Chroma Keying

How do I remove the canvas from my background?

This requires some knowledge of the device.  We always start each draw cycle by drawing a rectangle in the area that we are preparing to update.  The color of that rectangle is the color of the canvas in Altia Design.  For transparency between layers to work, this color needs to be something very unique and then the hardware is told to render this as 100% transparent for our layer.  It's called chroma keying.  The same concept as green screen in video editing.  The color to make transparent is sometimes called the chroma color.  The hardware must be told the chroma color for our layer.  Otherwise, our layer will show the Altia Design canvas color everywhere that objects are not drawn and this will cover up the pixel colors of the background layer.  Additionally, the hardware is also given an alpha value to apply to the pixels of our layer to make them semi-transparent in front of the background layer.  In summary, there's a chroma color for our layer to make our canvas color completely transparent and then there is an alpha mask to make other colors of our layer semi-transparent.

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