Lowpass, Highpass and how I use them

What is the difference between Darktable’s overlay and softlight blend modes?

Overlay adds contrast and softlight is a more gentle version. In this video tutorial I dive deeper into the questions I get asked most often about how I utilize Darktable's lowpass and highpass modules. I show different effects realized by combining them with overlay and softlight blend modes.

After the discussion and examples, I get to edit a contributor's RAW file. Thanks Sergei for the beautiful shot to work with.

Here is the link to the Darktable.org blog post "Using lowpass filter to recover shadows"

The RAW files used in this video are available here
Lava Steam
Flower
Vineyard

Complete Show Text

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Hi everybody and welcome to Weekly Edit.

I am going to talk about Lowpass Filters for a while.

We've had requests, and I'd like to nail down this subject.

Before I get started on that, I want to talk about Blend Modes.

There are two that I use a lot: I use Overlay and I use Softlight.

Overlay is a way of adding contrast to the way that you apply an effect, for instance a Gaussian Blur.

Softlight is a more gentle version of Overlay.

Let's look at Overlay.

This is a bunch of fuzzy dots that I made.

I want to use them to demonstrate some concepts.

First, let's look at a basic S-curve for Contrast.

It might be something like this.

Now, according to Wikipedia, an Overlay is an awful lot like an S-curve when you're applying an image to itself.

We'll take a Snapshot, turn that off, take the original image and apply it to itself with Overlay.

As we look at the two, aside from the color, they're very similar.

We've increased the Contrast.

This is what Wikipedia is talking about when they say we're increasing the Contrast.

Now, the Overlay method and the Softlight method don't increase the Contrast much right in the mid-tones.

The Softlight specifically protects shadows and highlights to a greater extent, too.

The areas where you're going to see more differentiation is between your mid-tones and your shadows and between your midtones and your highlights, with both of these.

What they do is take these middle values and push them toward the edges.

By doing so, they expand the contrast in these middle values and give you more information in those areas.

So, what effect does this give us? The first effect I want to show you is controlling highlights and shadows.

Let's imagine these darker parts are shadows and these brighter parts are highlights.

I'd like to bring the highlights down a little; I'd like to bring the shadows up a little.

One of the ways I can do that smoothly, without having halos around the edges or an uncomfortable transition zone is to use a Gaussian Blur.

A Gaussian Blur makes the whole image a little fuzzy.

The larger the radius, the more fuzzy it becomes.

If I make my radius pretty large and apply it with the Overlay method, and make my Contrast be zero, I can slowly bring my Contrast down.

As I do this, the darker parts get lighter and the lighter parts get darker.

I'll take a Snapshot and we'll take a look.

You can see that this part, which was lighter, is now darker than the original.

And this part that was dark, is now lighter than the original.

If I decrease my Contrast further, it will have even more effect.

So, the lighter parts are even more dark.

And the darker parts are even more light.

Notice how, because of the way the Gaussian Blur works, we don't lose our edge; we still have our contrast on our edge.

You can see this: it's going from a light to a dark in a transition zone here.

That's based on the size of this Gaussian Blur.

Look at where it's bright and where it gets dark.

Now I'll turn off the Overlay and we'll look at the original Blur.

You'll see it's the same size.

That's what's going on there.

We use that to control shadows and highlights.

Let's look at a real world example.

I have a shot here with a bright and a dark area.

Let's use the Lowpass Filter to fix that.

I'm going to take the Radius and turn it way up so that I'm not changing the tone in little areas, like in the clouds.

Nice and big.

Now I've got a general dark area and a general light area.

So if I take my Contrast and bring it down to zero, like we did with the dots -- once again, we're using our Overlay method -- then, if I take my Contrast and turn it negative, you'll see that the darker parts of the image got brighter and the brighter parts got darker.

Here's a great place to read about using a Lowpass Filter to recover shadows.

It's an article from a few years ago, but it's an excellent article.

It's on Darktable's site.

They talk about using the Overlay method and reversing the Contrast, and the size of the Gaussian Blur, and all the cool stuff.

It's a great article: lots of fun.

I put a link to it in the show notes.

The next thing I'd like to show you is Tones.

I'm using the same module: the Lowpass Filter with the Gaussian Blur, but I'm going to use it in a different way.

