An electronic graphic system for modifying image pixels includes a framestore 21 and a pipeline brush processor 22. Image pixels are read out of the framestore 21 in batches and applied to an input of the brush processor 22. Other data representing colour shape and pressure of a line drawn by a user on a touch tablet 17 by way of a stylus 18 is also input to the processor 22. As the pixels pass through the processor 22 they are modified by the other data. Batches of modified pixels are written back to the framestore 21. The reading and writing of pixels from the framestore 21 is interleaved with the reading of pixels for display as an image on a monitor 16. The system also includes an arrangement for interpolating stylus pressure to remove visible discontinuities or other changes in the drawn image resulting from significant changes in stylus pressure. An improved zoom facility enables a substantially continuously variable zoom factor to be defined thereby to avoid large changes in scale during a zooming operation. An arrangement for adjusting numerical values associated with some functions or features of the system allows the user to control the values solely by movement of the stylus over the touch tablet.