T90908: Reduce loading times when extracting thumbnails from Blendfiles.

Previously when loading an thumbnails for an asset the whole file was
read. Reason this was done was perhaps a future idea to load
all thumbnails inside a blendfile in a single go. This was never implemented
and currently unneeded disk and cpu cycles was spend with finding out what
preview to load.

This patch adds an early break when the thumbnail that the caller is
interested in has been found. This improves the thumbnail extraction
when looking into large files.

Reviewed By: mont29

Maniphest Tasks: T90908

Differential Revision: https://developer.blender.org/D12312

1 files changed, 91 insertions, 27 deletions

diff --git a/source/blender/blenloader/intern/readblenentry.c b/source/blender/blenloader/intern/readblenentry.c
index 44a26b9bf85..f67ff0f7ac7 100644
--- a/source/blender/blenloader/intern/readblenentry.c
+++ b/source/blender/blenloader/intern/readblenentry.c
@@ -217,6 +217,96 @@ LinkNode *BLO_blendhandle_get_datablock_info(BlendHandle *bh,
 }
 
 /**
+ * Read the preview rects and store in `result`.
+ *
+ * `bhead` should point to the block that sourced the `preview_from_file`
+ *     parameter.
+ * `bhead` parameter is consumed. The correct bhead pointing to the next bhead in the file after
+ * the preview rects is returned by this function.
+ * \param fd: The filedata to read the data from.
+ * \param bhead: should point to the block that sourced the `preview_from_file parameter`.
+ *               bhead is consumed. the new bhead is returned by this function.
+ * \param result: the Preview Image where the preview rect will be stored.
+ * \param preview_from_file: The read PreviewImage where the bhead points to. The rects of this
+ * \return PreviewImage or NULL when no preview Images have been found. Caller owns the returned
+ */
+static BHead *blo_blendhandle_read_preview_rects(FileData *fd,
+                                                 BHead *bhead,
+                                                 PreviewImage *result,
+                                                 const PreviewImage *preview_from_file)
+{
+  for (int preview_index = 0; preview_index < NUM_ICON_SIZES; preview_index++) {
+    if (preview_from_file->rect[preview_index] && preview_from_file->w[preview_index] &&
+        preview_from_file->h[preview_index]) {
+      bhead = blo_bhead_next(fd, bhead);
+      BLI_assert((preview_from_file->w[preview_index] * preview_from_file->h[preview_index] *
+                  sizeof(uint)) == bhead->len);
+      result->rect[preview_index] = BLO_library_read_struct(fd, bhead, "PreviewImage Icon Rect");
+    }
+    else {
+      /* This should not be needed, but can happen in 'broken' .blend files,
+       * better handle this gracefully than crashing. */
+      BLI_assert(preview_from_file->rect[preview_index] == NULL &&
+                 preview_from_file->w[preview_index] == 0 &&
+                 preview_from_file->h[preview_index] == 0);
+      result->rect[preview_index] = NULL;
+      result->w[preview_index] = result->h[preview_index] = 0;
+    }
+    BKE_previewimg_finish(result, preview_index);
+  }
+
+  return bhead;
+}
+
+/**
+ * Get the PreviewImage of a single data block in a file.
+ * (e.g. all the scene previews in a file).
+ *
+ * \param bh: The blendhandle to access.
+ * \param ofblocktype: The type of names to get.
+ * \param name: Name of the block without the ID_ prefix, to read the preview image from.
+ * \return PreviewImage or NULL when no preview Images have been found. Caller owns the returned
+ */
+PreviewImage *BLO_blendhandle_get_preview_for_id(BlendHandle *bh,
+                                                 int ofblocktype,
+                                                 const char *name)
+{
+  FileData *fd = (FileData *)bh;
+  bool looking = false;
+  const int sdna_preview_image = DNA_struct_find_nr(fd->filesdna, "PreviewImage");
+
+  for (BHead *bhead = blo_bhead_first(fd); bhead; bhead = blo_bhead_next(fd, bhead)) {
+    if (bhead->code == DATA) {
+      if (looking && bhead->SDNAnr == sdna_preview_image) {
+        PreviewImage *preview_from_file = BLO_library_read_struct(fd, bhead, "PreviewImage");
+
+        if (preview_from_file == NULL) {
+          break;
+        }
+
+        PreviewImage *result = MEM_dupallocN(preview_from_file);
+        bhead = blo_blendhandle_read_preview_rects(fd, bhead, result, preview_from_file);
+        MEM_freeN(preview_from_file);
+        return result;
+      }
+    }
+    else if (looking || bhead->code == ENDB) {
+      /* We were looking for a preview image, but didn't find any belonging to block. So it doesn't
+       * exist. */
+      break;
+    }
+    else if (bhead->code == ofblocktype) {
+      const char *idname = blo_bhead_id_name(fd, bhead);
+      if (STREQ(&idname[2], name)) {
+        looking = true;
+      }
+    }
+  }
+
+  return NULL;
+}
+
+/**
  * Gets the previews of all the data-blocks in a file of a certain type
  * (e.g. all the scene previews in a file).
  *
@@ -264,33 +354,7 @@ LinkNode *BLO_blendhandle_get_previews(BlendHandle *bh, int ofblocktype, int *r_
 
           if (prv) {
             memcpy(new_prv, prv, sizeof(PreviewImage));
-            if (prv->rect[0] && prv->w[0] && prv->h[0]) {
-              bhead = blo_bhead_next(fd, bhead);
-              BLI_assert((new_prv->w[0] * new_prv->h[0] * sizeof(uint)) == bhead->len);
-              new_prv->rect[0] = BLO_library_read_struct(fd, bhead, "PreviewImage Icon Rect");
-            }
-            else {
-              /* This should not be needed, but can happen in 'broken' .blend files,
-               * better handle this gracefully than crashing. */
-              BLI_assert(prv->rect[0] == NULL && prv->w[0] == 0 && prv->h[0] == 0);
-              new_prv->rect[0] = NULL;
-              new_prv->w[0] = new_prv->h[0] = 0;
-            }
-            BKE_previewimg_finish(new_prv, 0);
-
-            if (prv->rect[1] && prv->w[1] && prv->h[1]) {
-              bhead = blo_bhead_next(fd, bhead);
-              BLI_assert((new_prv->w[1] * new_prv->h[1] * sizeof(uint)) == bhead->len);
-              new_prv->rect[1] = BLO_library_read_struct(fd, bhead, "PreviewImage Image Rect");
-            }
-            else {
-              /* This should not be needed, but can happen in 'broken' .blend files,
-               * better handle this gracefully than crashing. */
-              BLI_assert(prv->rect[1] == NULL && prv->w[1] == 0 && prv->h[1] == 0);
-              new_prv->rect[1] = NULL;
-              new_prv->w[1] = new_prv->h[1] = 0;
-            }
-            BKE_previewimg_finish(new_prv, 1);
+            bhead = blo_blendhandle_read_preview_rects(fd, bhead, new_prv, prv);
             MEM_freeN(prv);
           }
         }