Improve multi-frame image resizing (#2789)

* Ports vips_thumbnail logic to sharp 
* Deprecates the pageHeight output option for WebP/GIF

src/operations.cc

@@ -308,4 +308,98 @@ namespace sharp {
     return image;
   }
 
+  /*
+   * Split and crop each frame, reassemble, and update pageHeight.
+   */
+  VImage CropMultiPage(VImage image, int left, int top, int width, int height,
+                       int nPages, int *pageHeight) {
+    if (top == 0 && height == *pageHeight) {
+      // Fast path; no need to adjust the height of the multi-page image
+      return image.extract_area(left, 0, width, image.height());
+    } else {
+      std::vector<VImage> pages;
+      pages.reserve(nPages);
+
+      // Split the image into cropped frames
+      for (int i = 0; i < nPages; i++) {
+        pages.push_back(
+          image.extract_area(left, *pageHeight * i + top, width, height));
+      }
+
+      // Reassemble the frames into a tall, thin image
+      VImage assembled = VImage::arrayjoin(pages,
+        VImage::option()->set("across", 1));
+
+      // Update the page height
+      *pageHeight = height;
+
+      return assembled;
+    }
+  }
+
+  /*
+   * Split into frames, embed each frame, reassemble, and update pageHeight.
+   */
+  VImage EmbedMultiPage(VImage image, int left, int top, int width, int height,
+                        std::vector<double> background, int nPages, int *pageHeight) {
+    if (top == 0 && height == *pageHeight) {
+      // Fast path; no need to adjust the height of the multi-page image
+      return image.embed(left, 0, width, image.height(), VImage::option()
+        ->set("extend", VIPS_EXTEND_BACKGROUND)
+        ->set("background", background));
+    } else if (left == 0 && width == image.width()) {
+      // Fast path; no need to adjust the width of the multi-page image
+      std::vector<VImage> pages;
+      pages.reserve(nPages);
+
+      // Rearrange the tall image into a vertical grid
+      image = image.grid(*pageHeight, nPages, 1);
+
+      // Do the embed on the wide image
+      image = image.embed(0, top, image.width(), height, VImage::option()
+        ->set("extend", VIPS_EXTEND_BACKGROUND)
+        ->set("background", background));
+
+      // Split the wide image into frames
+      for (int i = 0; i < nPages; i++) {
+        pages.push_back(
+          image.extract_area(width * i, 0, width, height));
+      }
+
+      // Reassemble the frames into a tall, thin image
+      VImage assembled = VImage::arrayjoin(pages,
+        VImage::option()->set("across", 1));
+
+      // Update the page height
+      *pageHeight = height;
+
+      return assembled;
+    } else {
+      std::vector<VImage> pages;
+      pages.reserve(nPages);
+
+      // Split the image into frames
+      for (int i = 0; i < nPages; i++) {
+        pages.push_back(
+          image.extract_area(0, *pageHeight * i, image.width(), *pageHeight));
+      }
+
+      // Embed each frame in the target size
+      for (int i = 0; i < nPages; i++) {
+        pages[i] = pages[i].embed(left, top, width, height, VImage::option()
+          ->set("extend", VIPS_EXTEND_BACKGROUND)
+          ->set("background", background));
+      }
+
+      // Reassemble the frames into a tall, thin image
+      VImage assembled = VImage::arrayjoin(pages,
+        VImage::option()->set("across", 1));
+
+      // Update the page height
+      *pageHeight = height;
+
+      return assembled;
+    }
+  }
+
 }  // namespace sharp