Lanenga Curves Explainer Part 1

7/25/2019 Lanenga Curves Explainer Part 1

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Basic c !urves Explainer Part 1Lanenga - 2016

This explainer is meant to be a basic intro into how Curves in Photoshop work

and how to use them.

In order to understand why certain things work the way they do, I will do a littletechnical explaining first. You could skip this part and go straight into how touse Curves, but I would recommend trying to read and fully understand thislittle technical aspect as well as it will make understanding everything elsethat much easier.


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-- Begin technical intro --

In order demonstrate how Curves work in Photoshop I will use a 24 bits fullcolor image in RGB mode.

A 24bits full color RGB image is an image made up of three 8 bits colorchannels. A red channel, a green channel and a blue channel.Each channel can contain 8 bits of information. And since a single bit can

have two values, 8 bits comes down to 256 different possible values.2^8 = 256. And since 0(zero) is also a value, the possible values go from0 to 255, making a total of 256 different values.

Another important thing to understand is the difference of additive color mixingversus subtractive color mixing.

According to Wikipedia:"Additive color is color created by mixing a number of different light colors,

with Red, green, and blue being the primary colors...[]

Additive color is in contrast to subtractive color, in which colors are created by

subtracting (absorbing) parts of the spectrum of light..."

What this means is that when you are working with light, the three primarycolors are Red, Green and Blue.

As you might know, white light is comprised of the entire color spectrum.

Additive color mixing is mixing by adding colors together. And adding thesethree primary colors of light together will result in white light.Black is obtained by the absence of light.

Subtractive color mixing is mixing colors by remove colors from the lightspectrum. We know this from printing and painting back in kindergarten.Pigments or dyes are materials that absorb certain color frequencies and onlybounce back part of the light spectrum.A Red dye is a substance that absorbs all frequencies of white light and onlyreflects the Red frequencies.

A Green dye is a substance that absorbs all the frequencies and only reflectsthe Green frequencies.

White is achieved by reflecting the entire light spectrum back.And black is achieved by absorbing the entire light spectrum.

In subtractive color mixing the three primary colors are Cyan, Magenta and

Yellow. By adding these colors together, they will absorb more and more of

the light spectrum (subtraction) and eventually result in black (or near black).

As you might have noticed, this is where RGB and CMYK come from.RGB are the Red, Green and Blue of additive color mixing.


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CMYK are the Cyan, Magenta, Yellow and Black of subtractive color mixing.

The K in CMYK stands for Key. In traditional printing a fourth "color" is usedwhere no true blacks can be achieved by mixing Yellow, Magenta and Cyan.

So when you are working with light sources, you are doing additive colormixing.

When you work with pigments and dyes (inks, paints etc.) you are doingsubtractive color mixing.

And since a computer monitor is basically one big light with thousands of lightsources (the pixels) this is why we'll be working in RGB mode.We are adding light sources together in order to create different colors.

In digital imagery, there is also something called an Alpha channel.An Alpha channel is basically a mask.

According to Wikipedia:"An alpha channel is an additional channel that defines areas of

transparency..."

Alpha channels store how pixels are supposed to be blended or merged when

you lay them on top of each other.

-- End technical intro --


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photo: Wouter van der Linden. http://www.woutervanderlinde.com

In a 24bits image, the image is made of three 8 bits channels. Red, Greenand Blue.

The Red channel contains all the red in the image.The Blue channel contains all the blue in the image.And the Green channel contains all the Green in the image.

Each pixel in any of these three channels can have a value of 0-255.This value is the brightness of the pixel.


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If you look at the Channels panel in Photoshop, you will see just that.

Three different channels and a fourth RGB channel, which is a combination ofall three channels, added together.


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Because an image is made up of these three color channels, it is also

possible to break the image down into these three components.


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Method 1 uses solid color layers with layer masks applied.

The solid colors represent the three primary colors and the layer masksrepresent the corresponding (alpha) channels.

Method 2 looks more like the actual channels panel.

It is the same as Method 1 with the difference that the layer masks are appliedto the solid colors and merged onto a black backdrop.

In both breakdown methods the layers are added together using the LinearDodge (Add) blending mode.

A fourth black layer is needed in additive color mixing as a backdrop. Thisrepresents where no light is emitting.


