# Centering a Ball on the Screen

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Posted (edited)

I had a question earlier, but I have another question about something said in the post.  Once again, here is the image of the rotation  being wrong because the ball is not staying centered on the screen.

I asked how do I keep the ball on the center of the screen.  Here was the response:

Quote

Anyway, there are multiple ways you could get the behavior in that video. For example, you could transform an appropriate offset by the sphere's model/world transform to yield a camera position in world space, and then use a 'look at' transform to place the camera at that position and aim it at the sphere.

I an interested in all the ways to do this, and I 'm not understanding what this means, exactly.  When I try to interchange World space and Camera space (use the same values for camera and translate) the ball isn't centered.  Even when the camera is inverted.

Here's what it does:

EDIT: the camera uses lookAt.

Edited by Josheir

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Dont know the api and what means lookat but i assume that you have a point where camera looks at so:

vec3 bdir = normalize( lookatpoint-campos);

vec3 ballpos = campos + somedistancefrom_camera*bdir;

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52 minutes ago, Josheir said:

I asked how do I keep the ball on the center of the screen.  Here was the response:

I think that particular suggestion was from me, but I made some other suggestions later in the thread that I think were probably more relevant and/or would be easier to implement.

I think there are some conceptual issues at play here regarding the basics of transforms, what they mean, and how to manage them. I think in the last thread I and others tried to provide an overview of the three types of transforms in a simple scene (projection, view, and world) and how they relate to each other. Although you might get lucky and stumble on a solution, I think solving this problem will more or less require coming to grips with some of the higher-level concepts we were talking about in the other thread.

I remember that one of the points of confusion was what behavior you wanted exactly. You posted a video that I think you said represented the desired behavior, in which the target object (a humanoid character in the video) rotated in world space, while the camera remained fixed with respect to the target object. However, you've also said this is a golf-like game, for which I'd expect the target object (the ball) not to rotate, and for the camera to rotate around the target. Can you clarify what behavior you want? Do you really want the ball to rotate? Could you post another (or the same) example video for reference?

Meanwhile, I think the easiest solutions available to you would probably be based on a 'look at' function. I think a good first step would just be to successfully use a 'look at' function to place the camera at an arbitrary position and have it aim at the ball. Once you have that working, the problem reduces to choosing an appropriate camera position, as discussed in the other thread. Once you have the basic 'look at' camera working, maybe we can help you in computing the camera position to give you the behavior you're after.

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1 hour ago, Josheir said:

I an interested in all the ways to do this, and I 'm not understanding what this means, exactly.  When I try to interchange World space and Camera space (use the same values for camera and translate) the ball isn't centered.

As far as I understand it, you need to compute the position of the ball in the world, and compute a camera position from that.

That is different from "normal" where you are at some (camera) position in the world, and look around. The latter is what "lookAt" does, it rotates the camera towards the lookAt point, the camera doesn't move as a result of the lookAt call.

Alternatively, you could always move the world such that the ball ends up at a fixed point in the world, say (0, 0, 0), and setup the camera somewhat away from that position.

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Posted (edited)
7 minutes ago, Alberth said:

Alternatively, you could always move the world such that the ball ends up at a fixed point in the world, say (0, 0, 0), and setup the camera somewhat away from that position. ﻿

I think I know what you mean, and I had this working but when I was using the barycentric equation (to make the ball collide with the terrain's height) the ball was interacting with the terrain not where the ball was but where the ball would be located with the rotation!

Edited by Josheir

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1 hour ago, _WeirdCat_ said:

Dont know the api and what means lookat but i assume that you have a point where camera looks at so:

vec3 bdir = normalize( lookatpoint-campos);

vec3 ballpos = campos + somedistancefrom_camera*bdir;

Something like this is what I was looking for, not a lot of theory, however theory isn't bad.  I hope this works, how is somedistancefrom_camera determined.  It's a vector?

Josheir

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1 minute ago, Josheir said:

Something like this is what I was looking for, not a lot of theory, however theory isn't bad.  I hope this works, how is somedistancefrom_camera determined.  It's a vector?

That solution doesn't look right to me because it appears to compute the ball position from a known camera position, whereas it seems what you want is to compute the camera position from (perhaps among other things) a known ball position. Obviously I don't know for sure that it's not what you want, and you can always try it, but it doesn't seem consistent with what's been discussed elsewhere in this and your other thread. (For what it's worth, 'somedistancefrom_camera' would almost certainly be a scalar here. Distances are usually scalars, and a scalar would make the most sense in that particular example.)

I feel like in this and the other thread we're sort of avoiding talking about the actual problem and just sort of casting about for solutions randomly. To be sure, sometimes one can find a solution that way  But if you find that _WeirdCat_'s suggestion doesn't give you what you want, I'd strongly suggest backing up a bit and trying to get a higher-level view of things.

I maintain that what you want to do is compute an appropriate camera position, and then use a 'look at' function to place the camera at that position and aim it at the ball. Once you have that working, the only task will be to compute an appropriate camera position, which I'm sure we can help you with. But I think we have to clear some initial hurdles first and get some fundamentals in place.

Lastly, if you find yourself still needing a solution for this, I again suggest clarifying what behavior you want (e.g. ball rotates and camera is fixed, or ball is fixed and camera rotates), and perhaps posting a video showing the desired behavior. (I know you posted an example video before, but I think further clarification would be useful.)

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What's happening here :

vec3 ballpos = campos + somedistancefrom_camera*bdir; ﻿

Hoe does it work that a vector is multiplied by a scalar, what is this going to do?  A vector has three values and a scalar has only one?  And where do I take this distance value from?

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Posted (edited)
8 minutes ago, Josheir said:

What's happening here :


vec3 ballpos = campos + somedistancefrom_camera*bdir; ﻿ 

Hoe does it work that a vector is multiplied by a scalar, what is this going to do?  A vector has three values and a scalar has only one?  And where do I take this distance value from?

???

Element wise multiplication, results in a new vector. These are the very basics you must know, or any attempt to help you will fail. Sorry for being so blunt ...

Edited by Green_Baron

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vector multiplied by a scalar means you apply the vector that many times, ie multiply each element of the vector by the scalar, commonly known as elementwise multiplication, as the green baron said.

"somedistance" is such that it looks good I think. If you pick 0, the camera is in the ball, which could be nice, but likely not what you want. If you pick a larger number, the camera moves away from the ball, letting you choose between a ball of screen-filling size, or anything smaller, with infinity being less than a pixel in size.

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