TIP: A word about Paint On Mask mode.
Corel Photo-Paint's Paint on Mask mode opens up a whole new world in the use of masks.

But first, a little on Masks and Selections -
A mask prevents/protects an area of your image from being edited or changed. The unmasked area is called a selection and is the reverse of a mask - the part of your image which CAN be edited or changed. Therefore masks and selections are each other's reverse. The mask itself can be considered to be an 8-bit grayscale image overlaying your 'real' image. The grayscale values determine which areas are editable and which are protected. The black (grayscale value 0) areas are the protected, non-editable regions, while the the white (grayscale value 255) areas are the editable region/s, sometimes referred to as the selection. Regions containing intermediate grayscale levels are variably protected/editable according to the grayscale value.

Normally, masks in Photo-Paint are represented either as a marquee (marching ants surrounding the selection) or as a mask overlay (a transparent overlay in a preset colour, the tint of which indicates masked and selected areas). In Paint on Mask mode, the mask is presented as a true grayscale bitmap image.

The mask marquee is sufficient to visualise masks that have a sharp edge, where the transition between the selection (inside the marquee) and the mask (the area outside the marquee) goes straight from a value of white (fully editable) to black (fully protected).

However, if the mask is feathered ( Mask > Shape > Feather ) it blends more gradually between the editable and the protected states. It contains a region which has grayscale values in between 0 (black), and 255 (white). If you apply an effect or edit a selection with a feathered mask, the effect is diminished across the feathered region. The example below shows how a mask can constrain the effect of a ripple distort effect ( Effects > Distort > Ripple ), and how feathering a mask alters this constraint.

Paint On Mask:

When you switch a mask to Paint On Mask (POM) mode (Mask > Paint on Mask, CTRL+K or the icon on the Mask/Object Toolbar), it appears as a grayscale image. In POM mode the grayscale image of the mask itself can be edited just like most other images.

Most of the standard Photo-Paint tools and effects can be used. Fill and paint colours are however limited to grayscale equivalents. But all of the Fills - uniform, fountain, bitmap and textures - all have grayscale equivalents. Even the interactive fill tool can be used. Try changing the paint colours via the colour palette while in POM mode - the darker the colour or tint, the darker the grayscale value and vice versa. This make sense as colours have no place in masks except as grayscale values. All of the brush tools can be used in POM mode - the Paint Brush tool, the Effects Brush tool, the Clone Tool and even the Image Sprayer Tool.

Using the Paint Brush in POM mode.

In POM mode, the Image Sprayer Tool was used to paint leaves and butterflies (A).
After switching POM mode off, the mask was filled with a fountain fill gradient (B)

In fact, you can even use the Mask Tools while in POM mode to create and manipulate a mask over the grayscale POM bitmap, effectively providing you with a mechanism to have TWO active masks operating simultaneously! In this case, you are really creating a mask over a mask, because in POM mode, you aren't editing the actual image, just the selection and the mask that you've made on the image. The ability to use the Mask Tools in POM mode allows you to apply complex effects such as The Boss and the Glass Filters (which require a mask in order to work) to your POM mode edits.

A mask was created from a rectangular object (original) and then switched to POM mode.
A mask was then created from the POM object (Mask > Create from Objects) and then
the Boss Effect (Effects > 3D Effects > The Boss ) applied (all while in POM mode).
POM mode was then switched off, and a texture fill applied to the object.

Any secondary masks you create from your new grayscale object while in POM mode are discarded when you turn off the POM mode and return to normal editing.

Interestingly, the Mask Overlay mode is not available to the user while in POM mode - only the Mask Marquee. However, Photo-Paint versions 9 and 10 actually display the Mask Overlay during some secondary mask editing procedures (e.g. Mask > Shape > Feather ) while working in POM mode. This is very handy but it would be even better to have the Mask Overlay feature available for secondary masks by choice.

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I have seen many special effects created with Adobe PhotoShop that require direct manipulation of a mask channel. In Corel Photo-Paint the manipulation of channels is limited BUT, using Paint on Mask mode makes any mask channel available for full editing, and in fact allows even greater manipulation than PhotoShop's channel options. Therefore, if you even need to know how to convert a cool PhotoShop channel editing technique over to Photo-Paint, just load the mask/channel, click on POM button, and then you can do all of the same things. It took me ages to realise this but once I did I have not found any instances of PhotoShop channel procedures that cannot be done the same or even better in Photo-Paint. Paint on Mask is the key! Paint on Mask is really cool and very powerful!