how to insert png to pdf?

I have readed the pdf reference 1.6,i want to insert png to pdf.

i konw that the png consits of alpha and RGB,

31 0 obj<</BitsPerComponent 8/ColorSpace/DeviceRGB/Filter/DCTDecode/Height 154/Length 8781/Name/X/SMask 30 0 R/Subtype/Image/Type/XObject/Width 154>>stream

I know that 30 0 obj is alpha data after encompressed,buf i don't  konw the stream object in 31 0 obj, how to get the stream data from png?