fix(oiiotool): -i:ch=... didn't fix up alpha and z channels

src/libOpenImageIO/imagebufalgo_channels.cpp

@@ -76,9 +76,9 @@ ImageBufAlgo::channels(ImageBuf& dst, const ImageBuf& src, int nchannels,
         return false;
     }
 
-    // If channelorder is NULL, it will be interpreted as
+    // If channelorder is empty, it will be interpreted as
     // {0, 1, ..., nchannels-1}.
-    int* local_channelorder = NULL;
+    int* local_channelorder = nullptr;
     if (channelorder.empty()) {
         local_channelorder = OIIO_ALLOCA(int, nchannels);
         for (int c = 0; c < nchannels; ++c)

src/oiiotool/imagerec.cpp

@@ -293,6 +293,8 @@ ImageRec::read(ReadPolicy readpolicy, string_view channel_set)
             std::vector<std::string> newchannelnames;
             std::vector<int> channel_set_channels;
             std::vector<float> channel_set_values;
+            int new_alpha_channel = -1;
+            int new_z_channel     = -1;
             int chbegin = 0, chend = -1;
             if (channel_set.size()) {
                 decode_channel_set(ib->nativespec(), channel_set,
@@ -306,6 +308,10 @@ ImageRec::read(ReadPolicy readpolicy, string_view channel_set)
                              && channel_set_channels[c]
                                     != channel_set_channels[c - 1] + 1)
                         post_channel_set_action = true;  // non-consecutive chans
+                    if (channel_set_channels[c] == ib->spec().alpha_channel)
+                        new_alpha_channel = c;
+                    if (channel_set_channels[c] == ib->spec().z_channel)
+                        new_z_channel = c;
                 }
                 if (ib->deep())
                     post_channel_set_action = true;
@@ -349,6 +355,11 @@ ImageRec::read(ReadPolicy readpolicy, string_view channel_set)
             }
             if (!ok)
                 errorfmt("{}", ib->geterror());
+            if (channel_set.size()) {
+                // Adjust the spec to reflect the new channel set
+                ib->specmod().alpha_channel = new_alpha_channel;
+                ib->specmod().z_channel     = new_z_channel;
+            }
 
             allok &= ok;
             // Remove any existing SHA-1 hash from the spec.

src/tiff.imageio/tiffoutput.cpp

@@ -801,21 +801,24 @@ TIFFOutput::open(const std::string& name, const ImageSpec& userspec,
         TIFFSetField(m_tif, TIFFTAG_PREDICTOR, m_predictor);
 
     // ExtraSamples tag
-    if ((m_spec.alpha_channel >= 0 || m_spec.nchannels > 3)
+    if (((m_spec.alpha_channel >= 0 && m_spec.alpha_channel < m_spec.nchannels)
+         || m_spec.nchannels > 3)
         && m_photometric != PHOTOMETRIC_SEPARATED
         && m_spec.get_int_attribute("tiff:write_extrasamples", 1)) {
         bool unass = m_spec.get_int_attribute("oiio:UnassociatedAlpha", 0);
         int defaultchans = m_spec.nchannels >= 3 ? 3 : 1;
         short e          = m_spec.nchannels - defaultchans;
-        std::vector<unsigned short> extra(e);
-        for (int c = 0; c < e; ++c) {
-            if (m_spec.alpha_channel == (c + defaultchans))
-                extra[c] = unass ? EXTRASAMPLE_UNASSALPHA
-                                 : EXTRASAMPLE_ASSOCALPHA;
-            else
-                extra[c] = EXTRASAMPLE_UNSPECIFIED;
+        if (e > 0) {
+            std::vector<unsigned short> extra(e);
+            for (int c = 0; c < e; ++c) {
+                if (m_spec.alpha_channel == (c + defaultchans))
+                    extra[c] = unass ? EXTRASAMPLE_UNASSALPHA
+                                     : EXTRASAMPLE_ASSOCALPHA;
+                else
+                    extra[c] = EXTRASAMPLE_UNSPECIFIED;
+            }
+            TIFFSetField(m_tif, TIFFTAG_EXTRASAMPLES, e, extra.data());
         }
-        TIFFSetField(m_tif, TIFFTAG_EXTRASAMPLES, e, &extra[0]);
     }
 
     ParamValue* param;