HLI

The HLI is an HDR image format aimed to be simple as possible.

File extension .hli.

General structure

[magic number][1-byte header-size length][variable-length header-size][variable-length header][raster]

Magic number

The magic number of the HLI format is HLi.v1 at the beginning of the file.

Header

CBOR encoded data:

type Header struct {
	Width       int               `cbor:"width"`
	Height      int               `cbor:"height"`
	Depth       int               `cbor:"depth"`
	Format      string            `cbor:"format"`
	RasterMode  string            `cbor:"raster_mode"`
	Compression string            `cbor:"compression"`
	Metadata    map[string]string `cbor:"metadata,omitempty"`
}

See consts.go for possible values for each field.

metadata is optional

Default options:

Mode6 = &hli.Header{
    Depth:       32,
    Format:      hli.FormatLogLuv,
    RasterMode:  hli.RasterModeSeparately,
    Compression: hli.CompressionZstd,
}

Raster

The raster contains all the pixels of the image.

Compression

The raster is compressed with the specified algorithm in the header (typically: gzip)

Supported compression:

gzip
zstd (default)

Format

RGBE and XYZE

These formats are based on the Radiance RGBE encoding. A pixel is stored on a 4-byte representation where three 8-bit mantissas shared a common 8-bit exponent. It offers a very compact storage of 32-bit floating points. The net result is a format that has an absolute accuracy of about 1%, covering a range of over 76 orders of magnitude.

RGB and XYZ

These formats are based on the representation of a 32-bit floating points in bytes. A pixel is stored on a 12-byte representation where a channel is coded on 4 bytes in little endian order. It offers a great absolute accuracy.

LogLuv (used as default format)

This format is based on the LogLuv Encoding for Full Gamut. A pixel is stored on a 4-byte representation where a channel is coded on 4 bytes in.

# Original LogLuv representation

 1       15           8        8
|-+---------------|--------+--------|
 S       Le           ue       ve


# HLI LogLuv representation

    8        8        8        8
|--------+---------|--------+--------|
   SLe       le       ue       ve

It offers a great compression and with an absolute accuracy of about 0.3%, covering a range of over 38 orders of magnitude.

Raster mode

Uncompressed Raster is stored in several modes:

normal mode

Each pixels' bytes are stored in contiguous order.

RGBE example:

rgbergbergbe
rgbergbergbe

This is the same for XYZE.

RGB example:

rrrrggggbbbbrrrrggggbbbbrrrrggggbbbb
rrrrggggbbbbrrrrggggbbbbrrrrggggbbbb

This is the same for XYZ.

LogLuv example:

SLeleueveSLeleueveSLeleueve
SLeleueveSLeleueveSLeleueve

separately mode

The color channels are stored separately in order to improve the compression ratio.