Stream-Based Visually Lossless Data Compression Applying Variable Bit-Length ADPCM Encoding

. 2021 Jul 5;21(13):4602. doi: 10.3390/s21134602

Algorithm 5 Variable bit-length ADPCM encoder.

Require:P // N bit Pixel element.
Require: M // bits of the current ADPCM value.
Require:

I n i t W

// if the predicted offset is initialized or not.
Require:

W_{i n i t}

// a predicted offset to be initialized.
Require:

I n i t P

// if the

P_{p r e v}

is initialized or not.
Require:

P_{i n i t}

// an original data to be set to

P_{p r e v}

.
Ensure: p // ADPCM value.
if

M = N

then

p \leftarrow P

return
end if
if

M \neq M_{p r e v}

then
if

M > M_{p r e v}

then

w \leftarrow w ≪ (M - M_{p r e v})

else

w \leftarrow w ≫ (M_{p r e v} - M)

end if
end if
if

I n i t W = t r u e

then

w \leftarrow W_{i n i t}

end if
if

I n i t P = t r u e

then

P_{p r e v} \leftarrow P_{i n i t}

end if

d \leftarrow P - P_{p r e v}

s t e p \leftarrow (w \times 2) / 2^{M - 1}

d > 0

then
if

| d | / s t e p > 2^{M - 1} - 1

then

p \leftarrow 2^{M - 1} - 1

else

p \leftarrow c e i l (d / s t e p)

end if
else
if

| d | / s t e p > 2^{M} - 1

then

p \leftarrow 2^{M} - 1

else

p \leftarrow c e i l (- d / s t e p) + 2^{M - 1}

end if
end if

f_{p r e v} \leftarrow f_{p r e v} \times C [p & ((1 ≪ M) - 1)] / 64

w \leftarrow f_{p r e v}