>> Welcome to another
Unreal Tips and Tricks. In this episode we'll discuss
the exposure controls available
in Unreal Engine, which will allow us to adjust
exposure using settings that accurately
mimic real-world cameras. Before we dive
into those settings, let's take a quick look at how
exposure values are determined. Exposure is simply
the amount of time that light is allowed to hit the sensor
or film back of the camera and it controls how dark
or bright the final image is. Exposure is calculated
using two values, shutter speed and f-stop
or aperture diameter. Shutter speed is simply
the length of time in seconds that the shutter stays open. The longer it's open,
the brighter the image will be, but it will also produce
more motion blur. F-stop determines how wide
the aperture is opened. The wider the aperture,
the more light is allowed in, and thus again, a brighter image
will be produced. Keep in mind
that lower f-stop values represent wider
aperture openings, how wide the aperture
is open will also affect the depth of field
of your images and finally, ISO represents the sensitivity
of your film or sensor. Most cameras have a base
ISO of 100 and this is usually
the best setting to use. Higher ISO values
can brighten the image, but they'll also cause
more film grain to be noticed. Now that we've got a basic
understanding of exposure, let's jump into
Unreal Engine. The first thing we need to do
is take a look at the Exposure Settings
in our Project Settings. You'll find these
under Engine, Rendering. Then scroll down
to the Default rollout. First up, we have Auto Exposure
or eye adaptation controls. We can enable or disable
Auto Exposure and change the method used
for the entire project here. Below this is an option
that allows us to extend
the range of luminance available for the minimum
and maximum exposure values. And finally, Pre-Exposure will
apply exposure in the shaders before the scene color
is written. This option can help improve the quality of basic
auto exposure. Changing these options will
require a restart of the Engine and once it's restarted,
shaders will likely need to be recompiled
for the project, which can take
a bit of time. To use the exposure controls
we'll either need a Camera or a Post-Process Volume
added to the scene. In this example, we'll be working
with a post-process volume. Be sure to set it to Unbound
so that the effect is applied whether we are inside
the Volume or not. We'll find the exposure settings on the volume
under the Lens rollout. We'll open both Exposure
and Camera as we'll be working with both.
Before we do though, let's take a look at
the viewport exposure controls. You'll find these on
the viewport view modes button at the bottom under exposure. Game Settings tells the viewport
to use the exposure settings as defined in the post-process
volume or camera being used. Turning this off will instead allow us to tweak the exposure
value using a simple slider. Keep in mind that this is only a
preview option for the viewport. The setting of EV100 refers
to an exposure value using an ISO or film
sensitivity of 100. The lower the exposure value,
the brighter the image will be. Conversely, higher
EVs produced darker images. Remember the EV represents
the F-stop and shutter speed. An EV of zero represents
an F-stop of one or a wide-open aperture and a shutter speed
of one second, producing a bright image
as more light is allowed in over
a longer period. Some other example of EVs
include an EV of five to seven for
a typical home interior, seven or eight
for a typical office space, and finally 15 for
a typical outdoor sunny day. Let's go ahead and return
to Game Settings now and discuss the options
available to us in the Post-Process Volume. Under Exposure, the first option
we have is Metering Mode. This gives us a drop down
with three different selections. The first one
is Auto Exposure Histogram. The Auto Exposure
Histogram option gives us access to a lot of advanced settings allowing us to really fine
tune the exposure here. A histogram essentially
is just a bar graph that represents
the frequency of values in this case exposure values. Below this we have
Auto Exposure Basic. Selecting it is going to
turn off the advanced options and allows for a quicker,
simpler way to adjust exposure. And then finally we have Manual which will disable
all of these settings and just give us controls
using the cameras, shutter speed, ISO, and f-stop. And as you can see
at the current settings, the scene gets very dark.
Let's go ahead and jump back up however to the Auto
Exposure Histogram. Next up, we have Exposure
Compensation. We have a text entry field as well as Exposure
Compensation Curve here. The curve is beyond the scope
of this tips and tricks so we're going to skip
over that for now, but the Exposure
Compensation field here will allow us to use
a logarithmic adjustment to tweak the exposure values. Of course, generally you want
to use this setting last, but for now we're going
to go ahead and look at that, how it works. If I put in a value
of negative one, that's going to make
the scene twice as dark because remember
this is logarithmic. For instance, if I put
in a value of negative two then the scene is now
four times darker than it was and conversely,
putting in a value of one; positive one, is going to make
the scene twice as bright, but we'll leave that
at a value of zero for now and use no compensation. Next step we have the Min
and Max EV100 values. Now normally these are Min
and Max Brightness, but remember at the beginning we went
into the Project Settings and we enabled the Extend
Default Luminance Range option which changes these options
to Min and Max EV100. For a minimum EV100 we want to find a dark area
such as this closet and it will adjust
this value here. You can see as I bring
the value up higher, the scene will get darker,
and for controlling max EV100, you'll want to find
a bright area and it adjust it from here. And you can see
as I bring this value down, the scene is going to get
much brighter. In the Unreal documentation, it recommends keeping
these values close to zero
for the best effect. And generally, you want to keep
them in the positive range. Next up we have Speed Up
and Speed Down. These control how long it takes
the camera to adjust when moving from a bright area
to a dark area and vice versa. Speed Up controls moving from
a dark area to a light area and generally that takes longer
for your eye to adjust. So, this takes three seconds,
and then Speed Down controls moving from a light area
to a dark area. Let's look at
the visualization tool we have available to us
in the viewport. If we go up to the
Show button here and then go down to Visualize, we can enable
HDR Eye Adaptation. This is going to give us
the information that we're currently using here
in our Post-Process Volume as far as all
the exposure settings, we also have a picture
in picture here showing us a color-coded
representation of the exposure values.
