Either you must correct the file in photoshop so that "pure" white becomes a light gray that the printer is capable of actually printing or you must place the highest highlights so high that the printer does not put down any ink and the media shows through unhindered.
The first option entails lowering the highlights to a colour that the printer can print. In photoshop this is usually done with levels or curves. Since the eye really only sees color in relation to the neighboring colors having "white" as a light grey will look fine provided there is no pure white nearby. The same thing is usually done to the shadows to make a solid black really a very dark grey.
The 2nd option only really works if the image has no gradual transitions to white since there will be a line where the printer is no longer capable of the appearance of a continuous tone. Also if the media itself isn't pure white you can't get a pure white in the first place. If you were reprinting a poster with a white background, for example, you would basically make sure that white is 255,255,255 (pure white) in the image before printing using the curves or levels dialog.
Good luck...
Doug
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Do not forget that the printer profile used still can shift the 255,255,255 white in your image to other numbers if that profile was made to compensate media white (yellowish most of the time) to neutral white.
If you do a profile>profile conversion in Photoshop with that printer profile on that image and check what the white numbers are after that conversion you will know more. You could as a fast and dirty method then bring the whites back to 255x3 in PS and send that image to the printer with CM off. In the end that profile needs to be replaced or tweaked for the media you use. Canvas with varnish will never be completely white but some canvasses are more white than others. UV blocking varnish is more yellow too than normal varnish and will reduce the optical brightener's effect in canvas and inkjet coating.
If the idea is to get a bluish white snow on an off-white canvas then you end with a bluish grey snow more or less. The eye can be fooled though with illumination light level and color + shifting the non-snow part color and tone in the image. The white snow in an oil painting isn't as white as in nature or on a poster with optical brighteners in the paper and coating.
Ernst Dinkla
try: [a href=\"http://groups.yahoo.com/group/Wide_Inkjet_Printers/]http://groups.yahoo.com/group/Wide_Inkjet_Printers/[/url]