From a418ed9506c23323131871c95ff270ed5e1ffce5 Mon Sep 17 00:00:00 2001
From: Danny Greenstein <danny.gr@gmail.com>
Date: Thu, 14 May 2026 21:42:33 -0700
Subject: [PATCH] oiiotool: Add --nchannels flag for parity with maketx

Signed-off-by: Danny Greenstein <danny.gr@gmail.com>
---
 src/doc/oiiotool.rst                          |   8 ++++
 src/oiiotool/oiiotool.cpp                     |  35 ++++++++++++++++++
 testsuite/oiiotool/ref/out.txt                |  12 ++++++
 .../oiiotool/ref/single_channel_black.tif     | Bin 0 -> 753 bytes
 .../oiiotool/ref/single_channel_good.tif      | Bin 0 -> 3198 bytes
 .../oiiotool/ref/single_channel_good_r.exr    | Bin 0 -> 4601 bytes
 .../oiiotool/ref/single_channel_good_y.exr    | Bin 0 -> 9083 bytes
 testsuite/oiiotool/run.py                     |  22 +++++++++++
 testsuite/oiiotool/src/single_channel_r.exr   | Bin 0 -> 3791 bytes
 testsuite/oiiotool/src/single_channel_tif.tif | Bin 0 -> 3198 bytes
 testsuite/oiiotool/src/single_channel_y.exr   | Bin 0 -> 8624 bytes
 11 files changed, 77 insertions(+)
 create mode 100644 testsuite/oiiotool/ref/single_channel_black.tif
 create mode 100644 testsuite/oiiotool/ref/single_channel_good.tif
 create mode 100644 testsuite/oiiotool/ref/single_channel_good_r.exr
 create mode 100644 testsuite/oiiotool/ref/single_channel_good_y.exr
 create mode 100644 testsuite/oiiotool/src/single_channel_r.exr
 create mode 100644 testsuite/oiiotool/src/single_channel_tif.tif
 create mode 100644 testsuite/oiiotool/src/single_channel_y.exr

diff --git a/src/doc/oiiotool.rst b/src/doc/oiiotool.rst
index d06a21f6b8..ab1289fc72 100644
--- a/src/doc/oiiotool.rst
+++ b/src/doc/oiiotool.rst
@@ -2385,6 +2385,14 @@ current top image.
     name, nor value), then this will just leave the images as-is, without
     any unnecessary expense or pointless copying of images in memory.
 
+.. option:: --nchannels <n>
+
+    Replaces the top image with a new image whose channels are the the first *n*
+    channels of the input image. If *n* is less than the number of
+    channels in the input image, the extra channels will simply be ignored.
+    If *n* is greater than the number of channels in the input image, the
+    additional channels will be filled with 0 values.
+
 .. option:: --chappend
 
     Replaces the top two (or more) images on the stack with a single new
diff --git a/src/oiiotool/oiiotool.cpp b/src/oiiotool/oiiotool.cpp
index c86f3449f5..5c756d5fd1 100644
--- a/src/oiiotool/oiiotool.cpp
+++ b/src/oiiotool/oiiotool.cpp
@@ -2836,6 +2836,38 @@ action_channels(Oiiotool& ot, cspan<const char*> argv)
 
 
 
