustment layer.
If the currently active layer has an active selection, then that selection will be copied and used as a mask for the adjustment layer. If there is no active selection, then there will be no mask and the adjustment will apply to the entire extent of the layers under the adjustment layer in the current group. There is no way of adding a mask to an existing adjustment layer.
If there is a mask in the adjustment layer, you can edit the mask using the ordinary painting tools and painting operations.
chalk composites the layers bottom to top, within each layer group. The aggregate -- or the projection as it is also called -- is then filtered by the adjustment layer. If there are layers on top of the adjustment layer, those are composited onto the projection. chalk converts all layer data before compositing, so if the bottom-most layer in an image is grayscale, all layers are converted to grayscale before compositing -- and that means that the adjustment layer projection will be grayscale, too.
With this knowledge you'll understand why chalk can often offer better performance working with layers on top of an adjustment layer which is on top of a complex layer structure: chalk uses the projection and doesn't even look anymore at the layers under the adjustment layer. Unless, of course, you change one of them.
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