1. A method for generating an alpha hint in a system memory to reduce memory access, the method comprising: rendering, by a graphical processing unit (GPU), graphical objects into a group of pixels stored in an on-chip buffer on a same chip as the GPU; reading an alpha value of each pixel from the on-chip buffer, the alpha value indicating transparency of the pixel; generating the alpha hint in the system memory for the group of pixels according to alpha values of the group of pixels, the alpha hint representing an aggregate of the alpha values; and storing the group of pixels into a frame buffer in the system memory, wherein generating the alpha hint further comprises concurrently setting different bits of the alpha hint according to the alpha values of two or more pixels in the group.