Are we still talking about PNG? Nowhere in the PNG standard is a description of changing the actual content of an image to make it compress better with DEFLATE. If you are thinking about the filter stage you should know that in this context an image filter is just creating a different representation of the same image, because it is fully reversible.
How would you make a lossy PNG? Just introduce errors randomly? That makes no sense.
By reading google or the wiki
Process data so DEFLATE works more efficiently
Are we still talking about PNG? Nowhere in the PNG standard is a description of changing the actual content of an image to make it compress better with DEFLATE. If you are thinking about the filter stage you should know that in this context an image filter is just creating a different representation of the same image, because it is fully reversible.