pokeglitch/pce

Pixel Countdown Encoding

Pixel Countdown Encoding

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Interface

NOTE : Requires the 'pngjs' module

pce.import.png(<input>)

Input can be one of:

• String (filepath to a PNG image)
• object with the following keys:
  • height (number)
  • width (number)
  • pixels (array of [r,g,b,a] values)

Output is a <DecodedImage>

NOTES:

• Each pixel must be grayscale (r=g=b & a=0) or entirely transparent (a=255)
• Grayscale pixels are converted into one of the following colors: (based on r/g/b value)
  • Black < 48
  • Dark < 128
  • Light < 208
  • White < 256

pce.batch.encode(<input>)

Input must be an object with the following key/value pairs:

• : <DecodedImage>

Output is an object with the following key/value pairs:

• table : <EncodedTable>
• images : object with the following key/value pairs
  • : <EncodedImage>
    • The matches the input

pce.batch.decode(<input>)

Input must be an object with the following key/value pairs:

• table : <EncodedTable>
• images : object with the following key/value pairs
  • : <EncodedImage>

Output is an object with the following key/value pairs:

• : <DecodedImage>
  • The matches the input

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Classes

Properties:

• height
• width
• pixels
  • The pixels is an array of one of 'W','L','D','B','A'
• hasAlpha
• encode()
  • Will encode and return an <EncodedImage>

Properties:

• data
  • The encoded value as a binary string
• decode()
  • Will decode and return an <DecodedImage>

Properties:

• data
  • The encoded value as a ByteArray

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Encoding Formats

Sprite Encoding (.pce)

Header

• height (8 bits)
• width (8 bits)
• hasAlpha (1 bit)
• initialColor ()
• initialCountdown
  • Remaining colors in the following format (in order of appearance)

Body

• Sequence of s

Whenever a pixel color changes, the next in the sequence belongs to the color that just ended

If an encoded sprite is self contained, the represents the counter If the encoded sprite uses a table, the represents the index in the lookup Table (and the table contains the counter)

The counter is the number of pixels until that color appears again

Table Encoding Format (.pcet)

The table contains all referenced counter values sorted by appearance count

The counter values in the table are not encoded

The table is separated into 2 sections:

• Single Byte section (contains all counters which fit into a single byte)
• Double Byte section (contains all counters which don't fit into a single byte)

Header

• Size of the 'single byte' portion of the table (8 bits)

Body

• Single Byte section
  • Each value uses 1 byte
• Double Byte section
  • Each section uses 2 bytes

Color Encoding Format

• The value of an encoded color is the index of the color in the following array:
  • [White, Light, Dark, Black, Alpha]
• Length is 3 bits if hasAlpha, otherwise 2 bits

Value Encoding Format

Value are encoded in the following way:

< Header >< 0 >< Data >

The Header is simply an array of consecutive set bits. The Header is finished when the unset bit (<0>) is encountered.

The bit length of the header indicates both the offset and the bit length of the Data.

The offset is the (summation of 2^n)-1 for n=0 to x, where x=bit lenth of the header.

The Data is a simple binary encoding of a number, with the bit length of x+1

The final value of the encoded is the sum of the offset and the data value.

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Future Work:

• Importing more image formats
• Reading/Writing to file (both encoded and decoded)
• Work with non-grayscale