Add functions to set and get framerate (#48)

- Mostly taken from #32.

v4l2.go

@@ -3,6 +3,7 @@ package webcam
 import (
 	"bytes"
 	"encoding/binary"
+	"fmt"
 	"unsafe"
 
 	"github.com/blackjack/webcam/ioctl"
@@ -75,6 +76,8 @@ var (
 	VIDIOC_QUERYBUF  = ioctl.IoRW(uintptr('V'), 9, unsafe.Sizeof(v4l2_buffer{}))
 	VIDIOC_QBUF      = ioctl.IoRW(uintptr('V'), 15, unsafe.Sizeof(v4l2_buffer{}))
 	VIDIOC_DQBUF     = ioctl.IoRW(uintptr('V'), 17, unsafe.Sizeof(v4l2_buffer{}))
+	VIDIOC_G_PARM    = ioctl.IoRW(uintptr('V'), 21, unsafe.Sizeof(v4l2_streamparm{}))
+	VIDIOC_S_PARM    = ioctl.IoRW(uintptr('V'), 22, unsafe.Sizeof(v4l2_streamparm{}))
 	VIDIOC_G_CTRL    = ioctl.IoRW(uintptr('V'), 27, unsafe.Sizeof(v4l2_control{}))
 	VIDIOC_S_CTRL    = ioctl.IoRW(uintptr('V'), 28, unsafe.Sizeof(v4l2_control{}))
 	VIDIOC_QUERYCTRL = ioctl.IoRW(uintptr('V'), 36, unsafe.Sizeof(v4l2_queryctrl{}))
@@ -203,6 +206,26 @@ type v4l2_control struct {
 	value int32
 }
 
+type v4l2_fract struct {
+	numerator   uint32
+	denominator uint32
+}
+
+type v4l2_streamparm_union struct {
+	capability     uint32
+	output_mode    uint32
+	time_per_frame v4l2_fract
+	extended_mode  uint32
+	buffers        uint32
+	reserved       [4]uint32
+	data           [200 - (10 * unsafe.Sizeof(uint32(0)))]byte
+}
+
+type v4l2_streamparm struct {
+	_type uint32
+	union v4l2_streamparm_union
+}
+
 func checkCapabilities(fd uintptr) (supportsVideoCapture bool, supportsVideoStreaming bool, err error) {
 
 	caps := &v4l2_capability{}
@@ -466,6 +489,29 @@ func setControl(fd uintptr, id uint32, val int32) error {
 	return ioctl.Ioctl(fd, VIDIOC_S_CTRL, uintptr(unsafe.Pointer(ctrl)))
 }
 
+func getFramerate(fd uintptr) (float32, error) {
+	param := &v4l2_streamparm{}
+	param._type = V4L2_BUF_TYPE_VIDEO_CAPTURE
+
+	err := ioctl.Ioctl(fd, VIDIOC_G_PARM, uintptr(unsafe.Pointer(param)))
+	if err != nil {
+		return 0, err
+	}
+	tf := param.union.time_per_frame
+	if tf.denominator == 0 || tf.numerator == 0 {
+		return 0, fmt.Errorf("Invalid framerate (%d/%d)", tf.denominator, tf.numerator)
+	}
+	return float32(tf.denominator) / float32(tf.numerator), nil
+}
+
+func setFramerate(fd uintptr, num, denom uint32) error {
+	param := &v4l2_streamparm{}
+	param._type = V4L2_BUF_TYPE_VIDEO_CAPTURE
+	param.union.time_per_frame.numerator = num
+	param.union.time_per_frame.denominator = denom
+	return ioctl.Ioctl(fd, VIDIOC_S_PARM, uintptr(unsafe.Pointer(param)))
+}
+
 func queryControls(fd uintptr) []control {
 	controls := []control{}
 	var err error

webcam.go

@@ -5,9 +5,10 @@ package webcam
 
 import (
 	"errors"
-	"golang.org/x/sys/unix"
 	"reflect"
 	"unsafe"
+
+	"golang.org/x/sys/unix"
 )
 
 // Webcam object
@@ -153,6 +154,16 @@ func (w *Webcam) SetControl(id ControlID, value int32) error {
 	return setControl(w.fd, uint32(id), value)
 }
 
+// Get the framerate.
+func (w *Webcam) GetFramerate() (float32, error) {
+	return getFramerate(w.fd)
+}
+
+// Set FPS
+func (w *Webcam) SetFramerate(fps float32) error {
+	return setFramerate(w.fd, 1000, uint32(1000*(fps)))
+}
+
 // Start streaming process
 func (w *Webcam) StartStreaming() error {
 	if w.streaming {