<p>It is bigger because it fills the same view and it ts further away. It would be smaller if it wouldn't fill the same view but then it wouldn't fill the camera view and it wouldnt work.</p>

<p>So the inverse relation applies. If you want something to stay constant in size in the view then it needs to grow as it gets further. </p>

<p>Your just comparing apples with oranges. </p>

CC BY-SA 3.0
2017-05-29T11:05:43.390Z
2017-05-29T11:10:49.933Z
2017-05-29T11:10:49.933Z
joojaa
<p>if you place your hand by your ear, you can't see it ... now then if you move it forward, suddenly you'll start seeing it ... this is what the frustrum represents, as in your second picture... </p>

CC BY-SA 3.0
2017-05-29T12:32:11.353Z
2017-05-29T12:32:11.353Z
simon b
<p>It might help to think of it this way:</p>

<p>Both the near plane and the far plane are the size that will fit onto the screen you are viewing on. The further away something is, the bigger it can be and still fit on the screen. Close up, a playing card can fill the whole screen, but in the distance, an entire building can fit on the screen.</p>

<p>Consider the example image:</p>

<p><a href="https://i.sstatic.net/jBNlH.png" rel="noreferrer"><img src="https://i.sstatic.net/jBNlH.png" alt="image of near and far plane"></a></p>

<p>If a playing card exactly fills the near plane, then you will be able to fit 4 playing cards on the far plane. This reflects the fact that when a playing card is twice as far from the camera, it appears to have half the height and width.</p>

<p>Think of the near plane and the far plane as simply the area that will fit on your screen. The further away you point your camera, the more will fit on the screen. This matches up with the fact that things look smaller in the distance - this means you can fit bigger things on the screen if they are further away.</p>

CC BY-SA 3.0
2017-05-29T13:22:58.040Z
2017-05-29T13:22:58.040Z
trichoplax is on Codidact now
<p>I'm following <a href="https://learnopengl.com/#!Getting-started/Coordinate-Systems" rel="noreferrer">this tutorial</a> on the perspective projection.</p>

<p><strong>Question:</strong> If the point of a perspective projection is to create the perspective effect (where objects faraway appear to be smaller):</p>

<p><a href="https://i.sstatic.net/W1LpU.png" rel="noreferrer"><img src="https://i.sstatic.net/W1LpU.png" alt="enter image description here"></a></p>

<p>then why does it generate a frustrum in which the <strong>back</strong> is larger than the <strong>front</strong>? Should it be the other way around?</p>

<p><a href="https://i.sstatic.net/jBNlH.png" rel="noreferrer"><img src="https://i.sstatic.net/jBNlH.png" alt="enter image description here"></a></p>

<projections><perspective>
Why is the back of a perspective frustrum larger than the front?
George
# Why is the back of a perspective frustrum larger than the front?

(Asked by: username_0 on 2017-05-29)

I'm following [this tutorial](https://learnopengl.com/#!Getting-started/Coordinate-Systems) on the perspective projection.

**Question:** If the point of a perspective projection is to create the perspective effect (where objects faraway appear to be smaller):

[![enter image description here](https://i.sstatic.net/W1LpU.png)](https://i.sstatic.net/W1LpU.png)

then why does it generate a frustrum in which the **back** is larger than the **front**? Should it be the other way around?

[![enter image description here](https://i.sstatic.net/jBNlH.png)](https://i.sstatic.net/jBNlH.png)

**Tags:** projections, perspective


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## 3 answers

### Answer by username_3 on 2017-05-29. Score: 5 | Accepted answer

It might help to think of it this way:

Both the near plane and the far plane are the size that will fit onto the screen you are viewing on. The further away something is, the bigger it can be and still fit on the screen. Close up, a playing card can fill the whole screen, but in the distance, an entire building can fit on the screen.

Consider the example image:

[![image of near and far plane](https://i.sstatic.net/jBNlH.png)](https://i.sstatic.net/jBNlH.png)

If a playing card exactly fills the near plane, then you will be able to fit 4 playing cards on the far plane. This reflects the fact that when a playing card is twice as far from the camera, it appears to have half the height and width.

Think of the near plane and the far plane as simply the area that will fit on your screen. The further away you point your camera, the more will fit on the screen. This matches up with the fact that things look smaller in the distance - this means you can fit bigger things on the screen if they are further away.


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### Answer by username_1 on 2017-05-29. Score: 3

It is bigger because it fills the same view and it ts further away. It would be smaller if it wouldn't fill the same view but then it wouldn't fill the camera view and it wouldnt work.

So the inverse relation applies. If you want something to stay constant in size in the view then it needs to grow as it gets further.

Your just comparing apples with oranges.


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### Answer by username_2 on 2017-05-29. Score: 1

if you place your hand by your ear, you can't see it ... now then if you move it forward, suddenly you'll start seeing it ... this is what the frustrum represents, as in your second picture...


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