HDR Aquisition - Introduction

Last week I went to a VES event on HDR usage in visual effects. After talking with a friend I realized that I have not written much of anything on HDR acquisition.

Most VFX Supervisors and artists now about HDRs. But they might not know how to capture and compile one. Lucky for us the tools have gotten much better in both areas!

Getting a good set of usable HDRs takes a bit of practice, familiarity with some basic photography rules, the right gear, and good communication with the First A.D. on set. Typically in pre-pro they're like yeah just let me know and we'll get you in there. But the reality is the second the "VFX Guy" drops his tripod on set to start taking HDRs the First A.D. is already yelling to "CLEAR SET!".

That's when practice and having the right tools on set pays off. And taking a fast and usable HDR data set will make your life a lot easier when you get back to the office.

I'm going to assume that we all know what HDRs are and how we use them in post production. But if you need a bit of a primer check out these links.

• Paul Debevic's webpage.
• Christian Blocks site hdrlabs.com and his book "The HDRI Handbook".

To make this a bit more digestible I'm going to break the whole thing down into separate entries.

1. Gear - what kind of gear do you need to take HDRs.
2. Setup - how to setup and and be "Johnny on the spot" on Set.
3. HDR image capture - the process of actually taking the pictures.
4. Stitching the HDRs together.
5. Processing the HDRs and getting them into your software of choice.

The process is generally the same but there are many variations largely depending on what equipment you have. I'm going to break down my process using my gear but the general theory applies across many different gear sets.

Feel free to hit me up with any questions along the way and I'll do my best to answer them!

Cross Polarization Photography and Skin Texures

Hello Everyone,

Here is a good trick for capturing only the diffuse component of a person for the basis for diffuse texture maps in a sub surface scattering shading model.

Bascially what you need to do is filter both your lens and your light source. In this case I am filtering a pair for canon 430 EX speedlights with linear polarizing film. The speedlights are mounted on a custom bracket. The purpose of the bracket is to align speedlights with the camera lens in order to evenly light our subject. (me)

First lets look at the results.


(note: the small amount of reflection/specular light on the right hand image is the uncorrected lights in my kitchen. Ideally you would take your photo in a more controlled setting. Also for this example I did not have 2 flash units available so I just used one directly on the hot shoe of the camera)

Why does it work?
Well when I first learned this trick back in the day.. I was just told "it works". But I wanted to know why so I did some digging. Here is what I've found.

I am paraphrasing The Handbook of Cosmetic Science and Technology, Third Edition. Cross polarization photography lets you differentiate between regular glare and back scattered light. The back scattered light is the complextion within the skin.

Both the camera lens and the flash have a filter placed in front of them. When the orientation of the filters are perpendicular (cross polarization) the regular glare is blocked and only the back scattered light is captured.


But when the filters are parallel only the reflected polarized light passes though the filter to be captured by the camera.

What do I need to take photos like this?

The Basics

• Camera
• Circular Polarizer for your camera
• Linear Polarizing Film (to filter your flash)

More Advanced Setup

• Camera
• Circular Polarizer for your camera
• External Flash units (I used 2)
• Linear Polarizing Film (to filter your flash)
• Flash diffusers (make sure the one you buy fits your flash)
  
• Flash bracket (My good friend and boss Les Ekker helped me make a nifty bracket)
• An external hot shoe extensions
• An adapter to connect more than one flash to your PC sync port
• Extra tripod mounting plate
• some standard "quarter/20" screws (1/4 inch screws with a 20 degree pitch threads). This will fit a standard camera mounting hole.
  

The Flash Bracket


Les had some extra 1/4 aluminum stock laying around so after some carefull planning we drilled a bunch of through holes. 2 hole for the hot shoe adapters. 1 hold for the camera, and the last through hole we threaded and attached a standard tripod mounting plate.

After we did all that... we threw the thing in a vice and gave it a good twist. This was to align the flashes as best as possible with the front of the lens.

presto chango.... hand dual flash bracket! (when I say we...I mean Les, he did most of the work as I drank a beer in his shop.)

