# LOCO

This page is originally based on a description written by User:Kostya.

LOCO codec written by Mohammad Rezaei and based on the JPEG-LS lossy/lossless compression scheme, originally known as LOCO-I.

More information on LOCO-I can be found on:

## Decoding Process

1. get predictor
2. decode residue
3. output predictor minus residue as new pixel

## Prediction

A pixel (X) can predict from up to 3 surrounding pixels: up (A), left (B), and up-left (C):

```  +---+---+
| C | A |
+---+---+
| B | X |
+---+---+

```

The prediction logic works as follows:

• A,B,C are unavailable (X is top left pixel) - use 128 as prediction value
• A and C are unavailable (X is in the top line) - use B as predictor
• B and C are unavailable (X is in the first column) - use A as predictor
• A,B,C are available (this one is the JPEG-LS prediction scheme):
• if C >= max(A,B) then use min(A,B) as predictor
• if C <= min(A,B) then use max(A,B) as predictor
• else use (A+B-C) as predictor

## Residue Decoding

Residue decoding is based on Rice codes. There are some differences though:

• Numbers are stored as their absolute value in Rice code and one trailing bit shows their sign - 0 for positive, 1 for negative integers
• Rice parameter is not static but updates every time

If there is lossy compression mode, then add loss value:

```if(val > 0)val += loss
if(val < 0)val -= loss

```

## Rice Code Decoding

• Get standard Rice code with given parameter
• It new code is zero
• If we have some saved bits:
• Get Rice(2) code meaning run length of zeroes
• If run length is more than one then add (run_length + 1) bits to saved bit, else substract 3 from saved bits
• Else increment decode_run_length
• If new code is non-zero:
• If decode_run_length > 2 then add it to saved bits, else substract 3 from saved bits
• Set decode_run_length to zero
• Return new code

## Rice Code Parameter Updating

Rice decoder choses its parameter based on two things - sum of absolute values of decoded residues and their number. To avoid overflow if count went up to sixteen then halve both count and sum.

Rice parameter is log2(sum/number) clamped to interval [0..9]