Instead of turning my Contrast negative and using an Overlay, I'm going to use Softlight, which is a more gentle version of the Overlay.

Let's see what it does to our fuzzy dots.

Take our Saturation down to zero.

Take our Radius and turn it up a little.

Now we've got rounded dots.

If we apply this with Softlight, we get a dome effect here.

I'll turn my Contrast all the way up and my Brightness all the way down so you can really see this.

It goes from darker in the middle to a little bit lighter on the outsides.

That gives us a tonal effect, and a more 3-dimensional look because the shading is more continuous instead of just going --BAM!-- from one color to another.

If I change this Radius -- if I make it larger, it looks even more like a little ball.

If I make it smaller, the transition is sharper.

How does this affect our real-world example? Let's give that a shot.

Here's our image.

We equalized the shadow and the highlight.

Now we're going to add a Lowpass Filter with the Softlight.

So, Saturation down to zero again.

Make the Radius be such that the clouds are the right size.

I want them to look like little balls, like the dots in the example did.

I'll change my Radius to achieve the nice, smooth shading.

Look at this: I don't have much distance between my lighter and my darker tones here.

In order to get more effect, I want my tonal range in the Histogram to be closer to this midpoint.

I'll fix that right now.

I'm adjusting my Radius now to give nice, billowing clouds with the shading.

If I apply this with the Softlight...

Here's our original.

It looks a little flat.

Here's our new version.

See, we've got some shading here, and we've got more 3-dimensionality here and up here.

I'll take a Snapshot.

There's our original on the bottom, and with the shading on top, there's our new one.

Before, and after.

Let's try that with a flower, too.

We'll do the same thing.Take our Lowpass Filter.

Saturation down to zero.

Take our Radius and adjust it to get the right size dots here.

We want these drops to show up really well.

I'll set my Radius so they show up real well.

If I go too far, I'm looking at the individual pores on the leaf.

I don't want that.

If I go too large, I don't see the individual drops.

I'll get to just the right size where I've got just the drops.

Now, I'm going to apply this Gaussian Blur with the Softlight Filter.

That's what that looks like.

Without it, and with it.

We'll take a Snapshot and look at that closely.

This is our original on the bottom, and this is our new version on top.

See, it's adding contrast over a distance.

Now, we're going to look at applying tones with the Highpass Filter.

It's a lot like the Lowpass Filter, but it works on finer-grained details.

If I take my Sharpness and turn it all the way up, you'll see I get some nice shading.

I've got some lightness in here going out to darkness.

I've got some nice shading around the outside of these.

If I apply that with the Softlight, it gives me a lot of the same effect that I got with the Lowpass Filter.

So, I'll take my Lowpass filter and work with it, take my Radius all the way down to about seven or eight pixels, and then I'll switch over and do the finer details with the Highpass Filter.

I'll show you a trick with the Highpass Filter and the Softlight.

Let's look at a real-world example here.

The finer details I want to bring out are all in these brighter parts of the image.

Let's try our trick with the Highpass Filter.

We'll bring the Sharpness way up.

Then we'll bring our Contrast up until we get the shading we want.

That looks great in here.

Now, when I apply that with the Softlight, it gives me a lot of detail in these highlights.

But, it makes the darks too contrasty.

So, the trick is to apply it with a Parametric Mask based on the L Channel.

Voila: our contrast isn't too high and we've got these beautiful details in our highlights.

Let's look at before and after.

This is what we started with.

Concentrate on these brighter areas and the detail in them.

See how that brings it out? This is before.

This is a good place to look, right in here.

And after.

Now, these halos around the edge: they're too much for me.

That usually happens, so I'll bring my amount down a little bit to get rid of them.

Sometimes you have to bring the Sharpness down a little too: whatever it takes to get rid of the halos.

There we go; those are gone.

Let's see if we still have a nice effect though.

Before, and after.

Oh, yeah, especially right in here.

And down here.

It adds a lot.

Let's look at it on the flower too.

Use our Highpass Filter.

Bring our Contrast way up to get some of this nice shading here.

Bring our Contrast up, but not so much that we get halos.

We'll apply that with Softlight.

Here's our before, and our after.

Now, see, it's awful crunchy in these darker pixels.

So, I'm going to use a Parametric Mask based on the L Channel.

There we go: our drops look more 3-dimensional and we don't look crunchy.