Making the flash filter



The flash filter was simple to make. I ordered a sheet of the polarizing film and simply used a sharp exacto knife to cut out a shape that matched the end of the diffuser. We simply taped the film to the end of the diffuser with some common "silver" tape that you can find at the hardware store. I think its generally used to repair dryer hoses. The silver tape will help contain any light leaks from the flash and focus as much light out through the filter.


Shooting Conditions

When shooting you want to control as much as the light as possible and have little or no reflective/specular light sources. (light fixtures, computer monitors, windows etc...)

make sure to focus and have fun.


Now what do I do with these...

Once you've taken a set of photos in each position (perpendicular and parallel) you can do some fancy photoshop action by either inverting or using difference matting techinques to derive a specular only map. These 2 images will take you a long way.


But wait there's more....

This technique works on all kinds of stuff. I've used it to shoot rubber tires, plants, leather and a bunch of other stuff...


here is a link to a page that I came across in my researching for this project.... just wanted to give some credit to naturescapes.com

Here is a flickr gallery that has more pictures of the rig.
enjoy

Andy

The Integration Company

I wanted to give a quick shout out to Tim Conway and Del DePierro over at The Integration Company. These guys are top notch on set Supervisors and Integration specialists. They offer a wide variety of services ranging from on set supervision, data acquisition, HDRI, camera tracking, scene layout and more. Swing by their site to see some of the great work they've done.

If your looking to outsource some integration work, I wouldn't hesitate to use these guys. In fact pick up the phone and call them now!

I'll be sitting down with these guys in the coming weeks to squeeze some good tips from them, and it would be "terrible" if this all happend over a pint of beer or two!

until then!

Andy

HD Lens to 35 mm equivalents

While on a shoot with an HD camera you will almost definetly here the question

"What is the 35mm equivalent of this lens?" Well you can do some fancy math... or use a handy cheat sheet from Panavision. Of course it's only accurate for Panavision lens but it give you a pretty good idea of what your seeing in a 35mm equivalent across the board.


You can down load the full PDF and a simular one here from Panavision's website. This was another gem from Les. He has a photo copy of a photo copy of this document and I did some digging with the help of Johnny B at work and found this on the web.

Enjoy!

The perfect tracking marker

Hey everyone,

Are you ever stuck with the problem of choosing the right tracker size for the shot? This tiny formula is useful since it works for every scenario. It takes into account the different parameters that can effect the camera angle and the number of pixels in the image.

This tracking marker tip comes from Francois Lord. Francois has been tracking since the 3d equalizer days and has spent a lot of time on set. He has devised a very creative and exact solution on how to calculate the perfect tracking marker size. This formula has been refined over the years by Francois and he was kind enough to share it with us. And now here is ....

The Flord Formula


Where :
T = Tracker size (in same units as D)
P = Prefered tracker size on screen (in pixels) between 5 and 10 pixels depending on the software you will use for tracking
B = Filmback (in millimeters) see table below for reference
D = Distance from camera (in units of your choice)
F = Focal length (in millimeters)
R = Horizontal Resolution (in pixels)

So... If you are shooting a scene with a 35mm camera and a 18mm lens, transferred to NTSC, and the background is at 3 meters from the camera... you can use the following formula...



...to find out that you need to put trackers that are 5 centimeters in diameter on the background to have them appear as 8 pixels big on screen.

Here is a list of different filmbacks that are often used in production. Please let me know if any of this information is inaccurate and I will update it.

Format Name	x size (mm)	y size (mm)
35mm (Super 35) - Full Aperture	24.89	18.67
35mm (Super 35) - 3-Perf	24.89	14.20
35mm (Super 35) - Transfered to HDTV	24.89	14.00
35mm - Academy	21.95	16.00
35mm - Slide	36.00	24.00
16mm - Standard	10.26	7.49
16mm - Super 16	12.52	7.42
70mm - IMAX	70.39	52.62
Video HDTV - 2/3" 16:9 sensor	9.59	5.39
Video HDTV - 1/3" 16:9 sensor	5.23*	2.94*
Video NTSC - 1/2" 4:3 sensor	6.40	4.80
Video NTSC - 1/3" 4:3 sensor	4.80	3.60
Video NTSC - 2/3" 4:3 sensor	8.80	6.60

* I'm not 100% sure about this one yet.

