This is a graphic design question.
Because it is a little difficult to explain, i've attached an image of what i want.

Suposse you have a door, and this door has an opening animation. Well, because of the perspective feeling, graphic size of frames must be different. In fact, the first frame and the last frame must have a difference of 32 pixels (the height of a floor tile).

Now my question is: Suppose you have a 3d modeling program called X. How could you set the perspective settings to set the exact difference of Y pixels between first frame and last frame? (In this case, Y=32 pixels)?.

The desired door and arrows are painted with a program called Z

Thanks in advance..

I think the door upper end also "extends" to Y-- as much it goes down.
But this might start to look weird if the door is above central hor-line.

If i understood you, what you say is to let size constant, don't you?
This is weird. Is like a "painted" door in the floor and in the wall.

It depends. Let's say this is the closed door in your picture:

tttr
fffr
fffr
fffr

(f for front) [edit: added t and r to make clearer]

With an orthographic projection, the opened door would look like this:

t
t
r
r
r
r

(t is top, r is right side)

I.e., the opened door would still have the same height in pixels, but you could see the door's top when it's open.

With perspective projection, it might look like this:

r
r
r
r
r
r

I.e., the side of the door now close to you gets bigger than the height of the closed door was before.

Take a piece of paper, draw the thing and start using triginometry to find all the needed variables (camera position and FOV).

I was thinking a easiest idea is to capture the screen of what i want, set it as background in the viewport, and do manually the perspective changes.

Thank you for all.

MSPaint does just fine...

Hope this help, btw cool graphics, do you have a demo or something?

That's an animation simmilar to what i want .

Yay, thank you paul .
I'm working on it. I must have a demo in one or two months, depending of the time making the rest of the enemy graphics.

[EDIT]
Sorry, Steve, i haven't seem your attached file. Pretty good solution. Thank you very much .