add alpha plane output to JS wrapper

libheif/heif_emscripten.h

@@ -162,6 +162,18 @@ static emscripten::val heif_js_decode_image(struct heif_image_handle* handle,
  break;
  }
  result.set("data", std::move(data));
+
+ if (heif_image_has_channel(image, heif_channel_Alpha)) {
+ std::basic_string<unsigned char> alpha;
+ int stride_alpha;
+ const uint8_t* plane_alpha = heif_image_get_plane_readonly(image,
+ heif_channel_Alpha, &stride_alpha);
+ alpha.resize(width * height);
+ unsigned char* dest = const_cast<unsigned char*>(alpha.data());
+ strided_copy(dest, plane_alpha, width, height, stride_alpha);
+ result.set("alpha", std::move(alpha));
+ }
+
  heif_image_release(image);
  return result;
 }

post.js

@@ -34,6 +34,11 @@ HeifImage.prototype._ensureImage = function() {
  this.data = new Uint8Array(StringToArrayBuffer(img.data));
  delete img.data;
  this.img = img;
+
+ if (img.alpha !== undefined) {
+ this.alpha = new Uint8Array(StringToArrayBuffer(img.alpha));
+ delete img.alpha;
+ }
 };
 
 HeifImage.prototype.get_width = function() {
@@ -54,12 +59,30 @@ HeifImage.prototype.display = function(image_data, callback) {
  var h = this.get_height();
 
  setTimeout(function() {
- this._ensureImage();
- if (!this.img) {
- // Decoding failed.
- callback(null);
- return;
- }
+
+ // If image hasn't been loaded yet, decode the image
+
+ if (!this.img) {
+ var img = libheif.heif_js_decode_image(this.handle,
+ libheif.heif_colorspace_YCbCr, libheif.heif_chroma_420);
+ if (!img || img.code) {
+ console.log("Decoding image failed", this.handle, img);
+
+ callback(null);
+ return;
+ }
+
+ this.data = new Uint8Array(StringToArrayBuffer(img.data));
+ delete img.data;
+ this.img = img;
+ }
+
+ if (img.alpha !== undefined) {
+ this.alpha = new Uint8Array(StringToArrayBuffer(img.alpha));
+ delete img.alpha;
+ }
+
+ const has_alpha = (this.alpha !== undefined);
 
  var yval;
  var uval;
@@ -80,6 +103,7 @@ HeifImage.prototype.display = function(image_data, callback) {
  var u = this.data.subarray(stridey * h, stridey * h + (strideu * h2));
  var v = this.data.subarray(stridey * h + (strideu * h2), stridey * h + (strideu * h2) + (stridev * h2));
  var dest = image_data.data;
+
  while (i < maxi) {
  x2 = (xpos >> 1);
  yval = 1.164 * (y[yoffset + xpos] - 16);
@@ -89,7 +113,7 @@ HeifImage.prototype.display = function(image_data, callback) {
  dest[(i<<2)+0] = yval + 1.596 * vval;
  dest[(i<<2)+1] = yval - 0.813 * vval - 0.391 * uval;
  dest[(i<<2)+2] = yval + 2.018 * uval;
- dest[(i<<2)+3] = 0xff;
+ dest[(i<<2)+3] = has_alpha ? this.alpha[yoffset + xpos] : 0xff;
  i++;
  xpos++;
 
@@ -98,7 +122,7 @@ HeifImage.prototype.display = function(image_data, callback) {
  dest[(i<<2)+0] = yval + 1.596 * vval;
  dest[(i<<2)+1] = yval - 0.813 * vval - 0.391 * uval;
  dest[(i<<2)+2] = yval + 2.018 * uval;
- dest[(i<<2)+3] = 0xff;
+ dest[(i<<2)+3] = has_alpha ? this.alpha[yoffset + xpos] : 0xff;
  i++;
  xpos++;
  }