Add support for extracting EXIF data from PNG files.

Closes evanoberholster#41.

exif2/reader.go

@@ -33,7 +33,7 @@ func Decode(r io.ReadSeeker) (Exif, error) {
 	return ir.Exif, nil
 }
 
-func (ir *ifdReader) DecodeTiff(r io.Reader, h meta.ExifHeader) error {
+func (ir *ifdReader) DecodeTiff(_ io.Reader, h meta.ExifHeader) error {
 	ir.buffer.clear()
 	// Log Header Info
 	if ir.logInfo() {

png.go

@@ -0,0 +1,25 @@
+package imagemeta
+
+import (
+	"io"
+
+	"github.com/evanoberholster/imagemeta/exif2"
+	"github.com/evanoberholster/imagemeta/png"
+)
+
+// DecodePng decodes a PNG file from an io.Reader returning Exif or an error.
+func DecodePng(r io.ReadSeeker) (exif2.Exif, error) {
+	header, err := png.ScanPngHeader(r)
+	if err != nil {
+		return exif2.Exif{}, err
+	}
+
+	ir := exif2.NewIfdReader(r)
+	defer ir.Close()
+
+	if err := ir.DecodeTiff(nil, header); err != nil {
+		return ir.Exif, err
+	}
+
+	return ir.Exif, nil
+}

png/png.go

@@ -0,0 +1,66 @@
+// Package png reads PNG Header metadata information from image files before being processed by exif package
+package png
+
+import (
+	"encoding/binary"
+	"io"
+
+	"github.com/evanoberholster/imagemeta/imagetype"
+	"github.com/evanoberholster/imagemeta/meta"
+)
+
+func ScanPngHeader(r io.ReadSeeker) (header meta.ExifHeader, err error) {
+	// 5.2 PNG signature
+	const signature = "\x89PNG\r\n\x1a\n"
+
+	// 5.3 Chunk layout
+	const crcSize = 4
+
+	// 8 is the size of both the signature and the chunk
+	// id (4 bytes) + chunk length (4 bytes).
+	// This is just a coincidence.
+	buf := make([]byte, 8)
+
+	var n int
+	n, err = r.Read(buf)
+	if err != nil {
+		return
+	}
+
+	if n != len(signature) || string(buf) != signature {
+		err = meta.ErrNoExif
+
+		return
+	}
+
+	for {
+		// 5.3 Chunk layout
+		n, err = r.Read(buf)
+		if err != nil {
+			break
+		}
+
+		if n != len(buf) {
+			break
+		}
+
+		length := binary.BigEndian.Uint32(buf[0:4])
+		chunkType := string(buf[4:8])
+
+		switch chunkType {
+		case "eXIf":
+			offset, _ := r.Seek(0, io.SeekCurrent)
+
+			return meta.NewExifHeader(meta.BigEndian, 8, uint32(offset), length, imagetype.ImagePNG), nil
+
+		default:
+			// Discard the chunk length + CRC.
+			_, err := r.Seek(int64(length+crcSize), io.SeekCurrent)
+			if err != nil {
+				return header, err
+			}
+		}
+	}
+
+	return header, meta.ErrNoExif
+}