Okay, we've got two more things to talk about.

One is separating things from other things.

I like to do that with the Highpass Filter and the Overlay Blend Mode.

I pick a Radius that determines the general shape of what I want.

Like, these circles are about that size.

Don't forget to take your Saturation down to zero.

When I apply this with Overlay, it gives me more contrast than I had to start with.

That makes these separate from the background.

Let's see what that looks like on this.

I would like to separate this smoke from this background.

Take our Lowpass.

Turn off our Saturation.

Adjust the Radius of our Gaussian Blur so that I'm seeing the general shape that I want here and here.

Then apply this with an Overlay mode.

Now, that really takes this smoke and separates it from the background.

As a matter of fact, it does it too much on this side.

So, once again, we use our heads and decide how to apply the effect.

I'm going to use a Drawn Mask so that a little bit of it gets used on the left, but more of it gets used as we go to the right.

There we go.

Before, and after.

It takes these large shapes and separates them from the background.

Let's try that with the flower too.

I'd like to separate this general flower from this background with the green.

We'll bring up a Lowpass.

Turn our Saturation down to zero.

Make our Radius large enough that we're just seeing the general outline of the flower.

I think that's about as good as I can do.

That gives me a little bit of separation down here and over here, and a little bit up here, but not much.

Okay, Overlay method.

Now, let's look at our original.

And let's look at our new one.

See how separate we are from the background now? The last thing I wanted to talk about is edges.

The reason I started with fuzzy circles is so that we could talk about edges.

I'm going to zoom in on this fuzzy circle.

These Highpass Filters: if I turn them all the way down, I can get JUST the edge of the circle.

Oops, too far.

Take my Contrast and turn it down.

Now, I've got just this border.

Overlay was the more contrasty method, so that's what I'm going to use.

I'll take Overlay and apply it to give me a nice sharp edge.

Where it was soft...

...and now it's sharper.

Let's see it in a real-world example.

Let's try and define our edges on the edge of the cloud.

We'll zoom in here and look.

We'll start up another Highpass Filter.

Take our Sharpness and go all the way down, so that we just have these edges that we want to enhance.

Bring our Contrast down -- like that.

Overlay method.

Before, and after.

It's almost like a sharpening filter, isn't it? We'll turn that off.

Take a Snapshot, and look at the difference.

On top is after; on the bottom is before.

As you can see, it makes such a difference that it's too much.

We actually have to turn it down some.

I usually end up using about half as much.

That's what half as much looks like.

So, here's where we started on this image, and here's where we ended, just by using Highpass and Lowpass Filters.

Let's see where our flower is at.

We're going to try and get edges around these water drops.

See how they're a little soft? The water drops are nice and rounded, but we don't have sharp edges.

Okay, Highpass Filter: build a new one.

Take our Sharpness and turn it down so that we have those edges.

We don't want to turn it down so far that we lose the edges.

There we go.

Take my Contrast and turn it down so I just have those edges.

Now I'll apply this with Overlay.

Here's what we got.

Once again, we got a little too much; we're a little crispy.

I'll turn it down.

Here's our before.

Here's our after.

It sharpened it up and gave us nicer edges.

We'll zoom in for a look.

Turn it off.

Take a Snapshot.

And turn it on.

Our edge is a little soft, and doesn't have much contrast.

After this effect, our contrast is increased and we have more delineation.

Before, and after.

From here, to here, just by using Lowpass and Highpass Filters.

I hope that answers some questions.

Now it's time for an edit.

This week, for Edit my RAW, Sergei from France sent us what looks like a vineyard picture with some mountains and some clouds.

To start, on this picture, I want to drive the highlights and shadows toward the mid-tones to get something to work with.

First, we'll look at our Input Color Matrix.

Let's see: Standard, Enhanced -- I don't see much difference there.

SRGB -- nope.

And Adobe -- nope.

Okay, we'll go with Enhanced.

That's a good starting point.

Next: Demosaic-ing.

Let's see if that makes any difference.

Starting with PPG...

Oh, boy, that Amaze looks better, doesn't it? And VNG...

that looks good too.

I'll have to take a Snapshot and take a look.

It's hard to tell which looks better: the VNG or the Amaze.

I like the Amaze better (on the right).

