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alpha values for alpha blending mixer

Altera_Forum
Honored Contributor II
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hi,  

 

I'm using the alpha blending mixer, with two input streams, (the background layer is 800x600 and the overlay is 640x480), and also the alpha blending ports are enabled. 

 

I have been able to use the 640x480 input stream's "startofpacket" signal inorder to find the videodata packet and determine the start of active data. Also, I have used line and row counters as reference inorder to provide the correct alpha values for the 640x480 stream. 

 

However, when using signaltap, I can see that the timing isn't quite right with regard to the numbers of rows and lines in the 640x480 stream, the numbers don't quite add up.  

 

What I need is a pointer on how to provide correct alpha values to the 640x480 stream, when to start, when to stop etc. I tried using the valid, startofpacket, endofpacket signals from the 640x480 stream and provide them directly to the alpha blending port of the 640X480 stream (while setting the stream data value to zero), but this resulted in a blank screen
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