Image::Write(): accept more channel formats when writing to RGB-only images

For 3-channel images, write out the channels, whatever they are.
For 4+, write R, G, B channels only (and error if not present)

src/pbrt/util/image.cpp

@@ -918,46 +918,59 @@ bool Image::Write(std::string name, const ImageMetadata &metadata) const {
     if (HasExtension(name, "exr"))
         return WriteEXR(name, metadata);
 
-    if (NChannels() > 4) {
-        Error("%s: unable to write an %d channel image in this format.", name,
-              NChannels());
+    // This copy will sometimes be wasteful but simplifies logic below...
+    Image outImage = *this;
+
+    switch (NChannels()) {
+    case 1:
+        // all good; onward
+        break;
+    case 2:
+        Error("%s: unable to write a 2 channel image in this format.", name);
         return false;
+    case 3: {
+        ImageChannelDesc desc = outImage.GetChannelDesc({"R", "G", "B"});
+        if (!desc)
+            // Still go for it with 3 channels.
+            Warning("%s: image has 3 channels but they are not R, G, and B. Image may be "
+                    "garbled.",
+                    name);
+        else
+            // Reorder them as RGB.
+            outImage = outImage.SelectChannels(desc);
+        break;
     }
-
-    const Image *image = this;
-    Image rgbImage;
-    if (NChannels() == 4) {
-        ImageChannelDesc desc = GetChannelDesc({"R", "G", "B", "A"});
-        if (desc) {
-            rgbImage = SelectChannels(GetChannelDesc({"R", "G", "B"}));
-            image = &rgbImage;
-        } else {
-            Error("%s: unable to write an 4 channel image that is not RGBA.", name);
+    default: {
+        ImageChannelDesc desc = outImage.GetChannelDesc({"R", "G", "B"});
+        if (!desc) {
+            Error("%s: multi-channel image does not have R, G, and B. Unable to write to "
+                  "this format.",
+                  name);
             return false;
+        } else {
+            std::string err = StringPrintf("%s: ignoring additional channels for this "
+                                           "image format: ",
+                                           name);
+            for (const auto &ch : outImage.ChannelNames())
+                if (ch != "R" && ch != "G" && ch != "B")
+                    err += StringPrintf("\"%s\" ", ch);
+            Error("%s", err);
         }
+
+        outImage = outImage.SelectChannels(desc);
+        break;
     }
-    if (NChannels() == 3 && *metadata.GetColorSpace() != *RGBColorSpace::sRGB)
-        Warning("%s: writing image with non-sRGB color space to a format that "
-                "doesn't store color spaces.",
-                name);
-
-    if (NChannels() == 3) {
-        // Order as RGB
-        ImageChannelDesc desc = GetChannelDesc({"R", "G", "B"});
-        if (!desc)
-            Warning("%s: 3-channels but doesn't have R, G, and B. "
-                    "Image may be garbled.",
-                    name);
-        else {
-            rgbImage = SelectChannels(desc);
-            image = &rgbImage;
+    }
+    CHECK(outImage.NChannels() == 1 || outImage.NChannels() == 3);
+
+    ImageMetadata outMetadata = metadata;
         }
     }
 
     if (HasExtension(name, "pfm"))
-        return image->WritePFM(name, metadata);
+        return outImage.WritePFM(name, outMetadata);
     else if (HasExtension(name, "png"))
-        return image->WritePNG(name, metadata);
+        return outImage.WritePNG(name, outMetadata);
     else {
         Error("%s: no support for writing images with this extension", name);
         return false;