computer-graphics-university

General Information

Media codec with the possibility of result representation.

Setup

Prerequisites

In the case of the usage with MacOS operational systems it's important to remember to allow the usage of SDL2 library on your local machine using privacy settings.

Installation

All setup related operations are processed via Makefile(which operates CMake configurations) placed in the root directory.

In order to build the application into project local bin library it's required to execute the following command.

make build

Specification

Metadata

All the output images of CGU type have metadata part, which is located after EOF flag. It's used for internal CGU properties interpretation and its further usage as parameters for different kind of operations.

It's important to remember, that all the metadata values are defined with the new line flag!

All the available metadata flags:

Name	Offset	Size	Description	Mandatory
Compatibility	1	1 byte	Compatibility flag, which is required to be present, so CGU can perform decode operation	true
Conversion	2	1 byte	Describes conversion type used for image encoding	true
Width	4	2 bytes	Describes image width	true
Height	6	2 bytes	Describes image height	true
Dithering	7	1 bytes	Describes if dithering is enabled for the image encoding	true
Indeces size	11	4 bytes	Represents size of the given color indeces	false
Indeces	11 + (4 * N)	4 bytes * N	Represents color indeces generated for palette conversions	false

Native conversion

Every pixel has 7-bit color. That color consists of the following compounds:

4 states for RED color.
8 states for GREEN color.
4 states for BLUE color.

The general color is equal to RRGGGBB.

RGB

The values of RED and BLUE compounds look like this:

0 - 0
1 - 85
2 - 170
3 - 255

The values of GREEN compound look like this:

0 - 0
1 - 36
2 - 73
3 - 109
4 - 146
5 - 182
6 - 218
7 - 255

BW

All the values are interpreted as an intensity of a grey scale, where 0 means the color is black and 255 means that the color is white.

Dithering

Both RGB and BW support additional feature, which is called dithering.

Dithering - an intentionally applied form of noise used to randomize quantization error, preventing large-scale patterns such as color banding in images

In the case of native conversion there are the next possible options of dithering algorithm to be used:

Floyd-Steinberg

Palette conversion

In order to perform color conversion it's required to have generated reduced bit color map of the colors of the source image. It should not exceed 128 colors.

Detected palette conversion

Detected palette conversion uses previously generated bit color map, but renders only palette colors themselves. Each color is placed in a rectangle, which are evenly spaced in the whole image surface.

MedianCut

MedianCut is the algorithm used for the color quantization operation. With the help of the generated 7-bit color palette saved in the metadata header and indeces placed as data image, it can process the given image and return the generated one.

MedianCut algorithm has a restriction. It works only if the given image has at least 128 unique colors.

RGB

BW

Image data

Image data is placed under the metadata header, after the offset(11 + (4 * N)). All the data is written in the HEX notation, but can be converted to usual RGB.

RGB color chunk for Native RGB looks in the following way.

RGB(0, 36, 85)
RGB(0, 0, 85)
RGB(0, 36, 0)
RGB(0, 0, 85)
RGB(0, 0, 85)
RGB(0, 36, 85)
RGB(0, 0, 85)
RGB(0, 36, 85)

RGB color chunk for Native BW(grey scale) looks in the following way.

RGB(7, 7, 7)
RGB(12, 12, 12)
RGB(40, 40, 40)
RGB(78, 78, 78)
RGB(153, 153, 153)
RGB(172, 172, 172)
RGB(78, 78, 78)
RGB(136, 136, 136)

Data collection

Image data consists of chunks. Each chunk has 8 pixels. The whole chunk size of 7 byte. The sequence of retrieved raw data chunks looks like this:

(0, 8), (8, 15), (15, 23), ...

Each chunk is written in the new line. Output structure looks like this:

(1, 8)
(2, 8)
(3, 8)
...

During the decoding process the chunk column is decerialized into row based one, which was known during the encoding process.

Use cases

Encoding

In order to encode input image, it should be of the supported extension type:

jpeg
jpg
png
bmp

Codec obliges to select conversion mode during encoding operation:

native_rgb
native_bw
palette_rgb
palette_bw

Also, there is an option to enable dithering mode during the encoding operation. This mode will enable the usage of Floyd-Steinberg dithering algorithm. To enable it, it's required to add --dithering flag.

Examples

The encoding for BMP to CGU with palette_rgb command looks like this:

./bin/cgu encode --from="./examples/1.bmp" --type=bmp --conversion=palette_rgb --to="1.cgu"

The encoding for BMP to CGU with native_rgb command looks like this:

./bin/cgu encode --from="./examples/2.bmp" --type=bmp --conversion=native_rgb --dithering --to="2.cgu"

Decoding

In order to decode input image, it should be of CGU extension.

Output image can be one of the following extensions:

jpeg
jpg
png
bmp

Codec obliges to select conversion mode during decoding operation:

native_rgb
native_bw
palette_rgb
palette_bw

Also, there is an option to enable debug mode during the encoding operation. This mode will enable dedicated palette overlay. To enable it, it's required to add --dithering flag.

The decoding for BMP to CGU with palette_rgb command looks like this:

./bin/cgu decode --from="./1.cgu" --type=bmp --to="1.bmp"

The decoding for BMP to CGU with native_rgb command looks like this:

./bin/cgu decode --from="./2.bmp" --type=bmp --to="2.bmp"

The decoding for BMP to CGU with native_rgb command and --debug flag looks like this:

./bin/cgu decode --from="./2.bmp" --type=bmp --to="2.bmp" --debug

Viewer

Viewer mode enables user to preview CGU encoded images.

The viewer command for CGU encoded image looks like this:

./bin/cgu view --from="1.cgu"

The viewer command for CGU encoded image with --debug flag looks like this:

./bin/cgu view --from="1.cgu" --debug

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