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Now that we have a visual image of how an RGB images is made up, we can

start to look at how Curves work.

The Curves filter is a mapping filter. It will let you map a given input value to aspecific output value. I will explain what this means and how to do this here.

If you look at the Curves panel window you will see several things.

At the heart of the Curves panel is a square image containing a histogramwith a raster overlay and a diagonal line going from bottom left to top right.

Underneath this histogram is a gradient going from black to white from left toright.


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And to the left of the histogram is a gradient going from black to white from

bottom to top.

On the left of the histogram are several icons buttons.

Right above the histogram are two dropdown input fields. And an 'Auto'button.

As you can see in the image of the Curves panel, the top most dropdown fieldsays 'Default' and to the left is a label saying 'Preset:'.This is a presets dropdown. Here you can select previously stored Curvesadjustments. We will not cover this in this explainer.

Underneath the Presets dropdown is another dropdown field that says 'RGB'.This dropdown is the Channels dropdown and corresponds to the channels in

the Channels panel.

Right now we have the RGB channel selected and the histogram is showing ahistogram of the RGB channel. But we could use this dropdown to switch toanother channel. Each channel will show a corresponding histogram in thehistogram area. We will be using this dropdown more in this explainer.

The 'Auto' button is a button that will let you apply a Photoshop algorithm that

tries to autocorrect your image. We are not going to use this button and it isprobably best you never use this button at all. After this explainer there should

also be no need to use this button. Ever.


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As mentioned, at the heart of the Curves panel is a histogram.

According to Wikipedia:"A histogram is a graphical representation of the distribution of numerical

data."

What this means in Photoshop is that the Histogram shows you the amount ofpixels with a given brightness in your image.

The histogram is basically a bar graph showing the amount of pixels at agiven brightness in your image. If you look at the bottom of the histogram, youwill see a black and white gradient. The black of the gradient represents thedarkest value in your image. 0 (zero) value, no brightness.The white of the gradient represents the brightest value in your image. 255value, maximum brightness. One look at the histogram should give you an

idea of distribution of dark (blacks) and light (whites) in your image.

As mentioned before, the Curves filter is a mapping filter.What this means is that the histogram is a representation of your image at itscurrent state. The current state of your image is the Input value for the Curvesfilter. This is your image without the Curves applied.

To the left of the Histogram is another black and white gradient.

This gradient goes from black to white from bottom to top and represents theOutput value of the Curves filter. Output or output value might sound a bit

complicated at first but what this means is that the Curves filter takes yourcurrent image as input, pulls it through the Curves filter and outputs anupdated image. Here are two images to show what this means.


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Our input image. Or image before curves applied.


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And if I change the diagonal line on the histogram. This will be the output

image. Or image with curves applied.


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In order to understand what happened and what causes this change you willneed to know what this diagonal line on the Histogram is and what it does.


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The diagonal line on the Histogram represents the mapping of input values to

output values.

A default Curves filter has a diagonal line going from bottom left to top right.What this means is that:

the black input value(black part of the gradient at the bottom). Inputvalue 0, is mapped to the black output value (black part of the gradientat the left). Output value 0.

the white input value(white part of the gradient at the bottom). Input

value 255. Is mapped to the white output value (black part of thegradient at the left). Output value 255.

The center of the diagonal line is at the input value 128 (256/2). This is the

middle point or 50% gray point.

The gray input value. Input value 128. Is mapped to the gray output

value. Output value 128.

When a new default Curves filter is added there will be no difference in yourimage since all input values are by default mapped to the same output values.0 is mapped to 0, 10 is mapped to 10, 200 is mapped to 200 and 255 ismapped 255.

Therefore what we did in the previous image, is map the gray value of ourinput image to a higher, thus brighter, output value. We mapped 128 (gray), tothe higher 175. Making the overall image appear brighter.

NOTE:

You can add a point by simply clicking on the histogram. And you can removea point by clicking and dragging it out of the histogram area.


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As you can see in the image of the Curves, we did not only move our input

128 up but Photoshop drew an arc through our three points.Photoshop made a Curve from our blacks at (0,0) through (128,175) to(255,255). Hence the name Curves filter!

