BC4 interpolation formulas

interp.py

# ---- Intel decoder impl
# matches 2014 HSW laptop

weights8_intel = [round(256*x/7) for x in range(8)]
weights6_intel = [round(256*x/5) for x in range(8)]

def intel_lerp8(a, b, w):
    return ((256-w)*a + w*b + 128) >> 8

def interp_int_8col(a, b, x):
    # 16-bit interpolation, 8-color mode (e0 > e1)
    # not right yet!
    return ((7-x)*a + x*b) * 257 // 7

def interp_int_6col(a, b, x):
    # 16-bit interpolation, 6-color mode (e0 <= e1)
    # not right yet!
    return ((5-x)*a + x*b) * 257 // 5

def interp_int_8col_u8(a, b, x):
    # 8-bit interpolation, 8-color mode (e0 > e1)
    # this agrees exactly
    return intel_lerp8(a, b, weights8_intel[x])

def interp_int_6col_u8(a, b, x):
    # 8-bit interpolation, 6-color mode (e0 <= e1)
    # this agrees exactly
    return intel_lerp8(a, b, weights6_intel[x])