Hi,
Here are some observations on the Dirac spec (2.2.2) I made while
writing a toy-decoder for dirac:
13.4.2.2: subband_coeffs() is missing "quant_index" in the parameter list
13.4.3.2: codeblock() Uses a local variable "band" that's not set in
this function. (should be "band = coeff_data[level][orient]")
15.6.2: The lift2-4 functions use wrong parameters in the for statement.
lift1 is correct ("for i = -N to M" vs. "for i = D to L + D - 1")
Arithmetic decoder:
There is no need for the arithmetic decoder to use the variable
"state[LOW]". You can simply change
count = state[CODE] - state[LOW]
to
count = state[CODE]
and
state[LOW] += range_times_prob
to
state[CODE] -= range_times_prob
and simplify renormalise() to:
state[RANGE] <<= 1
state[CODE] <<= 1
state[CODE] += read_bitb()
(There is no need for the bit twiddling and the masking if state[LOW]
always stays zero.)
Short justification:
In the end, state[LOW] is only used to calculate "count", which is the
real state that the read_boola function is interested in. So anything
that produces the correct value for "count" is valid.
Now count is defined as:
count = code - low
The operation
low' = low + range_times_prob
changes "count" like this:
count' = code - low - range_times_prob = count - range_times_prob
The renormalise operation:
low' = low * 2
code' = code * 2 + read_bitb
changes "count" like this:
count' = code * 2 + read_bitb - low * 2 = (code - low) * 2 + read_bitb
= count * 2 + read_bitb
So, it is possible to use "count" directly as a state variable instead
of "code" and "low" and this greatly simplifies the renormalise function.
Regards,
Dennis Ranke
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