The VNG is a little soft in here, and you can see the Amaze is a little sharper.

Yes, definitely with the Amaze.

Okay, we've got Demosaic-ing; we've got our Input Color Profile.

We'll drive these shadows and highlights towards our mid-tones.

Let's try the Shadow and Highlights.

Hmmm...

doesn't do much for these deep shadows, and it's affecting all this area here too.

So, let's Compress it, so it's just affecting our deeper shadows.

Boy it isn't doing much, is it? We can go like that, and turn our Radius down.

See, our Radius is so big, it's got giant halos now.

Bring our Radius down.

Bring our Shadows to there...

Oh, we've still got halos everywhere.

This is terrible.

Okay: NOT the Shadow and Highlights slider; let's look at Tone Mapping.

Bring our Compression down.

Boy, that looks a lot better through here.

It does a terrible job on these highlights, though.

Maybe we can just get the darker places with a Parametric Mask.

First I'm going to turn way down the Spatial Extent to give it a more natural look.

That looks better.

Let's see if we can get rid of its effect here.

Then we'll reverse this mask.

Oh, I don't want this area to be affected; just the tree.

We'll start up here.

Alright, let's see what that looks like.

Wow, that really lightened it up.

Let's do a Blend -- maybe 50% Okay So, this is before and after.

That might even be a little strong; we can come back a little from 50%.

Let's come back to 34%.

Yes.

Okay, that helps.

We still have dark shadows, and our clouds don't have much contrast.

Also, there's not much contrast in the mountains here.

Well, there's a Gamma correction called Unbreak Input Profile.

Let's give that a shot.

Now, the way the Unbreak Input Profile works is that it's got a linear setting.

That determines where...

here, I'll show you with a Tone Curve.

It's like this: if I bring this Gamma up like this; if I bring it farther to the left, that's like that linearity, and then the distance up I go is the Gamma value.

So, let's give that a shot.

Well, that helps some.

We certainly have more data towards our mid-points now.

Let's look at our History and see how far we've come.

Alright, most of our data was over here, and now we look a lot more mid-tone-y.

Good.

I want to increase the contrast on the mountain and in the sky.

I'll try creating an L Channel with more contrast.

I'll go to the Channel Mixer, go to Lightness.

I'll take a Snapshot so I can compare, and I'll bring up my Blue.

That blue does not show much contrast; as a matter of fact, it decreases the contrast.

Okay: no help.

The Green? A little more contrast, but not much.

Let's see what the Red does.

Oh, now we're talking.

The clouds look more distinct and so does the mountain, but it makes the green on the bottom look unrealistic.

I'll turn the Green a little bit negative so it darkens the greens, and bring the Red up just a little.

Now I've got more contrast in the mountain and up here.

It still looks unrealistic down here and terrible here.

We'll try applying it with an L Mask.

That certainly looks better.

I want to still bring out the shadows here more and knock down my highlights more.

I'll use a Lowpass Filter.

Saturation down to zero.

Bring my Radius up, maybe like that.

Overlay.

And bring our Contrast negative.

That made our shadows brighter and knocked down the highlights.

Let's see what we got.

I don't see any halos, so I think we're good.

What color do we have in our whites here? Not quite white.

It looks like it's magenta and yellow.

We'll fix that.

Color Balance.

Gamma.

Yellow.

Move it towards the Blue.

As I do, I'm watching this B Channel go down.

Okay, there's my B Channel going down, and I've got way too much magenta, so I'll move that towards the Green.

There we go: I'm almost at zero and zero.

My whites look better now.

I've got a big black dot here.

Maybe it's a spot on the sensor? Well: Spot Removal.

No, that didn't match; it's too dark.

That's too light.

Oh, this is a tricky one.

I'm going to Blend it, and use only Lighten.

I can still see it.

Oh, that's frustrating.

Okay, Spot Removal, and bring this down.

Boy, that is a tricky spot.

Okay, I still don't like it; I can still see a circle there.

Oh, I think I'm finally happy.

I'll Export this and start working with tones.

I would like for these clouds to have more definition; I want this mountain to have more definition; I want to see details in the leaves of the vineyard, and I'd like to see this rock better.

I'd like to see this road delineated more, and I want to see more detail in these rocks.

We'll start with the larger objects first, so we'll start with the road and the mountain first, and then we'll work our way down to the clouds and then the finer details like the leaves here and the rocks here.

