feat(idiff): allow users to specify a directory as the 2nd argument (#4015)

Teach `idiff` to accept a directory as the 2nd argument.

Fixes #4009

## Tests

Functional tests for `idiff` don't currently exist. Manual testing
verifies it works as advertised.

---------

Signed-off-by: David Aguilar <davvid@gmail.com>

Author: davvid

src/doc/idiff.rst

@@ -26,12 +26,15 @@ Using `idiff`
 
 The `idiff` utility is invoked as follows:
 
-    `idiff` [*options*] *image1* *image2*
+    `idiff` [*options*] *input1* *input2|directory*
 
 Where *input1* and *input2* are the names of two image files that should be
 compared.  They may be of any format recognized by OpenImageIO (i.e., for
 which image-reading plugins are available).
 
+When a *directory* is specified instead of *input2* then `idiff` will use
+the same-named file as *input1* in the specified directory.
+
 If the two input images are not the same resolutions, or do not have the
 same number of channels, the comparison will return FAILURE immediately and
 will not attempt to compare the pixels of the two images.  If they are the

src/idiff/idiff.cpp

@@ -43,7 +43,7 @@ getargs(int argc, char* argv[])
     ArgParse ap;
     ap.intro("idiff -- compare two images\n"
              OIIO_INTRO_STRING)
-      .usage("idiff [options] image1 image2")
+      .usage("idiff [options] <image1> <image2 | directory>")
       .add_version(OIIO_VERSION_STRING)
       .print_defaults(true);
 
@@ -169,6 +169,24 @@ print_subimage(ImageBuf& img0, int subimage, int miplevel)
 }
 
 
+// Append the filename from "first" when "second" is a directory.
+// "second" is an output variable and modified in-place.
+inline void
+add_filename_to_directory(const std::string& first, std::string& second)
+{
+    if (Filesystem::is_directory(second)) {
+        char last_byte = second.at(second.size() - 1);
+        if (last_byte != '/' && last_byte != '\\') {
+#if defined(_MSC_VER)
+            second += '\\';
+#else
+            second += '/';
+#endif
+        }
+        second += Filesystem::filename(first);
+    }
+}
+
 
 int
 main(int argc, char* argv[])
@@ -181,7 +199,9 @@ main(int argc, char* argv[])
     ArgParse ap = getargs(argc, argv);
 
     std::vector<std::string> filenames = ap["filename"].as_vec<std::string>();
-    if (filenames.size() != 2) {
+    if (filenames.size() == 2) {
+        add_filename_to_directory(filenames[0], filenames[1]);
+    } else {
         print(stderr, "idiff: Must have two input filenames.\n");
         print(stderr, "> {}\n", Strutil::join(filenames, ", "));
         ap.usage();