Add support for creating an animated WebP

This initial implementation doesn't fully expose the `WebPAnimEncoderOptions`.
It also still needs some unit tests and examples.

webp/anim_encode.go

@@ -0,0 +1,121 @@
+package webp
+
+/*
+#include <stdlib.h>
+#include <string.h>
+#include <webp/encode.h>
+#include <webp/mux.h>
+
+int writeWebP(uint8_t*, size_t, struct WebPPicture*);
+
+static WebPPicture *calloc_WebPPicture(void) {
+	return calloc(sizeof(WebPPicture), 1);
+}
+
+static void free_WebPPicture(WebPPicture* webpPicture) {
+	free(webpPicture);
+}
+*/
+import "C"
+
+import (
+	"errors"
+	"fmt"
+	"time"
+	"image"
+	"unsafe"
+)
+
+// AnimationEncoder encodes multiple images into an animated WebP.
+type AnimationEncoder struct {
+	opts C.WebPAnimEncoderOptions
+	c *C.WebPAnimEncoder
+	duration time.Duration
+}
+
+// NewAnimationEncoder initializes a new encoder.
+func NewAnimationEncoder(width, height, kmin, kmax int) (*AnimationEncoder, error) {
+	ae := &AnimationEncoder{}
+
+	if C.WebPAnimEncoderOptionsInit(&ae.opts) == 0 {
+		return nil, errors.New("failed to initialize animation encoder config")
+	}
+	ae.opts.kmin = C.int(kmin)
+	ae.opts.kmax = C.int(kmax)
+
+	ae.c = C.WebPAnimEncoderNew(C.int(width), C.int(height), &ae.opts)
+	if ae.c == nil {
+		return nil, errors.New("failed to initialize animation encoder")
+	}
+
+	return ae, nil
+}
+
+// AddFrame adds a frame to the encoder.
+func (ae *AnimationEncoder) AddFrame(img image.Image, duration time.Duration) error {
+	pic := C.calloc_WebPPicture()
+	if pic == nil {
+		return errors.New("Could not allocate webp picture")
+	}
+	defer C.free_WebPPicture(pic)
+
+	if C.WebPPictureInit(pic) == 0 {
+		return errors.New("Could not initialize webp picture")
+	}
+	defer C.WebPPictureFree(pic)
+
+	pic.use_argb = 1
+
+	pic.width = C.int(img.Bounds().Dx())
+	pic.height = C.int(img.Bounds().Dy())
+
+	pic.writer = C.WebPWriterFunction(C.writeWebP)
+
+	switch p := img.(type) {
+	case *RGBImage:
+		C.WebPPictureImportRGB(pic, (*C.uint8_t)(&p.Pix[0]), C.int(p.Stride))
+	case *image.RGBA:
+		C.WebPPictureImportRGBA(pic, (*C.uint8_t)(&p.Pix[0]), C.int(p.Stride))
+	case *image.NRGBA:
+		C.WebPPictureImportRGBA(pic, (*C.uint8_t)(&p.Pix[0]), C.int(p.Stride))
+	default:
+		return errors.New("unsupported image type")
+	}
+
+	timestamp := C.int((duration + ae.duration) / time.Millisecond)
+	if C.WebPAnimEncoderAdd(ae.c, pic, timestamp, nil) == 0 {
+		return fmt.Errorf(
+			"Encoding error: %d - %s",
+			int(pic.error_code),
+			C.GoString(C.WebPAnimEncoderGetError(ae.c)),
+		)
+	}
+
+	ae.duration += duration
+	return nil
+}
+
+// Assemble assembles all frames into animated WebP.
+func (ae *AnimationEncoder) Assemble() ([]byte, error) {
+	// add final empty frame
+	if C.WebPAnimEncoderAdd(ae.c, nil, C.int(ae.duration / time.Millisecond), nil) == 0 {
+		return nil, errors.New("Couldn't add final empty frame")
+	}
+
+	data := &C.WebPData{}
+	C.WebPDataInit(data)
+
+	if C.WebPAnimEncoderAssemble(ae.c, data) == 0 {
+		return nil, errors.New("Error assembling animation")
+	}
+
+	return C.GoBytes(
+		unsafe.Pointer(data.bytes),
+		C.int(int(data.size)),
+	), nil
+}
+
+// Close deletes the encoder and frees resources.
+func (ae *AnimationEncoder) Close() {
+	C.WebPAnimEncoderDelete(ae.c)
+}