Trim transparent space without changing the pose
Trimming removes only outer rows and columns whose alpha values are all zero. A faint antialiased edge with nonzero alpha stays inside the crop. The tool records the original canvas size and the crop offset, so a moving character can retain its intended position when frames have different transparent borders.
An empty frame still matters to an animation. Instead of discarding it, the atlas stores a one-pixel transparent tile and retains the original dimensions. This representation keeps the frame in the sequence. Hidden RGB values beneath fully transparent pixels are not a reliable preservation target because the browser’s Canvas conversion may already have normalized them.
Separate empty padding from repeated edge pixels
Extrusion repeats the nearest edge pixel around the cropped rectangle, including the corners. Transparent padding sits outside that repeated ring. The frame coordinates point to the original cropped pixels and exclude both additions. Each setting is measured per side, which matters when you estimate how much extra space a small sprite needs.
A larger border can reduce contact between neighboring sprites when sampling reaches beyond an edge, but it consumes atlas area. It does not create mipmaps, perform color dilation, or guarantee a particular texture filter will never reveal a seam. Check the asset at the scales and camera distances that your game actually uses.
Use the preview to make a practical choice
Turn on the border outlines to see each allocation around its frame. Compare a modest extrusion ring with larger padding and watch how atlas dimensions change. A power-of-two output can also add unused area because each edge is rounded upward; the displayed frame-pixel percentage is a geometric ratio rather than a measure of file compression.
If the packer cannot fit the images, reduce unnecessary borders or increase the allowed atlas edge. Trimming can help when the source frames contain large transparent margins. Because rectangle packing is heuristic, neither a successful result nor a failed attempt establishes the mathematically optimal arrangement. Choose settings that survive your engine test, not only the smallest number shown in a preview.