Updating SubjectOnDisk to be able to read frames at a stride

dart/biomechanics/SubjectOnDisk.cpp

@@ -422,18 +422,22 @@ std::string SubjectOnDisk::readRawOsimFileText()
 ///
 /// On OOB access, prints an error and returns an empty vector.
 std::vector<std::shared_ptr<Frame>> SubjectOnDisk::readFrames(
-    int trial, int startFrame, int numFramesToRead, s_t contactThreshold)
+    int trial,
+    int startFrame,
+    int numFramesToRead,
+    int stride,
+    s_t contactThreshold)
 {
   (void)trial;
   (void)startFrame;
+  (void)stride;
   (void)numFramesToRead;
 
   std::vector<std::shared_ptr<Frame>> result;
 
   // 1. Open the file
   FILE* file = fopen(mPath.c_str(), "r");
 
-  // 2. Seek to the right place in the file to read this frame
   int linearFrameStart = 0;
   for (int i = 0; i < trial; i++)
   {
@@ -454,11 +458,13 @@ std::vector<std::shared_ptr<Frame>> SubjectOnDisk::readFrames(
     return result;
   }
 
-  int offsetBytes = mDataSectionStart + (mFrameSize * linearFrameStart);
-  fseek(file, offsetBytes, SEEK_SET);
-
   for (int i = 0; i < numFramesToRead; i++)
   {
+    // 2. Seek to the right place in the file to read this frame
+    int offsetBytes = mDataSectionStart + (mFrameSize * linearFrameStart)
+                      + (i * stride * mFrameSize);
+    fseek(file, offsetBytes, SEEK_SET);
+
     // 3. Allocate a buffer to hold the serialized data
     std::vector<char> serializedFrame(mFrameSize);
 

dart/biomechanics/SubjectOnDisk.hpp

@@ -95,6 +95,7 @@ class SubjectOnDisk
       int trial,
       int startFrame,
       int numFramesToRead = 1,
+      int stride = 1,
       s_t contactThreshold = 1.0);
 
   /// This writes a subject out to disk in a compressed and random-seekable

python/_nimblephysics/biomechanics/SubjectOnDisk.cpp

@@ -255,6 +255,7 @@ Note that these are specified in the local body frame, acting on the body at its
                 ::py::arg("trial"),
                 ::py::arg("startFrame"),
                 ::py::arg("numFramesToRead") = 1,
+                ::py::arg("stride") = 1,
                 ::py::arg("contactThreshold") = 1.0,
                 "This will read from disk and allocate a number of "
                 ":code:`Frame` "

stubs/_nimblephysics-stubs/biomechanics/__init__.pyi

@@ -2048,7 +2048,7 @@ class SubjectOnDisk():
         """
         This returns the timestep size for the trial requested, in seconds per frame
         """
-    def readFrames(self, trial: int, startFrame: int, numFramesToRead: int = 1, contactThreshold: float = 1.0) -> typing.List[Frame]: 
+    def readFrames(self, trial: int, startFrame: int, numFramesToRead: int = 1, stride: int = 1, contactThreshold: float = 1.0) -> typing.List[Frame]: 
         """
         This will read from disk and allocate a number of :code:`Frame` objects. These Frame objects are assumed to be short-lived, to save working memory. For example, you might :code:`readFrames()` to construct a training batch, then immediately allow the frames to go out of scope and be released after the batch backpropagates gradient and loss. On OOB access, prints an error and returns an empty vector.
         """

unittests/unit/test_SubjectOnDisk.cpp

@@ -686,4 +686,24 @@ TEST(SubjectOnDisk, HAMNER_RUNNING)
   EXPECT_EQ(subject.getNumTrials(), 4);
   EXPECT_GT(subject.readFrames(0, 7, 10).size(), 0);
 }
+#endif
+
+#ifdef ALL_TESTS
+TEST(SubjectOnDisk, HAMNER_RUNNING_READ_WITH_DATA_STRIDE)
+{
+  auto retriever = std::make_shared<utils::CompositeResourceRetriever>();
+  retriever->addSchemaRetriever(
+      "file", std::make_shared<common::LocalResourceRetriever>());
+  retriever->addSchemaRetriever("dart", DartResourceRetriever::create());
+  std::string path = retriever->getFilePath(
+      "dart://sample/subjectOnDisk/HamnerRunning2013Subject01.bin");
+
+  SubjectOnDisk subject(path);
+  EXPECT_EQ(subject.getNumTrials(), 4);
+  auto frames = subject.readFrames(0, 7, 10, 5);
+  EXPECT_GT(frames.size(), 2);
+  EXPECT_EQ(frames[0]->t, 7);
+  EXPECT_EQ(frames[1]->t, 7 + 5);
+  EXPECT_EQ(frames[2]->t, 7 + 10);
+}
 #endif