Is Image Transparent

This is a simple and dirty tool to check whether a given PNG or WebP image is completely transparent. A completely transparent image is an image which contains alpha data and where all pixels have an alpha (or opacity) value of 0.

This tool may be useful for optimising cases where empty images can be removed (e.g. determining whether an overlay image can be safely discarded).

Compiling

The tool requires libwebp and libpng (>=1.6) development libraries to be available. On Debian, you can use apt-get install libpng-dev libwebp-dev to get these.

Compiling can simply be done via the command: gcc -Wall -O2 -lwebp -lpng image_empty_check.c -o is_image_transparent

If you don’t want WebP or PNG support, this can be disabled by omitting the relevant linker flag (-l*) and adding -DNO_WEBP or -DNO_PNG to the compile command.

Usage

./is_image_transparent [file]

[file] can be a PNG or WebP file.

The result is in the exit/return code, which can be:

• 0: image is completely transparent

• 1: image has non-transparent pixels (is always the case if image contains no alpha information)

• 2: error occurred. Error message is written to stderr

Nothing is ever written to stdout, and only fatal error messages written to stderr.

Limitations

• The alpha channel is rendered in 8-bit, which means that if you’re using a higher bit depth (e.g. 16-bit alpha channel), there may be some accuracy loss

• By default, it will refuse to read in files larger than 32MB, or decoded images (32-bit ARGB format) larger than 128MB in size. You can adjust these limits by supplying values for the defines MAX_SIZE and/or MAX_IMAGE_SIZE during compile, for example, to set the max input size to 1MB, append to the compile -DMAX_SIZE=1048576