Adding an Xcode Build Rule to compress PNGs
Images in an iOS App can make up the majority of it size. This is especially true if it is a Universal App requiring it to have all image in triplicate (iPhone Standard, iPhone Retina iPad).
The best type of image to use in an iOS App is a PNG. When your designer created these images in Photoshop they are saved with a lot of meta data that can be discarded.
However you don't want to loose all that data completely as it's useful to designers if they need to alter the image.
PNGOUT Optimisation
There are several tools for optimising PNG files, but pngout seems to be the best option.
- Download the Mac Version of pngout.
- Copy the binary command-line application
pngout to a directory with in your project. Adding the binary to the project directory makes sure is is available to anyone building the project on any system.
Creating a Build Rule (Xcode 4)
Build rules are Target specific, so if you have more than one target you must copy the rule into each.
Add DEBUG and DISTRIBUTION macros to your build configurations. Optimising the PNGs is quite processor intense and as such you only want to do it on distribution builds. 
As you can see amongst others I have added DEBUG_BUILD=1 and DISTRIBUTION_BUILD=1.
Add a build rule for PNG files. A build rule simply processes a specific file (and/or) file type during the build process. The power of this is rules can be chained together. Build Rule - Xcode 4 http://tinypic.com/images/404.gif
Click the "Add Rule" button;
- Set Process to "Source files with names matching" and its value
*.png.
- Set Using to "Custom Script"
Paste this code into the script box
echo "----------------------------------------------------------------" >> "${DERIVED_FILES_DIR}/pngout-log.txt"
echo "${INPUT_FILE_PATH}" >> "${DERIVED_FILES_DIR}/pngout-log.txt"
echo "${DERIVED_FILES_DIR}/${INPUT_FILE_NAME}" >> "${DERIVED_FILES_DIR}/pngout-log.txt"
echo ${GCC_PREPROCESSOR_DEFINITIONS} >> "${DERIVED_FILES_DIR}/pngout-log.txt"
BUILD=`echo ${GCC_PREPROCESSOR_DEFINITIONS} | grep -o DISTRIBUTION_BUILD`
echo $BUILD >> "${DERIVED_FILES_DIR}/pngout-log.txt"
if [ "${BUILD}" == "DISTRIBUTION_BUILD" ]; then
echo "COMPRESS" >> "${DERIVED_FILES_DIR}/pngout-log.txt"
"${PROJECT_DIR}/build-process/pngout" -y -q -force "${INPUT_FILE_PATH}" "${DERIVED_FILES_DIR}/${INPUT_FILE_NAME}"
else
echo "COPY" >> "${DERIVED_FILES_DIR}/pngout-log.txt"
cp -f "${INPUT_FILE_PATH}" "${DERIVED_FILES_DIR}/${INPUT_FILE_NAME}"
fi
echo "...done." >> "${DERIVED_FILES_DIR}/pngout-log.txt"
There are several environment variables that are of note:
${INPUT_FILE_PATH} — full path to the image file${INPUT_FILE_NAME} — image file name (with extension)${DERIVED_FILES_DIR} — where Xcode stores build files etc${GCC_PREPROCESSOR_DEFINITIONS} — the macros you set above
The work is done on this line:
"${PROJECT_DIR}/build-process/pngout" -y -q -force "${INPUT_FILE_PATH}" "${DERIVED_FILES_DIR}/${INPUT_FILE_NAME}"
${PROJECT_DIR} is the full path to your project, the -y overwrites existing files, -q limits pngouts output and -force prevents pngout from exiting with status of 2 when a file can't be optimised and causing Xcode report build errors.
This script simply tests the ${GCC_PREPROCESSOR_DEFINITIONS} to see if it a DISTRIBUTION_BUILD if so it uses pngout to optimise the file, otherwise it copies it to the ${DERIVED_FILES_DIR} so Xcode can continue to process it.
Finally, don't forget to add ${DERIVED_FILES_DIR}/${INPUT_FILE_NAME} to the "Output files" list this is how Xcode knows how to find the files you have processed.
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