DX10 graphics far better than DX9 on old game

[#1]

To explain

I am playing Far Cry 2 as it is one of the few games of that type which will run on my laptop with an Intel HD5500 chip at 1920x1080 and I just realised that it was running DX9 on Windows 10.

Switching to DX10 and the improvement in graphics quality was astounding with no real change in fps in that the range of brightness/contrast etc. suddenly became well within the capability of the standard LCD screen with no white out

Is there a technical reason for this

[#2]

yes,

the gfx card and cpu are doing less work per frame so you get improved performance 

in dx9 every detail had to be mapped and for things like explosion profiles the cpu/gpu would draw up to 100 frames but only use 3 of them on screen, in dx10 they just calculate the bits that change per frame and then draw those changed into the couple of frames that are required which shifts the workload back and forth for different things, you might find in some parts of the game that its prone to getting all stutter and frame rates plummet but on the whole whilst moving through the maps it should be much smoother experience becasue of it:)

btw intel hd gfx are the absolute worst for becoming saturated and then starting to display edge lining as each frame refreshes... one of the primary reasons i buy AMD based laptops (since my budget has never stretched to nvida gfx) /

[#3]

Thanks

In this case there are two other factors (one of which I hadn't realised) a shader package I imported only works in Dx10

The other is that under Dx10 the game is prone to crash which is a nuisance but is liveable with