End date: May 13
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By Ernesto
#366571
Camera Height

Perspective is defined by many parameters.
Human perception can understand the scale of things by these parameters, although it is not a concious procedure.
In order to determine the missing data (in the case of La Ciudad de Las Ciencias by Calatrava) we can use the following pictures:

Here we can have an idea of the tile sizes.
Image

Image

Image

Here we have in the same frame, some walking humans, and the floor pattern, which could be clearly compared.
The white stick measures the height of the guy, and the gray horizontal stripe is the same measurement, to be compared to the floor tiles.

Image

In this enlargemente we can compare the height of the people with the floor pattern.
It seems that 16 floor tiles are similar to the height of the people in the picture.
Then we could assume that each floor tile is 10cm therefore the human height is about 160cm.

This is the key to determine the camera height in the backplate.

Image

In this image we can see that the height of the camera is similar to 6 tiles therefore 60 cm.
This is the key to human perception to understand that the ball is 120 cm diameter.
If we do not take this in considration at the moment to create the scene we might fake scale of objects in the scene.
Perhaps for the people who had never been in the Ciudad de las Ciencias, it will be irrelevant, but people who knows it, will find the composition unreal.

FOV

The FOV, or Field Of View, is measured in degrees and is the angle of view.
Some use a vertical FOV others use a horizontal FOV and in the photographic market it is used a diaonal FOV.
It doesn´t matter which we use, but we have to be clear in order to avoid comparing diferent FOVs.
The image of the chrome ball could be usefull to determine the FOV.
We will use floor tiles again.
I could count 15 floor tiles at the base of the image. This would be 150 cm wide.
We knew already that the camera height was at 60 cm.
And that is all we need to calculate the FOV.
Our frame is 12 floor tiles by 15 floor tiles.

If we measure the width of the frame just where the ball is standing, we could count: 40 floor tiles

And the distance between the popint where the ball is standing to the lower frame border is: 24 Floor tiles.

In the following sketch, the full construction is represented, all measurements are in "floor tiles"
Each floor tile is estimated in 10 cm x 10 cm

Image


By trigonometry we can know that the horizontal FOV is 55 degrees.
This could correspond to a 35 mm lens (35mm negative standard) but since the proportion of the frame is 1:33 we are only using a 40mm lens.

Then if we use a longer lens in our Maxwell camera, the 3d object will look flattened, and on the other side if we use a shorter lens, for instance a 12 mm lens, our object will look stretched in its depth.

It all depend on the degree of realism that is being intended.
The concepts realistic or unrealistic, has to do with these parameters, and deppending on the observer cultural background, they will judge our work, realistic or not. By cultural backround I am not meaning a professional degree or anything like that, I only mean the previous knowledge that this person has on the objects in the image. As deeper the knowledge, the hability to detect an inconsistency in the image.
The only way to do a professional compositing job, is taking into accunt these problems. Otherwise we will be working by eye, and the realism of the final product will be judged by a diferent eye.




Ernesto
Last edited by Ernesto on Fri Mar 29, 2013 11:16 pm, edited 1 time in total.
#366593
Just to add that checking the exif on the photo, it was taken using a PhaseOne camera with a P45+ filmback, which is 49.1x36.8mm. So users should enter first of all this data in the camera film back (or sensor, depending on what your main app calls it). This way any lens focal length Maground will give us will match and you won't have to do any conversions (in case you are using a matching ratio film back but smaller or larger than the actual one used in the camera that took the photo).
#366598
Mihai wrote:Just to add that checking the exif on the photo, it was taken using a PhaseOne camera with a P45+ filmback, which is 49.1x36.8mm. So users should enter first of all this data in the camera film back (or sensor, depending on what your main app calls it). This way any lens focal length Maground will give us will match and you won't have to do any conversions (in case you are using a matching ratio film back but smaller or larger than the actual one used in the camera that took the photo).
Image

Interesting Mihai!
Does it says the Focal Length of the Lens used? (usually in milimeters)
(for some reason I cannot see the eixf info)
If the previous FOV calculation is right, and based in the information you have just provided, it should be arround 47mm

Image
In their web site: http://www.phaseone.com/
It is stated that the camera system can use these Focal plane lens range

28mm AF f/4.5
Superb extreme wide angle lens for full format digital especially. Perfectly suited for landscape, architecture and creative looks.
35mm AF f/3.5
Compact and light weight wide angle lens. Ideal for street, landscape, interior or architecture photography.
45mm AF f/2.8
Using low dispertion glass, this wide angle lens offers minimal chromatic aberration and is ideal for high contrast shooting conditions.

