fix(png): We were not correctly suppressing hint metadata (#4983)

Each file writer that is capable of writing arbitrary metadata is
supposed to tace care to suppress hints meant for other writers, or for
OIIO in general, not interpret them as literal metadata to write to the
file. PNG was not doing so, and ended up writing hints as literal
metadata. This brings it in line with our behavior when writing OpenEXR
and other formats.

---------

Signed-off-by: Larry Gritz <[email protected]>

Author: lgritz

src/png.imageio/png_pvt.h

@@ -564,12 +564,30 @@ put_parameter(png_structp& sp, png_infop& ip, const std::string& _name,
         return true;
     }
 #endif
+
+    // Before handling general named metadata, suppress format-specific
+    // metadata hints meant for other formats that are not meant to be literal
+    // metadata written to the file. This includes anything with a namespace
+    // prefix of "oiio:" or the name of any other file format.
+    auto colonpos = name.find(':');
+    if (colonpos != std::string::npos) {
+        std::string prefix = Strutil::lower(name.substr(0, colonpos));
+        if (prefix != "png" && is_imageio_format_name(prefix))
+            return false;
+        if (prefix == "oiio")
+            return false;
+    }
+
     if (type == TypeDesc::STRING) {
+        // We can save arbitrary string metadata in multiple png text entries.
+        // Is that ok? Should we also do it for other types by converting to
+        // string?
         png_text t;
         t.compression = PNG_TEXT_COMPRESSION_NONE;
         t.key         = (char*)ustring(name).c_str();
         t.text        = *(char**)data;  // Already uniquified
         text.push_back(t);
+        return true;
     }
 
     return false;

testsuite/png-damaged/ref/out.txt

@@ -4,10 +4,10 @@ iconvert ERROR copying "../oiio-images/png/broken/invalid_gray_alpha_sbit.png" t
 PNG read error: IDAT: Read error: hit end of file in png reader
 libpng error: No IDATs written into file
 Comparing "../oiio-images/png/broken/invalid_gray_alpha_sbit.png" and "invalid_gray_alpha_sbit.png"
-libpng error: tEXt: Read error: hit end of file in png reader
-libpng error: tEXt: Read error: hit end of file in png reader
+libpng error: oFFs: Read error: hit end of file in png reader
+libpng error: oFFs: Read error: hit end of file in png reader
 idiff ERROR: Could not read invalid_gray_alpha_sbit.png:
 	Invalid image file "invalid_gray_alpha_sbit.png": Read error: hit end of file in png reader
-PNG read error: tEXt: Read error: hit end of file in png reader
+PNG read error: oFFs: Read error: hit end of file in png reader
 Invalid image file "invalid_gray_alpha_sbit.png": Read error: hit end of file in png reader
-PNG read error: tEXt: Read error: hit end of file in png reader
+PNG read error: oFFs: Read error: hit end of file in png reader