matrix help: how does postScale affect the translation part of a matrix?

Ive been trying to implement a limit to prevent the user from scaling the image too much in my multitouch zoom app. Problem is, when i set the max zoom level by dumping the matrix, the image starts to translate downward once the overall scale of the image hits my limit. I believe it is doing this because the matrix is still being affected by postScale(theScaleFactorX,theScaleFactorY,myMidpointX,myMidpointY) where theScaleFactorX/Y is the amount to multiply the overall scale of the image (so if the theScaleFatorX/Y is recorded as 1.12, and the image is at .60 of its origional size, the overall zoom is now .67). It seems like some sort of math is going on thats creating this translation, and was wondering if anyone knew what it was so i can prevent it from 开发者_运维知识库translating, and only allow the user to zoom back out.

Still unsure how postScale affects translation, but I fixed it by having an if statement saying as long as we are within the zoom limit i set, post scale as normal. Otherwise, divide the set zoom limit by the saved global zoom level recorded on ACTION_DOWN and set the scale so it will keep the image at the proper zoom level