Again, thanks to the Francois. And if anyone is savvy enough... I smell a really good iphone app or java application.

until next time.

Andy

Good Iphone apps for VFX from VFXHack.com

In my daily search for the end of the internet I swung by one of my favorite sites to see what is new. And vfxhack.com had a great little blog entry about good iPhone apps for on set VFX guys.

They are Clinometer, HDRhelper, Jott, Dropbox, and Google apps. To see how he uses them... cruz on by www.vfxhack.com.

Simple Survey: Suunto Tandem Inclinometer

Once piece of equipment that isn't in my bag yet but is very handy is the Suunto Tandem compass & inclinometer.

This simple piece of equipment can do many things. It can help you figure out where the sun is going to be at a certain time of day. It is very handy when setting up for golden hour shoots. It can help you figure out how many degrees a camera pans. It can also help you align your directTV satellite dish!

The tip I'm going to talk about today is how to use an inclinometer to do some simple and quick outdoor set surveys. All you need is the inclinometer/compass, a laser tape measure, and some simple math.

This piece of equipment is sometimes referred to as a "sighting" compass because the compass/inclinometer is read by looking through an eyepiece on the side of the unit. There are two eye pieces and two floating dials on the device. For purposes of this tip we're just using the top dial, the inclinometer.

Hold the inclinometer vertically so that you're looking through the eyepiece with one eye. It takes some getting used to reading because you actually read the inclinometer's scale with one eye, and sight your target with the other eye. If you're using this for the first time it may be simpler to close one eye first, focus on the scale, then open your other eye and sight your target. You need to relax your gaze so that you are looking at both the scale and your target that you want to measure. This is the most difficult to master when using this device.

Hold the scale to your dominate eye and sight with the other eye. Then simply tilt your head up and down and the scale will tell you what degree of incline you are sighting.

Ok, now that we know how to use it, let's measure something. In the above image I want to measure/survey the wall height.

In this example A, D, M, N are values that you can easily measure. B, C are difficult to measure. The Wall Height = B (calculated side of our triangle) + D (distance of our eyesight from the ground).

The first measurement we will take is "M". "M" is the angle from your eye to the top of the building. A good way not to use a lot of math is to use the 45/45/90 deg triangle method. Sight the top of your target and move closer or further away until the angle of sight is 45 degrees. Using simple math principles we know that that "N" is also 45 degrees.

From this position sight down to zero degrees. This is the base of our right angle triangle. Now we calculate distance A by using our handy Hilti laser tape range finder, measure the distance from where you are standing to the "Zero Degree incline" line you just measured.

A = 25'5". Because in a 45/45/90 degree triangle A = B. We also know that B = 25'5"

Now we measure the D. D is how far your eye line is from the ground. In this case D = 5'4".

B + D = Wall Height
25'5" + 5'4" = 29'9"

There you have it. Our wall is approximately 29'9". Now of course this isn't EXACT but its close enough to help us set a scene scale or create tracking geometry for this building.

If you can't for use the 45/45/90 degree triangle method you can still derive the distance of B but you'll need to do a bit more math. I'm not a mathemagician so I use cheat sheets. A buddy of mine (Mike Romey) showed me these Quick study laminated charts. These reference charts are another great addition to the tracking kit.

I'd like to thank Les Ekker for showing me this tip. Les is a VFX Supervisor at Zoic Studios and is an encyclopedia of VFX knowledge. I'm sure we'll see many more tips from Les.

And thanks to Max for helping me out and posing for the reference pictures.

If you have any tips that you would like to share please email me at vfxtips(at)wilkoff(dot)net.