So, Lowpass.

Bring our Contrast down a little, our Brightness up a little, Saturation down to zero, bring our Radius up until we can see the road and the rock.

Nice big features.

And the mountain.

Can we add a little tone to that mountain? A little bit, not much.

So, now I can see the rock and the road and the mountain, but I'm not lost in the details of the leaves and the rocks down here.

I'll apply that with the Softlight Blend Mode and use the Opacity.

That looks better.

Okay, one more time.

Go to finer and finer details each time.

I'm looking for more detail in the mountain and more detail in the clouds.

I'm having a tough time with these clouds, so before I go too far, I'll use my Tone Control to create more distance between the lighter and darker parts of the clouds.

So, here's a lighter part.

I'll bring that up just a little.

Then here's a darker part of the cloud.

We'll bring this down a little, give us a little more to work with.

This is making the tree awful dark, so I'll exclude that with the Parametric Mask.

Apply this with the Softlight and use my Opacity to determine how much gets used.

Okay, that looks good.

Now, the finer details.

The finer details, instead of using a Lowpass Filter, I'm going to use the Highpass Filter with the Softlight Blend Mode.

There we go.

I'll apply this with a Parametric Mask based on the L Channel and a Softlight Blend Mode.

Let's see what that looks like.

It certainly adds more contrast up here.

Let's see what it looks like in the rest of the photo.

Before, and after.

It's making the tree dark.

I'll exclude it from the darkest parts of the tree.

Turn on my Mask Indicator, and that excludes these darkest parts.

I want to add a little more color, but I don't want to add it everywhere.

I'll steepen my A and B Channels and use my Chromacity Channel to apply the steeper color channels to the areas of the image that have less color saturation to start with.

I'll first look at my A and B Channels, and bring these up to...

..about there...

Parametric Mask.

C.

Now, there's way too much of it in here and in here and over here, so let's turn on our Mask Indicator and bring our high end down until we start excluding the parts we don't want.

There we go.

Then we'll bring the upper limit all the way down to zero.

Now it's applying more of the effect, the steepening of the A and B Channels, to the areas that have the least color.

One of the nice things this does is reduce color noise a bit.

See, the areas that have more color are getting less of the effect.

Let's see what that looks like.

This is before, and this is after.

Before, and after.

Good.

It makes the road a little more red and doesn't get us out of gamut down here.

I'll save this and start over once again.

Here's our new version.

We're starting again with a zero on the History.

First, I'll add more contrast in the finer details.

Let's find some edges.

Use the Highpass filter and bring the Sharpness all the way down until we just find the edges we want.

Bring our Contrast down too.

That looks pretty good.

I'll apply this with the Overlay Blend Mode.

I'll have to attenuate it; it's usually too much.

There we go: right about like that.

Here's our before and our after.

It seems to sharpen things up; it finds edges for us.

That's starting to look good.

Let's see if we can sharpen things up more in here.

I'll use the Equalizer.

Well, that helps a little.

That helps more.

And that doesn't do much.

Okay, so we'll come up a little here and a little here.

That's on the L Channel; I'm not going to mess with the C Channel.

That looks pretty good.

It looks a little crunchy down here, so I'll mostly apply it up here.

That looks better.

This looks dark on the bottom.

I'll fix that before I carry on.

Oh, that looks more natural.

We want some color in here.

I want the road to be more red and the greens to pop more.

I don't want to enhance the blue.

How can I do that? Well, let's find out what these colors actually are, and then enhance them.

So, it's this much Magenta and this much Blue.

I'll pull up the high end.

There's my Blue.

And there's my Magenta.

That gives a little more contrast on the green and the red: nice.

It didn't touch our Blue too much, but it did some.

Yeah, that's already looking a little ballistic there.

I'll take our Blue down a little.

There we go.

Good.

Now what? I want more deeper saturation down here.

I'll use a Drawn Mask and a Parametric Mask combined.

I'll reverse the Drawn Mask so it's just down here.

I'll take my L Channel and use that as a mask also, and apply this with a Subtract method.

I won't need much; just a little.

There.

Here's before, and here's after.

Adds a little saturation.

I'd like more contrast in the mountain here and in the clouds, and I'd like to bring out the shadows more in the tree.

