A method for inserting a sparse, variable data carrying signal into a digital image, the method comprising: providing a synchronization signal component, the synchronization signal component facilitating a synchronization function of a variable data carrying signal, and comprising signal component values at coordinates within a synchronization two-dimensional embedding tile; providing a data signal component, the data signal component being modulated to carry a variable data signal, and comprising signal component values at coordinates within a data two-dimensional embedding tile, in which the values of the synchronization and data signal components are quantized into quantization bins and in which at least one of the synchronization or data signal components comprises a sparse signal component; or at least one of the synchronization or data signal components is a multi-valued signal and is converted to a sparse signal component by applying at least a first threshold; producing a sparse, variable data carrying signal comprised of sparse elements by combining signal elements at corresponding coordinates within the synchronization and data two-dimensional embedding tiles, where combining the quantized signal component values of the synchronization signal component and the quantized signal component values of the data signal component retains only synchronization and data tile component values falling within the same quantization bins to generate dark sparse elements on a lighter background or to generate a sparse signal with light holes against a darker background; wherein the signal elements correspond to adjustments to be made to corresponding spatial locations in the digital image; and inserting the sparse, variable data carrying signal into at least a first colour or ink channel of the digital image, by making the adjustments to the corresponding spatial locations in the digital image colour or ink channel. providing a synchronization signal component, the synchronization signal component facilitating a synchronization function of a variable data carrying signal, and comprising signal component values at coordinates within a synchronization two-dimensional embedding tile; providing a data signal component, the data signal component being modulated to carry a variable data signal, and comprising signal component values at coordinates within a data two-dimensional embedding tile, in which the values of the synchronization and data signal components are quantized into quantization bins and in which at least one of the synchronization or data signal components comprises a sparse signal component; or at least one of the synchronization or data signal components is a multi-valued signal and is converted to a sparse signal component by applying at least a first threshold; producing a sparse, variable data carrying signal comprised of sparse elements by combining signal elements at corresponding coordinates within the synchronization and data two-dimensional embedding tiles, where combining the quantized signal component values of the synchronization signal component and the quantized signal component values of the data signal component retains only synchronization and data tile component values falling within the same quantization bins to generate dark sparse elements on a lighter background or to generate a sparse signal with light holes against a darker background; wherein the signal elements correspond to adjustments to be made to corresponding spatial locations in the digital image; and inserting the sparse, variable data carrying signal into at least a first colour or ink channel of the digital image, by making the adjustments to the corresponding spatial locations in the digital image colour or ink channel.