img2gray

An HTTP server for benchmarking RGBA to gray scale image conversion using pure Go, CGO and CGO SIMD instructions. The project is to demonstrate what an image conversion pipeline entails when CGO SIMD instructions are used.

Overview

The server contains 4 different conversion types for benchmarking conversion performance:

• NativeGO: basic loop with standard library calls that allocate memory for color.Color struct per pixel.
• NativeGONoAlloc: similar to nativego but will prevent the per pixel allocation by modifying the data directly in the image buffer.
• CGOBasic: uses the cgo FFI to convert using C code. Pixel data is modified within the image buffer.
• CGOSIMD: uses the cgo FFI to call into C with conversion logic using SIMD instructions for loading and modifying multiple pixels at a time on the CPU.

NativeGONoAlloc, CGOBasic and CGOSIMD allow for zero-copy allocations when the image buffers are allocated prior to the calls, see grayscale_test.go for buffer allocations. The server currently allocates two buffers for the image and grayscale image buffers per request, but these could be eliminated by using a sync.Pool to reuse buffer pools to cut down on GC pressure in high-throughput environments.

Benchmark Results

Run the make target benchmark-all to get a baseline across the 4 different conversion types.

Method	Image Size	Time/op	Memory	Allocs
NativeGO	100×100	124.5µs	49KB	10,002
	500×500	3.02ms	1.2MB	250,002
	1000×1000	12.2ms	4.9MB	1,000,002
	1920×1080	25.4ms	10.1MB	2,073,602
	3840×2160	103.6ms	40.5MB	8,294,402
NativeGO (No Alloc)	100×100	9.2µs	0B	0
	500×500	231.2µs	0B	0
	1000×1000	918.5µs	0B	0
	1920×1080	1.89ms	0B	0
	3840×2160	7.49ms	0B	0
CGO Basic	100×100	2.2µs	0B	0
	500×500	56.2µs	0B	0
	1000×1000	215.0µs	0B	0
	1920×1080	431.0µs	0B	0
	3840×2160	1.71ms	0B	0
CGO SIMD	100×100	0.9µs	0B	0
	500×500	17.7µs	0B	0
	1000×1000	73.8µs	0B	0
	1920×1080	144.8µs	0B	0
	3840×2160	578.1µs	0B	0

Tested on Apple M3 Max (darwin/arm64)

note: see grayscale.c for implementation of the cgo calls.

Setup

Clone the repo and run make. After compilation a binary will be located in a build folder: ./build/img2gray.

Usage

% ./build/img2gray -h
Image to grayscale cgo testing server
Usage of ./build/img2gray:
  -host string
    	server listening host (default "localhost")
  -port string
    	server listening port (default "8080")

Running

% ./build/img2gray
2026-01-14T13:05:26.140-0800	INFO	img2gray/main.go:325	Server started	{"address": "localhost:8080"}