Also, I need to bring my White Point up and my Black Point down.

First, let's see what color is on the mountain.

The darkest parts are here and the lightest parts are here.

That makes it tough because I don't want to reduce contrast in here, and that's in the same area.

I might be stuck with the mountain.

I do have the ability to separate by where it is, so maybe I can get a little bit of an effect.

We'll bring this down.

We'll create a little separation in the mountain foliage itself.

I'll bring this down, there we go, and bring that up.

What does that look like? Get rid of that.

I think that helps.

Then, in the clouds, I want more separation.

I want more separation in the blue in general.

So, Parametric Mask based on the amount of blue.

I'll take my Eyedropper Tool, use Area, and look at the entire image.

So, my blue goes from here to here.

This is the least amount of blue, and this is the most amount of blue.

I want to bring down my blue level, so I want to apply most of this to the most blue.

I'll bring this slider over to the most blue there is in the picture, and I'll bring this slider over to where the least blue in the picture is, and then I'll create more contrast.

I'm bringing this over, watching my Black Point to make sure I don't clip, and bringing this end up, also watching my Black Point.

I don't want to get out of gamut here.

Look, I'm starting to get a little Magenta.

I've got to come back.

There we go.

Now it looks nice and white.

Before, and after.

Okay, and I want to bring up a little bit in these shadows.

Let's do that.

Keep our Black Point where it is and bring this up a little.

We'll just work on the Gamma for the whole image with an emphasis on these tones.

Ah, this got too bright in here.

Eyedropper.

And: too much.

Alright, Sergei; I hope you like it.

Thank you for sending it in.

You win the prize this week.

I will send you an autographed photo.

Everybody, have a great week; I'll see you next week.

Thanks so much.

Don't forget to send in your RAWs for me to edit.

Next week, we'll look at a different topic.

Bye, everybody.

11 thoughts on “Lowpass, Highpass and how I use them

  1. Harry, I just wanted to say I watched this video 3 times, understanding a bit more each time. I think I now have it in my brain just what these filters are doing and how to use them to best effect, so THANK YOU VERY MUCH!! 🙂

    • Wow. Thank you Les. That is an immense investment of time on your part; I’m honored. Glad it helped.
      –Harry

    • Aloha Mathieu,

      The lowpass, shadows/highlights, and equalizer all use similar methods. So you’re right about them being similar. But I find the lowpass method easier to visualize where the effects are happen. Also it’s gives a little separation with defocusing some elements vs others. But sometimes I don’t want that effect and I’ll use the shadows/highlights or equalizer. This would be a great idea for a video topic, comparing the effects of each.

      Thank you so much for writing,
      Harry

  2. Pingback: Paso bajo y paso alto con Harry Durgin - Apratizando

  3. I’ve created some notes based on this video and I’m sharing below to help others.

    “Overlay adds contrast and softlight is a more gentle version.”

    Overlay takes more effect in the mid tones (specially between the mid tones and the light tones, and between the mid tones and dark tones).
    Softlight protects the light and dark tones, taking effect on the mid tones.

    – Lowpass Filter

    – Recover Shadows (darker parts -> brither / lighter parts -> darker):
    – Overlay
    – Radius: any (control the transition zone)
    – Contrast: negative (control the effect)
    – Brightness: 0
    – Saturation: 0

    – Apply tones (more 3D look, less flat / shades more continuous):
    – Softlight
    – Radius: any (less -> sharper / more -> more continuous transitions) (adjust just enough to see the wanted details)
    – Contrast: positive (control the effect)
    – Brightness: 0
    – Saturation: 0

    – Separate things (more contrast / separate from background):
    – Overlay
    – Radius: any (adjust to large enough to see general only the shape of objects)
    – Contrast: 1
    – Brightness: 0
    – Saturation: 0

    – Highpass Filter

    – Apply tones (same as lowpass, but works on fine grain details / bring out fine details):
    – Softlight
    – Sharpness: 100% or lower
    – Contrat boost: any (control the shade)
    – Opacity: control the effect / remove hallos in the edges
    – Parametric mask: more effect on highlights

    – Sharpen edges:
    – Overlay
    – Sharpness: any (adjust to low enough to see only the edges of objects)
    – Contrat boost: any (same as Sharpness)
    – Opacity: control the effect

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