Batch processing of gain maps does not appear to be possible with IM

ImageMagick version

7.1.2-23

Operating system

MacOS

Operating system, version and so on

26.5

Description

Batch conversion with normal ImageMagick commands like mogrify -resize 50% *.jpg only uses the standard JPEG coder.

It sees and resizes the SDR/base JPEG, but it does not preserve the gain map JPEG aux image or metadata. The result is an SDR JPEG, not a gain-map JPEG (the HDR content is stripped).

Steps to Reproduce

Compile with gain map support:

PKG_CONFIG=/private/tmp/im-release-testing/tools/pkg-config \
IM_RELEASE_LOCAL=/private/tmp/im-release-testing/local \
CPPFLAGS="-I/opt/homebrew/opt/jpeg-turbo/include" \
LDFLAGS="-L/opt/homebrew/opt/jpeg-turbo/lib -L/private/tmp/im-release-testing/local/lib -Wl,-rpath,/private/tmp/im-release-testing/local/lib" \
/private/tmp/im-release-testing/src/ImageMagick-7.1.2-23/configure \
  --with-uhdr=yes \
  --prefix=/private/tmp/im-release-testing/install

Transcode a JPG gain map using mogrify, the output will no longer have a gain map.

Images

Any gain map could be used, nothing special about this source:

Start by reproducing the loss of the gain map with the provided ImageMagick 7.1.2-23 build, using mogrify on a gain-map JPEG compiled with --with-uhdr=yes. Trace the batch conversion path and verify that the resized output preserves the gain-map JPEG auxiliary image and metadata rather than producing SDR-only output.