A method of variable rate compression including: partitioning the image data into a plurality of blocks, each including channel-blocks; and for one of the one or more non-flat channel-blocks: generating a residual block including residual values corresponding to values of the one of the one or more non-flat blocks; in response to determining that a particular residual value of the residual values is equal to a corresponding previous residual value: incrementing a run counter; and in response to determining that the run counter is greater than a threshold, generating a run-mode flag and writing the run-mode flag after the particular residual value in a compressed output stream; and in response to determining that the particular residual value is not equal to a corresponding previous value: in response to determining that the run counter is greater than the threshold, generating a run-length after the run-mode flag in the compressed output stream.