Green represents mid-tone values and then blue represents
your darker values. We don't have any red values
in the scene here, but if we did, they would
represent the brightest values. Anything overblown basically. And then below
this we have a histogram. Remember a histogram
is just a bar graph that represents the frequency
of certain values and in this case those values
are our exposure values. So, on the far left here we have
an exposure value of -10. That is our minimum
exposure value currently defined in the Advanced options
for our auto exposure histogram. If you look down here, you'll
see Histogram Min EV100 -10. So, if we change this to say -5, you can see now the histogram
has been adjusted. Likewise, on the top here, if we unselect
our Post-Process Volume, we'll be able to see that
it is set to a value of 20. We'll go back to our
Post-Process Volume and Histogram Max EV100 we'll change that
to a value of 10. Now in the center here,
this white line represents our current exposure value
and of course the bar graph here represents all the different
exposure values in the scene. So, as we move
our viewport around, we can see those bars
are changing based on the information
being displayed at the time. So, at the current Speed Up
and Speed Down settings, if I look towards a darker area, you can see how long it takes
for the adjustment to occur and to bring us to
the current level of exposure. And if I go back
to a brighter area, you can see it happens
much quicker. So, to change that, I'm going to
reduce the Speed Down to a much lower value here, to 0.1, which is going to adjust
that speed by a factor of 10. And so, if I look
at the dark area now you can see it's taking a lot longer,
10 times as long to adjust
the relative exposure to match the exposure
that it should be at. Under Advanced we'll also
find Low and High Percent. These two values represent
a range of luminance values taken from the scene
or from the histogram. These values are then
averaged together to determine the current
luminance value. The Unreal documentation
generally recommends keeping these between 70 and 90% so we'll go ahead and leave
these at the default values. Put simply, the auto exposure
system works by first determining
the average luminance value using the low
and high percentage. From here, the eye adaptation
takes place using that current luminance value, and the eye adaptation
will be clamped between the Min
and Max EV100 values. Let's look now at using
manual exposure controls. Remember when we do that using
the current camera settings, the scene's going
to get very dark. Let's go ahead and tweak
those settings now. First of all, we'll go ahead and set our Shutter Speed
to one second and we'll change our F-stop
to a value of one. Remember having an f-stop of one and a shutter speed
of one second at ESA 100 gives us
an exposure value of zero. But what if we want to mimic
a typical sunny day? Well, this brings in the sunny
16 rule from photography. With the sunny 16 rule, you can essentially mimic
a sunny day in your photography, whether you have
a light meter or not. The way we represent
the sunny 16 rule in Unreal is to set our Shutter Speed
to a value of 100, our ISO should be at 100 and our F-stop
should be at 15. Of course, this returns our
scene to a very dark scene and that's because our lights
now need to be tweaked to match these camera settings. Before we go any further,
however, it's important to understand that when working
with manual exposure to mimic a real-world
cameras exposure, the settings we'll be using
will not work in all cases. There are some systems in Unreal
such as Fog Actors or Lightmass that simply won't respond correctly to real world
camera values. The settings I'm going to be
using will work in this case. Just keep that in mind
as you may not be able to use these real-world values
in your particular scene. So, we'll look
for our sunlight here and that's going to be
a Directional Light and you're going to notice
the Intensity is currently set
to a value of 8 lux. Now that is way too low
for a sun in a physically
accurate representation for our exposure settings. With an exposure value of 15 the
typical illuminance of the scene should be about 82,000 lux, so we're going to go ahead
and change our sunlight. Remember that value
is coming all from the sun. We're going to change
that to 82,000 and you'll see when we do that,
that brings our lights back in. Now remember, using real world
values like this for the sun
may cause Lightmass issues when baking Lightmaps
or other potential problems. So again,
it works fine in this scene, but your mileage may vary. And with a quick Lightmap bake,
we can see that the lighting is now beginning
to bounce around the room and lighten everything up and everything looks like
it's at the proper exposure. In fact, if we go
to our view mode options again and we turn off Game Settings,
we're going to see that at an exposure value of one,
the scene is much too bright. However, if we match up
the exposure value from our Post-Process Volume, we'll see that we get a much
closer representation of the same exposure value that we have
when we baked our Lightmap. Alright, so that's going
to do it for this episode. We hope you enjoyed watching it and we'll see you next time
on Unreal Tips and Tricks.
