Alpha channel in overlay
In addition to color channels (red, green, and blue), some image formats allow storing an alpha channel encoding image opacity. Four such formats are supported by PatchMaker: BMP, TGA, PNG, and TIFF. In images containing an alpha channel, each pixel is represented by four bytes (a total of 32 bits): three color components and an alpha value.
The alpha channel (also called matting) should be taken care of when creating an overlay. Depending on the matting value at the given pixel, the overlay color can entirely occlude the underlay or make it (partly) visible. A smooth blending of overlay and underlay colors near the edges of an opaque overlay region will normally yield better-looking results.
PatchMaker is designed to work with overlays that contain an alpha channel. If you try to load an overlay without an alpha channel, PatchMaker will issue a warning:
It is important to remember that different interpretations of matting
values are possible and frequently used. For example, an alpha value of
0 might mean total transparency of a pixel and a value of
255 its total opaqueness. Under a different interpretation,
0 would mean pixel opaqueness and 255 its transparency.
Naturally, the alpha channel interpretation in the image editor used to
create an overlay and in PatchMaker should be the same. In PatchMaker,
the overlay has a selectable Alpha Interpretation
property. The interpretations beginning with the word Inverted
refer to the first case and all the others to the second.
To make things even worse, color value can also have different interpretations with respect to matting. For example, color values stored in your image file can be already multiplied by the relative pixel opacity. The interpretations ending with the word Premultiplied correspond to such images. It is useful to understand, when creating an overlay, what interpretation its alpha channel is going to have.
All in all, there are four possible values for the Alpha Interpretation property. If you do not know which one to use, simply import the overlay and change the Alpha Interpretation property until it looks like it should. In Tutorial 1, there is an example of such a case (see Step 8).
If you usually create your overlays with the same alpha interpretation, its default value in PatchMaker can be adjusted by choosing the corresponding item of the menu:
