semi-transparent image convert issue

Hi guys,

I meet a problem when converting semi-transparent images. Because I want to reduce the start up time of the application ,I need to convert the png file to ktx file, but before converting, I need to convert the png file with Imagemagic first , here is the command described in document: 

convert MYORIGIMAGE ( +clone -alpha Extract ) -channel RGB -compose Multiply -composite MYPMIMAGE

After convert the png file, I find that, the converted image is different with the original one 

that make the look of the application a little bit strange.