Because Photoshop drew a curve trough or points, more than just the gray

values became brighter. Both values to the left and right of 128 gray becamebrighter as well, but to a lesser and lesser extend. Leaving our blacks andwhites untouched.


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The fact that our blacks and whites were not altered or touched is important.Had we changed those values too, our image would have looked a lotdifferent.


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This time I mapped our input 128 (gray) to the brighter 175 again, but I alsotouched the blacks. I mapped the blacks, of 0 (zero), of our input to a brighteroutput value of 95.

As you can see in the image above, mapping the blacks to a brighter valueresults in an output image that no longer contains any 'real' blacks.

What was black in our original image has now become some kind of gray inthe resulting image.

As you might have noticed, our whites are still untouched. This is because our

whites are already at their maximum value of 255.


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We cannot go any higher than 255. What we could do however is map more

input values to the maximum value of 255.


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Since I couldn't go any higher than the maximum of 255 I mapped more of ourinput values to the maximum 255 output value.What this means is that:

what was previously 180 suddenly became 255.

what was previously 190 suddenly became 255

what was previously 200 suddenly became 255

etc.


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This is what the Curves filter is. It allows you to map a certain input value to a

new output value.

And the output value does not have to be brighter. You can also map inputvalues to lower (darker) output values.


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So far we have only made changes to the RGB channel in the Curves panel.

Working on the RGB channel gives us the option to alter all three colorchannels at once and thus only influencing the overall brightness or darknessof the image. But as said before, the Curves filter allows us to switch between

color channels! We could switch to the Red channel for example and mapinput values of the Red color channel to new output values on the Red colorchannel.


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Let's have a look at what that means exactly.

When we switch to the Red color channel in our Curves panel, you will noticethat the histogram changes too. The histogram turned red, showing us that weare now working on the Red color channel. The shape of the Histogram also

changed and now represents the brightness values of the Red color channelonly.


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Now lets make the same gray point (128,128) changes as before and map our

input 128 (gray) to an output of 175 again.


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Our image is a lot redder now, but why? Why did this happen?


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In order to fully understand what exactly happened we will have to go back to

our previous image in the RGB channel.


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This is what we did before in the RGB channel.

When we mapped our gray midpoint (128,128) to (128,175), our overall imagebecame brighter. But when we make the same changes in the Red color

channel, our image becomes redder, but not brighter


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So why didn't our image become brighter?

Well, actually it did! But not in the way you would expect.

If you look back at this image


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You will see how we broke our image down in separate color layers, just like

how the Color Channels work.

Now if you look at Method 1. The Method in which we use solid color layerswith layer masks added to the solid color layers, you might start to understand

what happened.

What we did was increase the brightness of the layer mask!

Or to be more precise, we made changes to the Red color's Alpha channel.

As mentioned before, an alpha channel is a mask, just like a layer mask is amask. And masks tell Photoshop which part of an image or layer should betransparent and which parts should be opaque (the opposite of transparent).So what we did by mapping the gray value of the Red color channel in ourCurves filter to a higher value was in fact the same as mapping the gray value

on our Red color's Alpha channel to a higher output value.

This might be a bit tricky to understand, but we can simulate this and achievethe same result by using our breakdown image.


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If we look at Method 1 in our breakdown image and select the Layer Mask on

the Red layer.


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With the Red layer's Layer Mask selected, we can press CMD+M(OS X) or

CTRL+M (Windows) or go to Image > Adjustments > Curves.


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This will open a layer specific Curves window.NOTE: This is a destructive Curves adjustment filter.

Inside this Curves window we can make the same changes we made before.

We can map our 128 (gray) input value to the higher 175 output value.


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And when we click the 'OK' button we should get the exact same result as we

did before.

But this time we can actually see how these changes were applied to our

image.


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If we have a closer look at the Red layer's Layer Mask.

We can see that we actually made the Layer Mask itself brighter.

Layer Mask before Curves:

Layer Mask after Curves:


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Our Curves adjustments made the mid values of our Layer Mask brighter.

And brighter values in a mask or Alpha channel mean more transparency.Thus more of the Red channel is let trough or shown in our image!

This is the very basics on how Curves adjustments work.

In the next part I will show how we can use changes to individual ColorChannels to color and tone an image.

Documents

Lanenga Curves Explainer Part 1