Implement decoder

codecs/mozjpeg/dec/mozjpeg_dec.cpp

@@ -0,0 +1,74 @@
+#include <emscripten/bind.h>
+#include <emscripten/val.h>
+#include <inttypes.h>
+#include <setjmp.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include "config.h"
+#include "jpeglib.h"
+
+extern "C" {
+#include "cdjpeg.h"
+}
+
+using namespace emscripten;
+
+thread_local const val Uint8ClampedArray = val::global("Uint8ClampedArray");
+thread_local const val ImageData = val::global("ImageData");
+
+val decode(std::string image_in) {
+  uint8_t* image_buffer = (uint8_t*)image_in.c_str();
+
+  struct jpeg_decompress_struct cinfo;
+  struct jpeg_error_mgr jerr;
+  /* Initialize the JPEG decompression object with default error handling. */
+  cinfo.err = jpeg_std_error(&jerr);
+  jpeg_create_decompress(&cinfo);
+
+  jpeg_mem_src(&cinfo, image_buffer, image_in.length());
+  /* Read file header, set default decompression parameters */
+  jpeg_read_header(&cinfo, TRUE);
+  // Force RGB decoding, even for grayscale images
+  cinfo.out_color_space = JCS_RGB;
+  jpeg_start_decompress(&cinfo);
+
+  // Prepare output buffer
+  size_t output_size = cinfo.output_width * cinfo.output_height * 4;
+  auto output_buffer = new uint8_t[output_size];
+  uint8_t* row_ptr = &output_buffer[0];
+  auto stride = cinfo.output_width * 4;
+
+  /* Process data */
+  while (cinfo.output_scanline < cinfo.output_height) {
+    jpeg_read_scanlines(&cinfo, &row_ptr, 1);
+    // Turn that row from RGB into RGBA. I’m starting at the end so
+    // I can do it in-place.
+    for (int32_t x = cinfo.output_width - 1; x >= 0; x--) {
+      // RGB channels
+      for (auto i = 0; i < 3; i++) {
+        row_ptr[x * 4 + i] = row_ptr[x * 3 + i];
+      }
+      // Alpha channel
+      row_ptr[x * 4 + 3] = 255;
+    }
+    row_ptr += stride;
+  }
+  jpeg_finish_decompress(&cinfo);
+
+  /* Step 7: release JPEG compression object */
+
+  auto data = Uint8ClampedArray.new_(typed_memory_view(output_size, output_buffer));
+  auto js_result = ImageData.new_(data, cinfo.output_width, cinfo.output_height);
+
+  /* This is an important step since it will release a good deal of memory. */
+  jpeg_destroy_decompress(&cinfo);
+  delete[] output_buffer;
+
+  /* And we're done! */
+  return js_result;
+}
+
+EMSCRIPTEN_BINDINGS(my_module) {
+  function("decode", &decode);
+}