Lossless compression of simulated Gaussian CMB data, with and without application of the Kp0 mask (data-spheres are displayed using the Mollweide projection). The first column of panels shows the original simulated CMB data, with corresponding file sizes also specified. The second column of panels shows the residual of the original and decompressed CMB data reconstructed for a precision parameter of p = 3, with corresponding file sizes of the compressed data also specified. Note that the colour scale between the first and second column of panels is scaled by a factor of 100. RLE is applied when compressing the masked data to efficiently compress the runs of zeros associated with the masked regions. The third column of panels shows the trade-off between compression ratio and decompression fidelity with precision parameter p. Compression ratio (solid black line; left axis) is defined by the ratio of the compressed file size relative to the original file size, expressed as a percentage. The decompression error (dashed red line; right axis) is defined by the ratio of the MSE between the original and decompressed data relative to the rms value of the original data, expressed as a percentage.