A computing system may access first alpha values associated with first pixels in a first pixel region of an image and determine a bit budget for encoding the first alpha values. The computing system may then select a first alpha-encoding mode for the first alpha values to reflect a determination that the first alpha values are all fully transparent or all fully opaque, and encode the first alpha values by storing the selected first alpha-encoding mode as part of a metadata without using the bit budget to encode the first alpha values individually. The computing system may then update a record of unallocated bits available for allocation based on the bit budget unused in the encoding of the first alpha values, and allocate, based on the record of unallocated bits, bits to encode a set of alpha values different from the first alpha values.