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This is a paid asset, but now you can download Channel Packer Free.
Channel Packer
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Why use Channel Packer:
Channel Packer allows you to pack up to 4 different images in one texture, to manage and process them at the same time, saving memory.
Channel Packer works in two different ways:
– Single mode: used to generate a single texture, packing up to 4 input images;
– Batch mode: used to generate many textures in a unique operation, starting from a list of images; it allows to associate them to the needed channels and pack them in a texture for each association.
Further options:
You can invert images, use colors, select channels to use, and more.
It does NOT require Unity Pro
Read the full documentation.
Official forum thread