Here is an algorithm to blend two RGB565 colors together using ARM assembly. This is very efficient as it uses a minimal number of registers and executes in only 6 ARM operations.

RGB565 Channel Format

Color blender example

Code

# Average two RBG565 format colors together
#
# dst - 0x0000FFFF
# src - 0xFFFF0000
.macro blend_color_rgb565 dst, src

    mov   \dst, \dst, ror #11     @ Move RGB565 to GB-R 5-65
    adds  \dst, \src, ror #11     @ Add source color using carry for green channel overflow
    mov   \dst, \dst, ror #22     @ Rotate back to normal form plus shift right one for divide by two
    bic   \dst, \dst, #0x420      @ Clear underflowing green bit and high blue bit
    orrcs \dst, \dst, #0x400      @ Use carry to add back overflowed high green bit
    bic   \dst, \dst, #0x80000000 @ Clear rotated underflowing blue bit

.endm


# Example

mov    r0, #0xF800    @ Assign red to r0

mov    r1, #0xFF00
orr    r1, r1, 0x00DF @ Assign white to r1

# Call macro to blend colors together
blend_color_rgb565 r0, r1

# Use blended color r0 for application...