Go的PNG编码速度很慢

Question

I'm acquiring frames from my built in webcam with go-opencv. The time it takes to acquire an image from the camera is ~50ms. The time to encode the PNG is ~300ms. Encoding a JPEG is 3x faster, but still horribly slow.

Why is this running so slowly?

Note : I've written similar code against NodeJS, executed on the same machine, and have absolutely no problem hitting 30fps w/ additional image processing. For me, this removes hardware issues from the equation.

My code looks like this:

import (
    "fmt"
    "image/png"

    "github.com/lazywei/go-opencv/opencv"
)

camera := opencv.NewCameraCapture(0)
if camera == nil {
    panic("Unable to open camera.")
}
defer camera.Release()

for {
    if camera.GrabFrame() {
        img := camera.RetrieveFrame(1)
        if img != nil {
            frame := img.ToImage()
            buffer := new(bytes.Buffer)
            png.Encode(buffer, frame)
        } else {
            fmt.Println("Unable to capture frame")
        }
    }

}

Answer 1

Disabling compression improves encoding performance by an order of magnitude on my machine. That may be a start if you don't want to look for png packages outside the standard library.

I also tried the BufferPool (new in Go 1.9), but it didn't make a difference compared to an encoder with a nil BufferPool. Perhaps I did it wrong. The docs are unfortunately very, erm, concise.

package main

import (
    "bytes"
    "image"
    "image/png"
    "os"
    "testing"
)

func BenchmarkPNG_Encode(b *testing.B) {
    img, buf := loadImage(b)

    b.ResetTimer()

    for i := 0; i < b.N; i++ {
            buf.Reset()
            png.Encode(buf, img)
    }
}

func BenchmarkPNG_Encoder(b *testing.B) {
    img, buf := loadImage(b)
    enc := &png.Encoder{}

    b.ResetTimer()

    for i := 0; i < b.N; i++ {
            buf.Reset()
            enc.Encode(buf, img)
    }
}

func BenchmarkPNG_Encoder_NoCompression(b *testing.B) {
    img, buf := loadImage(b)
    enc := &png.Encoder{
            CompressionLevel: png.NoCompression,
    }

    b.ResetTimer()

    for i := 0; i < b.N; i++ {
            buf.Reset()
            enc.Encode(buf, img)
    }
}

func loadImage(b *testing.B) (image.Image, *bytes.Buffer) {
    // foo.png PNG 1920x1053 1920x1053+0+0 8-bit sRGB 251KB 0.000u 0:00.000
    f, err := os.Open("foo.png")
    if err != nil {
            b.Fatal(err)
    }

    img, err := png.Decode(f)
    if err != nil {
            b.Fatal(err)
    }
    f.Close()

    buf := &bytes.Buffer{}
    // grow the buffer once
    (&png.Encoder{CompressionLevel: png.NoCompression}).Encode(buf, img)

    return img, buf
}

Again, this was on my machine with a roughly 1920x1080 pixel image -- a random screenshot; not sure how much this will differ from, say, a photo. YMMV.

$ go test -v -bench . -benchmem 
goos: linux
goarch: amd64
BenchmarkPNG_Encode-8                         10         119289121 ns/op          884964 B/op         38 allocs/op
BenchmarkPNG_Encoder-8                        10         118001658 ns/op          884932 B/op         37 allocs/op
BenchmarkPNG_Encoder_NoCompression-8         100          13050664 ns/op          807156 B/op        212 allocs/op

It's interesting that no compression causes many more allocations than with compression though.

Answer 2

Why is this running so slowly?

Measure! Go has nice profiling tools built in eg into go test .

Most probably it is the PNG encoder which is pure, "unoptimized" Go. If this is not fast enough for you you might want to consider contributing a faster PNG encoder.