I am new to rendering in Zbrush …so whenever i apply any bpr filter it doesn’t come out in the viewport …i have to press render to preview so what’s wrong …i’ve seen all the tutorials previewing the effects whenever they change the sliders or the effect …it;s not going to work like that to press render after ever change so plz help 
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You need to press Render>BPR RenderPass>BPR first. Are you doing that? Once you’ve done a BPR you can adjust the filters and they will update in realtime, so long as you don’t click in the viewport (which would mean you’d need to render again).
It’s a little confusing but BPR is different from the rendering modes at the top of the Render palette. Preview is what you normally see, Best is pretty much a legacy mode but useful for some material effects, Flat and Fast are useful in limited circumstances.