
Philips Semiconductors
PNX15xx Series
Volume 1 of 1
Chapter 11: QVCP
PNX15XX_SER_3
Koninklijke Philips Electronics N.V. 2006. All rights reserved.
Product data sheet
Rev. 3 — 17 March 2006
11-19
2.6.2
Alpha Blending
Blending allows video and graphics to be combined with varying levels of
transparency. Blending is possible only when both current and previous Layer pixels
are valid. Either 16 or 256 levels of blend from one layer to another and vice versa are
available. The blend value may come from a layer’s alpha register or from the upper 4
or 8 bits of an incoming pixel or is a multiplication of both.
The blending is done according to the following equation:
Pixel_result = alpha x Pixel_current + (1-alpha) x Pixel_previous
Pre-multiplied pixel formats are supported. The Premult bit is set, which means the
incoming pixel stream is already pre-multiplied with the per-pixel alpha value. The
resulting alpha blend equation is as follows:
Pixel_result = Pixel_current + (1-alpha) x Pixel_previous
An additional per-component pre-multiply with a constant can be achieved by proper
programming of the color space matrix. Fading of alpha values is controlled by the
alpha_mix bit. If it is set, the pixel alpha gets multiplied by the xed alpha value/256.
Remark: Alpha=255 has the effect, in hardware, of making alpha equal to 1 in the
above equations.
2.7 Output Pipeline Structure
The input to the output pipeline comes from the mixers. The output pipeline houses
the formatter (FRMT) that produces the nal output stream in the required output
format.
Figure 6:
Mixer Block Diagram—Pixel Processing
Previous Pixel
New Pixel
Alpha
Invert_ROP
Select_ROP
Alpha_ROP
New Mixed Pixel
Previous Alpha
Alpha_REG
Current Pixel Extracted Alpha
Previous Alpha for
next MIXER Stage
Alpha
Alpha_Use_REG
Pixel Formatter Block
Alpha_Pass_ROP
Valid Pixel?
premult
Blend