Not sure how the word sparkle is being used here , ... but here are a few thoughts:
1. The image has plenty of spectral variation within the grass, ..but in your image, ..the overall dark saturation of the grass keeps you from seeing that. To me, getting the grass to look photorealistic is determined by letting the sun create variation of tint across the grass depending on the angle of grass blades.
The good news is that all of that work is already done via Maxwell -- you just can't see it. For illustration purposes, what I did was to adjust the saturation till the sun highlights appear on the grass. I also leaned(yellow) the grass towards a non-fertilized look (you could still go that way if you'd like, ie., ultra-green) and still have the highlights(sun).
Then, ...to add a hint more of realism, ... there's a subtle lens blur applied to the image that helps in blending the individual grass planes. Plus there's an added benefit of easing away the general CG-ness.
If you didn't want to affect the entire image, ..then I would just create an Object ID map, and just affect the grass alone.
ps.
on grabbing V2.
ps2: You have to remember, ..a normal viewer simply doesn't scrutinize the grass the way we do. So, all we have to do is create the 'feeling' of lush grass. And that means having a ton of creative license in altering it's color, tint, focus etc.. all of which can be 'non-correct', ..but in the end, ..will do a better job of creating that 'feeling' of grass and not so much the proper 'viewing' of grass.
I say, who cares if it
looks like grass, but more importantly ---- does it
feel like grass?? ... or does it feel like a trillion little spikes (most renders) ??
You get me ???
- By jurX