go-pixelart

Transform picture into pixel art

🎖️ Awesome tiny project getting fourth place 4️⃣ at GoHack Hackathon 2022 (1011 participants)

The picture is cut into square blocks - the slices are counted along the short or the long edge - every block is replaced by the average color of the pixels it covers, and that color is mapped to a retro palette (CGA, EGA, VGA...) or to a grayscale/contrast filter.

Two ways to use it: an HTTP API (with a small web page) and a CLI.

---

Quick start

Run the image from the GitHub Container Registry

docker run -it --rm -p 8080:8080 ghcr.io/fairhive-labs/pixelart

Build and run it locally

docker build -t fairhivelabs/pixelart . && docker run -it --rm -p 8080:8080 fairhivelabs/pixelart
# or
docker compose up --build
# or, without docker
make run

Then visit http://localhost:8080 🚀 and pixelize your pictures ;)

---

HTTP API

Method	Path	Description
GET	/	web page to upload and pixelize a picture
POST	/pixelize	transform a picture, returns HTML or JSON
GET	/health	liveness probe, returns ok
GET	/favicon.ico	favicon, cached and revalidated with an ETag

POST /pixelize

multipart/form-data, every field is required:

Field	Type	Values	Description
file	file	PNG or JPEG	the picture to transform
slices	integer	1 to 1000, and not more than the smallest side of the picture	number of blocks cut along the selected edge
edge	string	short, long	edge the slices are counted on
filter	string	cga2, cga4, cga16, ega, vga, identity, dark-contrast, dark-gray, gray, invert, light-gray, xray	color palette applied to each block

Query	Values	Description
mime	html (default), json	response format

The pixel art is returned in the same format as the picture that was uploaded (a PNG in, a PNG out).

HTML response

Returns 201 Created and a page displaying the pixel art, which can be rotated and resized:

curl -X POST "http://localhost:8080/pixelize" \
  -F "file=@pictures/wfvr.png" \
  -F "edge=short" \
  -F "slices=100" \
  -F "filter=ega" \
  --output pixelart.html

JSON response

Returns 201 Created and the picture, base64 encoded:

curl -s -X POST "http://localhost:8080/pixelize?mime=json" \
  -F "file=@pictures/wfvr.png" \
  -F "edge=short" \
  -F "slices=100" \
  -F "filter=ega" | jq -r .data | base64 -d > pixelart.png

{
  "data": "iVBORw0KGgoAAAANSUhEUgAAB+QAAAp4CAIAAADg...",
  "encoding": "base64",
  "filter": "ega",
  "length": 36192
}

Status codes

Code	When
201 Created	the pixel art was produced
400 Bad Request	a field is missing or invalid, or slices is larger than the picture
413 Content Too Large	the upload is over MAX_UPLOAD_BYTES, or the picture declares more than MAX_PIXELS
415 Unsupported Media Type	the file is not a PNG or a JPEG, or cannot be decoded
503 Service Unavailable	MAX_CONCURRENT_JOBS transformations are already running, retry later (Retry-After)

The API is public and CORS is open to any origin (POST, OPTIONS), without credentials.

---

Filters

Applied to the average color of each block. pictures/wfvr.png is the source of every sample below.

Filter	Description	Sample
cga2	2 colors CGA palette, black and white	wfvr_cga2.png
cga4	4 colors CGA palette	wfvr_cga4.png
cga16	16 colors CGA palette	wfvr_cga16.png
ega	64 colors EGA palette	wfvr_cga64.png
vga	18 bits VGA color, 6 bits per channel	-
identity	true color, no palette	wfvr_pixel80_shortedge_truecolor.png
gray	grayscale, average of the channels	-
dark-gray	grayscale using the darkest channel	wfvr_dark_gray.png
light-gray	grayscale using the brightest channel	-
dark-contrast	darkens the two brightest channels	wfvr_dark_contrast.png
invert	inverted colors	-
xray	inverted, then brightest channel	-

Transparent pixels stay transparent with the palette filters.

---

CLI

make cli          # builds ./bin/pixelart
./bin/pixelart -slices 80 -edge long -filter cga16 pictures/wfvr.png
# or without building
go run ./cmd -slices 80 -edge long -filter cga16 pictures/wfvr.png

Flag	Default	Description
-slices	100	number of slices cut along the selected edge
-edge	short	short or long
-filter	cga4	any filter of the table above
-out	<picture>_<timestamp>.<ext>	output file
-quality	75	JPEG output quality, from 1 to 100

👉 Source file "pictures/wfvr.png" opened
🤖 Image DECODED - Format is "png"
🖼  Original Dimension = [ 2010 x 2679 ]
👾 Processing Transformation...
✅ Transformation is over
💾 Pixel Art saved in file "pictures/wfvr_20260804-233956.png"

Exit codes: 0 on success, 2 on a usage error, 1 when the transformation fails.

---

Configuration

Every setting is an environment variable read at startup:

Variable	Default	Description
PORT	8080	port the server listens on
MAX_UPLOAD_BYTES	10485760 (10 MiB)	largest accepted request body
MAX_PIXELS	40000000 (40 MP)	largest accepted picture, checked before decoding
MAX_CONCURRENT_JOBS	number of CPUs	transformations running at the same time, further requests get a 503
GIN_MODE	release	set to debug for gin's verbose mode

The dimensions are read from the picture header before it is decoded, so an oversized picture is rejected without allocating memory for it.

---

Development

make build   # builds the API into ./bin/api
make run     # builds and runs it on :8080
make cli     # builds the CLI into ./bin/pixelart
make test    # go test ./...
make race    # go test -race ./...
make bench   # filter benchmarks
make vet     # go vet ./...

Go 1.25 or later is required (see go.mod).

Layout:

Path	Content
api/	HTTP server, embedded templates and assets
cmd/	command line tool
internal/filter/	pixel, basic and convolution filters, CGA/EGA/VGA palettes
internal/colorutils/	color helpers
pictures/	sample pictures

The filters are parallelized over the available CPUs; internal/filter/equivalence_test.go keeps the original implementations as an oracle and checks the output pixel per pixel, so an optimization that changes the result fails the build.

---

Deployment

Docker

The image builds a static binary and runs it as an unprivileged user on a pinned alpine, with a health check on /health:

docker build -t fairhivelabs/pixelart .
docker run -it --rm -p 8080:8080 -e MAX_UPLOAD_BYTES=20971520 fairhivelabs/pixelart

GitHub Container Registry

Every push on main publishes ghcr.io/fairhive-labs/pixelart (see .github/workflows/docker.yml).

Heroku

.github/workflows/heroku.yml deploys the Docker image on every push on main. The deployment is skipped when the HEROKU_API_KEY secret is missing. Set the HEROKU_APP_NAME and HEROKU_EMAIL repository variables to target another app - the health check URL follows the app name.

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License

MIT