80mm AF f/2.8
Great general purpose lens with outstanding optical performance. All scenes are captured with amazing details and accuracy.
120mm AF Macro f/4.0
For beauty, portraits and close-up macro work, this lens will deliver amazing results. The limiter feature ensures fast auto focus in the relevant focus range, whether working with close-ups or at further distances.

More info on lens: http://en.wikipedia.org/wiki/Normal_lens

The closest to the calculation based in the image, seems to be a 45 mm lens, which is the NORMAL lens for this format.

Ernesto
#366603
A warning for Maya Users:
No matter if you are working in cm, the sensor/frame size is in INCHES.
So if we need to match the curent sensor size of 49,1mm x 36,8 mm, we should convert to INCHES which is 1.931 x 1.447.
Strangely the focal length is in milimeters.

Ernesto
#366640
Hello, here I will try to complete this thread.
I wish I could illustrate what I am going to explain, with rendered images by Maxwell...
Unfortunately for obvious reasons I will no be able to do this since my platform is Maya.
http://www.maxwellrender.com/forum/view ... 01&t=40339
http://www.maxwellrender.com/forum/view ... 32&t=40349
Anyway, in order to finish this topic properly, I have prepared some images created in photosoftware.

The consequences of not matching the FOV:

Here in this first image, I tried to illustrate a white disc on the ground.
As it is a perfect circle, the diameter should be the same no matter where you measure it.
In this case the diameter in 6 floor tiles (therefore about 60 cm)
Image
You can see that this is the same measured in both directions of the floor plane (x and y)
And this is how it should look if you have matched the right FOV

Here in this other illustration I tried to show, what happens when you use a wider FOV
This happens when you use a wider lens than the used in the original photograph.
Do not confound FOV with focal lenght. When FOV is wider is because you are using a shorter Focal lenght.
Image
In other words, this is what happens using a shorter focal length, or a wider FOV which is the same. All considerations are related to the proper Focal Lenght and the proper FOV, which were used at the time of taking the picture.
The disc in this image looks like an ellipse, 6 floor tiles by 15 floor tiles.

The following is the opposite case:
Image
In other words, this is what happens using a longer focal length, or a narower FOV which is the same.
Now it looks an ellipse 6 tile floors by 3 tile floors.

The consequences of not matching the camera height:

Here in this image, I tried to paste a human figure, in the right scale.
I based this in the assumtion that the flloor tiles are 10 cm by 10 cm, so the height of the woman, is about 17 floor tiles measured in the point where she is standing.
Image
This gives the user, a sense of scale, so we could say that the ball is about one meter diameter.
We can have the sense that the camera height is about 60 cm, by looking at the aproximated intersection between the vertical axis of our woman, and the ideal horizon of the image.

In the next image, I tried to show, what happens if you use a wrong camera height.
Image
Here I used a higher camera height. and what happens is that the woman looks like a dwarf, She seems to be a miniature, and now she looks about the same height as the ball. This are the human perceptions. If you want to verify in a more geometrical way, count the tiles under her feet, and compare with her height.

In the next image, we have the oposite situation:
Image
The camera height il lower than it should be, therefore we have a giant of 30 o more meters tall.

Of course these samples were exagerated to show clearly the effects. But when your errors in FOV or Camera Heigh are not that evident, the image transmits a sense of fakeness. The normal observer will not understand what is happenning, but it will produce rejection, since the human perception knows there is something wrong there.
We as professionals of the image, shouldn´t work by eye. Instead the best ap`roach is to work in a more cientific way.
This is way safer, since our eye (which is usually a very trained eye) could be fooled by certain circumstances called optical illusions.
So our only way to provide an accurate result is, working professionally.

This is the reason Maground should provide this information together with the images. This will rise the product from amateur to professional, independently from the attitude of the user.

Ernesto
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