Imagination PowerVR SDK Blog

Scissor rect error on Windows

pvrvframe

#1

Using the Windows PC SDK I am seeing odd things with glScissor. When using glScissor on the main window surface everything is ok. However if Scissoring is used on an offscreen area (FBO) then I seem to be off by the height of the windows titlebar (28 pixels or so). Are there any issues with Windows and glScissor?





Brian


#2

Our PVRVFrame engineer is looking at this just now and will get back to you after his investigation.





Due to the way PowerVR architecture works we advise developers to favour stencil operations over scissor calls - stencil ops can be done per-tile and on-chip so they are very efficient and can be more flexible than scissor operations. This subject is dealt with in the revision of our performance guidelines that should be available early next month.





What are you using the scissors for?


#3

Hi Brian, I’m having trouble reproducing this. The emulation layer essentially just forwards any calls to glScissor to the desktop drivers so there’s not much room for error. It would be especially helpful if you could provide a minimal example project which exhibits the behavior you describe.


#4

I am trying to narrow this down to a smaller application for you. I did implement the stencil clipping and this seems to work well and gave me a performance boost on the embedded target I use.