Transparency notes

On all systems currently supported by Retro Graphics Toolkit at the time of writing this (20151017) the first color for each palette row is transparent.

When you import a PNG file it may contain transparent data. It is better to use a PNG file with transparency than a mask color, however a mask color will also work if the mask color is the same color as the first color in affected palette rows.

When the same color occurs multiple times in the same palette row the last color will be used. You can take advantage of this if you want the transparent color to be the same as a solid color. In Sonic the Hedgehog One, the sprite of Sonic uses black for transparency and solids. This also applies to all sprites that share the same palette with Sonic. The reason this works is due to the fact that transparent pixels are always assigned to color 0. Therefore any non-transparent pixels will not use said transparent color.