+// --nchannels
+static void
+action_nchannels(Oiiotool& ot, cspan<const char*> argv)
+{
+    if (ot.postpone_callback(1, action_nchannels, argv))
+        return;
+
+    // Get the requested number of channels
+    int n = Strutil::stoi(ot.express(argv[1]));
+    if (n < 1) {
+        ot.errorfmt(argv[0], "nchannels must be at least 1 (got {})", n);
+        return;
+    }
+
+    // We build the indices into a vector, then join them into a string.
+    std::vector<int> indices;
+    for (int i = 0; i < n; ++i)
+        indices.push_back(i);
+
+    // Convert to ustring to ensure the underlying char* is interned.
+    // This guarantees the pointer remains valid even if action_channels
+    // defers execution (postpones) beyond the scope of this function.
+    ustring channel_list(Strutil::join(indices, ","));
+
+    // Hand off to action_channels with our generated index list.
+    // this ensures we are always doing the same thing as --ch
+    const char* fake_argv[] = { argv[0], channel_list.c_str() };
+    action_channels(ot, fake_argv);
+}
+
+
+
 // --chappend
 static void
 action_chappend(Oiiotool& ot, cspan<const char*> argv)
@@ -7277,6 +7309,9 @@ Oiiotool::getargs(int argc, char* argv[])
     ap.arg("--ch %s:CHANLIST")
       .help("Select or shuffle channels (e.g., \"R,G,B\", \"B,G,R\", \"2,3,4\")")
       .OTACTION(action_channels);
+    ap.arg("--nchannels %d:N")
+      .help("Force the current image to have N channels (padding with 0.0 as needed)")
+      .OTACTION(action_nchannels);
     ap.arg("--chappend")
       .help("Append the channels of the last two images")
       .OTACTION(action_chappend);
diff --git a/testsuite/oiiotool/ref/out.txt b/testsuite/oiiotool/ref/out.txt
index 679adc453e..9b01433faa 100644
--- a/testsuite/oiiotool/ref/out.txt
+++ b/testsuite/oiiotool/ref/out.txt
@@ -62,6 +62,10 @@ oiiotool ERROR: -o : Non-existent output directory: folder1/folder2
 Full command line was:
 > oiiotool --create 2x2 1 -o folder1/folder2/out.tif
 folder1/folder2/out.tif :    2 x    2, 1 channel, float tiff
+WARNING: --ch: Unknown channel name "R", filling with 0 (actual channels: "Y")
+WARNING: --ch: Unknown channel name "R", filling with 0 (actual channels: "Y")
+    channel list: Y
+    channel list: R
 Comparing "filled.tif" and "ref/filled.tif"
 PASS
 Comparing "autotrim.tif" and "ref/autotrim.tif"
@@ -204,3 +208,11 @@ Comparing "box_over_missing2.tif" and "ref/box_over_missing2.tif"
 PASS
 Comparing "box_over_missing3.tif" and "ref/box_over_missing3.tif"
 PASS
+Comparing "single_channel_black.tif" and "ref/single_channel_black.tif"
+PASS
+Comparing "single_channel_good.tif" and "ref/single_channel_good.tif"
+PASS
+Comparing "single_channel_good_y.exr" and "ref/single_channel_good_y.exr"
+PASS
+Comparing "single_channel_good_r.exr" and "ref/single_channel_good_r.exr"
+PASS
diff --git a/testsuite/oiiotool/ref/single_channel_black.tif b/testsuite/oiiotool/ref/single_channel_black.tif
new file mode 100644
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diff --git a/testsuite/oiiotool/ref/single_channel_good.tif b/testsuite/oiiotool/ref/single_channel_good.tif
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diff --git a/testsuite/oiiotool/ref/single_channel_good_y.exr b/testsuite/oiiotool/ref/single_channel_good_y.exr
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diff --git a/testsuite/oiiotool/run.py b/testsuite/oiiotool/run.py
index 6d9b711a70..f182ae19d2 100755
--- a/testsuite/oiiotool/run.py
+++ b/testsuite/oiiotool/run.py
@@ -266,6 +266,24 @@
 command += oiiotool (f"--create-dir --create 2x2 1 -o {root_folder}/folder2/out.tif")
 command += oiiotool (f"--info {root_folder}/folder2/out.tif")
 
+
+# in a tif the single channel doesn't really have a name so oiio gives it the name Y
+# so if you naively do --ch R you'll get a black image (with a warning which we capture in out.txt)
+command += oiiotool ("src/single_channel_tif.tif --ch R -o single_channel_black.tif")
+# using nchannels 1 will always do the right thing
+command += oiiotool ("src/single_channel_tif.tif --nchannels 1 -o single_channel_good.tif")
+# exrs have explicit channel names, but you don't need to know the name if you are using nchannels
+# so you can easily process a mix of frames without thinking about the name of the channels
+# this can also be accomplished with --ch 0, but --nchannels 1 is a bit more intuitive
+# and matches maketx
+command += oiiotool ("src/single_channel_y.exr --nchannels 1 -o single_channel_good_y.exr")
+command += oiiotool ("src/single_channel_r.exr --nchannels 1 -o single_channel_good_r.exr")
+
+# the info -v will print out the channel names and make sure they came out good in the output
+command += oiiotool ("--info -v single_channel_good_y.exr | grep 'channel list'")
+command += oiiotool ("--info -v single_channel_good_r.exr | grep 'channel list'")
+
+
 # To add more tests, just append more lines like the above and also add
 # the new 'feature.tif' (or whatever you call it) to the outputs list,
 # below.
@@ -312,6 +330,10 @@
             "const5-rgb.tif",
             "box_over_missing2.tif",
             "box_over_missing3.tif",
+            "single_channel_black.tif",
+            "single_channel_good.tif",
+            "single_channel_good_y.exr",
+            "single_channel_good_r.exr",
             "out.txt" ]
 
 #print "Running this command:\n" + command + "\n"
diff --git a/testsuite/oiiotool/src/single_channel_r.exr b/testsuite/oiiotool/src/single_channel_r.exr
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