PNM

PNM is a collection of open image formats. They are also referred as portable anymap format(PNM). This library supports ppm(portable pixmap), pgm(portable graymap) and pbm(portable bitmap) formats. There is also an encoder and decoder implementations.

• Description
  Each file starts with a two byte magic number (in ascii) that identifies the type of the file it is (PPM, PGM, PBM) and its encoding (ascii/plain or binary/raw). The magic number is a capital P followed by a single-digit number. Below is a explanation table with ascii/binary magic number formats.

Type	Magic Number	Extension	Color
Portable Bitmap	P1/P4	.pbm	0-1(White & Black)
Portable GrayMap	P2/P5	.pgm	0-255(gray scale), variable, black to white range
Portable PixMap	P3/P6	.ppm	16 777 216 (0-255 for each RGB channel), some support for 0-65535 per channel

The ascii/plain format allow for human readability and easy transfer to other platforms. The binary/raw formats are more efficient in size but will be dependent of platforms.

• Encoder usage

import (
    "math/rand"
    "os"
    "github.com/jcbritobr/pnm"
)

func main() {
    image := pnm.NewPGMImage(800, 800, 255, pnm.PGMBinary)
    file, err := os.Create("testdata/synimage.pgm")
    if err != nil {
        panic(err)
    }
    encoder := pnm.NewEncoder(file)

    imgbuf := image.Buffer()

    for i := range image.Buffer() {
        data := rand.Intn(255-0) + 0
        imgbuf[i] = byte(data)
    }

    err = encoder.Encode(image)
    if err != nil {
        panic(err)
    }
}

• Decoder usage

import (
    "fmt"
    "os"

    "github.com/jcbritobr/pnm"
)

func main() {
    var image pnm.PGMImage
    file, err := os.Open("testdata/synimage.pgm")
    if err != nil {
        panic(err)
    }
    decoder := pnm.NewDecoder(file, pnm.PGMBinary)
    err = decoder.Decode(&image)

    if err != nil {
        panic(err)
    }
}

• Using ImageBuffer and color.Model (RGBA)

import (
    "os"
    "github.com/jcbritobr/pnm"
    "github.com/jcbritobr/pnm/buffer"
    "github.com/jcbritobr/pnm/color"
)

func main() {
    var image pnm.PPMImage
    file, err := os.Open("testdata/tree_1.ppm")
    if err != nil {
        t.Errorf("fail with %v", err)
    }
    defer file.Close()

    decoder := pnm.NewDecoder(file, pnm.PPMBinary)
    err = decoder.Decode(&image)
    if err != nil {
        t.Errorf("fail to decode image %v", err)
    }

    imbuf := buffer.NewImageBuffer(image)
    rgba := color.RGBA{}
    newbuf := []byte{}
    for {
        n, _ := imbuf.Read(rgba[:])
        if n <= 0 {
            break
        }
        r, g, b, _ := rgba.RGBA()
        r = 255 - r
        g = 255 - g
        b = 255 - b
        newbuf = append(newbuf, r)
        newbuf = append(newbuf, g)
        newbuf = append(newbuf, b)
    }

    image.SetBuffer(newbuf)

    fe, err := os.Create("testdata/tree_3.ppm")
    if err != nil {
        t.Errorf("fail to create file %v", err)
    }
    defer fe.Close()

    encoder := pnm.NewEncoder(fe)
    err = encoder.Encode(&image)
    if err != nil {
        t.Errorf("Encode() = %v want %v", err, nil)
    }
}