Alpha Blending - Samsung S3C2416 User Manual

S3C2416X RISC MICROPROCESSOR

4.6 ALPHA BLENDING

Alpha Blending combines the source color and the destination color in the frame buffer to get the new destination
color.
The conventional alpha blending equation is: final data = src * alpha + dest * (1.0 − alpha). 2D uses 8-bit integer to
represent the alpha value, with 0 indicating 1/256 and 255 indicating 1.0. The equation of converting 8-bit ALPHA
value to the actual fractional alpha value is: alpha = (ALPHA+1) / 256.
The internal computation of alpha blending and fading is as follows:
User-specified alpha value: ALPHA (given by ALPHA_REG, from 0 to 255)
[Alpha Blending]
data = ( source * (ALPHA+1) + destination * (255-ALPHA) ) >> 8
[Fading]
data = (( source * (ALPHA+1) ) >> 8) + fading offset
Per-pixel alpha blending: ALPHA (given by the source image, from 0 to 255)
[Alpha Blending]
data = ( source * (ALPHA+1) + destination * (255-ALPHA) ) >> 8
[Fading]
data = ((source * (ALPHA+1) ) >> 8 ) + fading offset
4.6.1 Related Registers
ROP_REG
ALPHA_REG
Alpha blending configurations: alpha blending disable/enable, per-pixel alpha
blending disable/enable, fading disable/enable.
Alpha value and fading value.
2D
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