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DRA72XEVM: DSS DISPC Layer blending doesn't appear to work with lowest zorder layer

Michael Van Reenen
Expert 1085 points

Part Number: DRA72XEVM

While trying to display a PNG file with transparency we have found that the transparency (via alpha blending) does not appear to be working when the image is placed on the layer that is configured to have the lowest zorder.   Simply changing the zorder of that layer via the ATTRIBUTES register and leaving all other register settings the same the transparency/blending works as expected.      We would expect to see the background color blended with the image as the format is ARGB-8888 and our test has only 1 layer active at a time.

Looking in the TRM at "Figure 11-76. DISPC Alpha Blending Architecture With Premultiplied Alpha Support"   It shows both A & RGB available for all layers 2,3,4 but layer 1 only shows RGB.  We'd like to know if this is the expected behavior of the HW or possibly something missed in our QNX display driver.

If you require more information please let me know.

Thanks

Mike