Chapters: 1: Introduction 2: Simple example 3: Invocation 4: Finer Control 5: X-Y Plots 6: Contour Plots 7: Image Plots 8: Examples 9: Gri Commands 10: Programming 11: Environment 12: Emacs Mode 13: History 14: Installation 15: Gri Bugs 16: Test Suite 17: Gri in Press 18: Acknowledgments 19: License Indices: Concepts Commands Variables | ### 9.3.24: ``mask`'
    `mask the image [to {uservalue .u.}|{imagevalue .i.}]'
Examine both the image and the mask pixel by pixel. For any pixels which have a mask value of 1 (which indicates an invalid region of the image), change the image value. If no ``to`' phrase is present, change the image value to 0 in pixel units. If the ``to uservalue .u.`' phrase is present, change the pixel to hold the imagevalue that corresponds to this uservalue (see ``set image range`' command for a discussion of this correspondance). If the ``to imagevalue .i.`' phrase is present, change the pixel to hold that imagevalue (in range 0 to 255 inclusive for 8-bit images).