DX8 - Getting a pixel colour and Thanks to all for help

[danday1974]

OK all this should be pretty much my last question for these forums (phew says Z) unless I get stuck coding my project although Im feeling pretty confident now (even though it will probably only run at 1 FPS ).

Is there any FAST way of returning the pixel colour of a screen coordinate? I know it can be done by locking the backbuffer although as far as I am aware this is a v slow process and I dont know how to do it anyway. If not I guess I can live without it.

Cheers all and especially to Z thanks very much for all your help, I couldnt have done it wivout u .

Dan

PS I will be back to help others as my way of saying thanks so dont think u can get rid of me that easy .

[Zaei]

Other then locking the backbuffer, no, there is no way to do it.

Z.

[danday1974]

Z

Would u agree that this method is slow?

[Zaei]

Pretty slow... but if you need to do it, do it. Experiment =).

Z.

[Sastraxi]

Using the array is fast, using GetLockedPixel is slower.

[danday1974]

Well I wanna do it to check wether the 'sun' is visible or not. If the pixel colour at the suns screen pos is not that of the sun then I know it is obscured otherwise it is visible. If so draw a lens flare if not dont.

Can any1 give me some VB code for this please (just the locking the backbuffer and getting the pixel colour part).

Cheers

Dan

[Zaei]

Ack! BAD reason to lock the back buffer... To see if the sun is onscreen, just use the D3DXVec3Project() function, and project the sun's position onto the screen. Then just check to see if the returned point is within the screen's boundries.

Z.

[Zaei]

As far as I can see, there are only 2 reasons that you would lock the back buffer. The first, is to apply some sort of post render image filter (like a blur). You could blur by using a texture as a render target, and then using that texture to cover a screen-sized quad, scaled up (the texture filter will blur for you).

The other is to take a screen shot.

Some people will use the abck buffer to do culling, but I think this is a bad method.

Z.

[Sastraxi]

The blur would look pretty bad though, Zaei!!

[Zaei]

Depends on the card, and the blur filter applied =).

Z.

[danday1974]

D3dXVec3Project would tell u if it was on screen or not but it wouldnt tell u if it was obscured by other geometry or not right? You wouldnt want a lens flare showing if u couldnt see the sun even if it was on screen.

Probably is a bad approach though.

Dan

[Zaei]

That is what ray casting is for =). Use D3DXVec3Project, and find the screen coordinates of the sun. If it is onscreen, use D3DXVec3Unproject to find the point on the near clip plane (z==0.0). Create a ray using the sun pos, and the point you just found, and see if that ray intersects any geometry. If it does not, draw your flare.

Z.

[danday1974]

lol @ z==0.0 u C++ man .

Can I be cheeky and ask for code for that please Z its just that Im desperate to have tomorrow off Ive been stuck to this thing all week and its back to college on Monday.

Cheers

Dan

[Zaei]

Its an engine thing. You are starting to realize why game engine design is such a hot topic. You need a way to get at your geometry, so that you can create planes from it, and then check to see if the intersection point on that plane is inside of the geometry triangle. You can use the D3DXLineIntersectPlane() function to check plane intersection, and D3DXPlaneFromPoints() to create the plane. Do a google search for a function to find out if a point is within a plane. Other then that, it is simply a matter of looping through each triangle in your geometry, and see if it is intersected by the ray.

And yes, C++ smacks VB around =).

Z.

[edit]
At lease I didnt use == 0.0f =).

[danday1974]

lol yeah dont I know it, why on earth did I start learning DX in VB O well no time to change now .

Those function names will give me something to work on. Cheers

Dan