Curves là gì?

Curves là một trong những công cụ cơ bản nhưng lại vô cùng mạnh mẽ. Curves được dùng để tăng độ tương phản sáng tối, cân bằng màu sắc và hiệu chỉnh phạm vi sắc độ cho từng vùng điểm ảnh trong hình.

Curve và Levels có chức năng khá giống nhau là điều chỉnh độ tương phản của ảnh, nghĩa là tăng giảm độ sáng riêng cho vùng pixel nào đó. Tuy nhiên Levels thì bạn chỉ chỉnh được 3 vùng là shadows, highlights và midpoints, trong khi curve bạn có thể chỉnh được đến 16 node trên đồ thị

• A - output tones - sắc độ sau khi chỉnh
• B1 → B2 - input tone - sắc độ trước khi chỉnh
  • B1- Shadows màu tối
  • B2 - Highlights màu sáng
• C - Đường thể hiện quan hệ giữa input tone và out put tone. Ở trạng thái mặc định thì nó là đường thẳng nghiêng 45 độ thể hiện các giá trị input tone và out tone bằng nhau ⟶ Ảnh không thay đổi. Nếu kéo 1 điểm nào đó lên trên là tăng độ sáng cho vùng pixel đó
• D - Histogram - mảng núi lởm chởm là biểu đồ độ sáng tối của ảnh (thể hiện số lượng pixel theo chiều cao). Ảnh minh hoạ trên có số lượng pixel sáng ở gần B2 là nhiều nhất

Dãy sáng (tonal range) trong thực tế môi trường luôn rộng hơn các dãy sáng mà nhà thiết kế tái tạo trên màn hình, hay lên trang giấy. Vì vậy cần phải qua 1 khâu nén (compress), scale đều nó nhỏ lại. 

Vì dãy sáng trên giấy nhỏ hơn dãy sáng thực tế nên độ tương phản của Midtone dễ được cảm nhận hơn khu vực ở 2 đầu (mảng shadows và highlights rất dễ bị mất khi nén), vì vậy hầu hết film và ảnh chụp đều sử dụng đường cong hình chữ S để tăng contract ở Midtone

Tất nhiên bạn không thể tăng sáng mãi 1 vùng nào đó để làm nó nổi bật lên, vì giá trị dãy sáng có hạn (<256). Thay vào đó, bạn cần tăng nơi này và giảm nơi kia để tách bạch sáng tối bằng cách uốn đồ thị thành hình chữ S --- tăng độ tương phản

Tham khảo:

https://idesign.vn/graphic-design/huong-dan-can-ban-khi-su-dung-curves-trong-photoshop-330515.html
https://pamarketing.vn/lenh-hieu-chinh-levels-va-curves-trong-nhom-adjusment-layer-photoshop/
https://www.techradar.com/how-to/photography-video-capture/cameras/photoshop-curves-tool-6-techniques-every-photographer-must-know-1320970
https://www.cambridgeincolour.com/tutorials/photoshop-curves.htm

Curves
NATALIA TAFFAREL·FRIDAY, JANUARY 15, 2016
In order to understand curves we need to understand color in the way that photoshop does.
Additive color theory
EVERYTHING in an image is made of 3 colors - RED - GREEN - BLUE

Each circle is painted on an individual, independent layer - The Red has the RGB values set to 255-0-0, the Green has the RGB values set to 0 - 255 - 0, and the Blue has the RGB values set to 0-0-255 (Primaries: Pure RED, pure GREEN, pure BLUE) - Each layer blending mode is set to screen - Over a Black BG
The interaction between two primaries create secondaries Cyan, Magenta, Yellow
The center creates pure white - 255 - 255 - 255
The variation of luminosity of each creates all the range of visible colors.
We also need to take into account the relationship between tones and we can see that in the RGB color wheel

We can see Primaries and the secondaries as their complimentary - RED and cyan - GREEN and magenta - BLUE and yellow.
Every color that we see in a screen is formed with only these 3 colors.

These apparent gradients are actually created with gradients of the 3 Primaries set to screen blending mode on black, creating the illusion of the whole range of hues (100%saturation, 100% luminosity) of course, we need all the ranges of Lum/sat to have all the colors and I will address that further on.
Every single one of the millions of tones we perceive in an image are created by the primary 3

This image was created filling each independent layer with the primary colors - The Red has the RGB values set to 255-0-0, the Green has the RGB values set to 0 - 255 - 0, and the Blue has the RGB values set to 0-0-255 (Primaries: Pure RED, pure GREEN, pure BLUE) - Each layer blending mode is set to screen - Over a Black BG - Then each channel luminosity was used as a mask of each of the layers to duplicate the appearance of the image without needing to have the image.
When I separate the 3 layers it looks like this:

Even What it´s perceived as Luminosity is color

To create white we need 255 -255-255 To create Black we need 0-0-0 To create Grey we need 128-128-128

Why is this important to understand curves?
Curves work with the 3 channels to modify the image. Even when you think, you´re only working with light.
How?
For example, when you pull the curve up to brighten up the image like this:

What you´re actually doing is pulling up the 3 individual channels up at the same time
Like this:

Lets take a look at the Curves and the different parts
The curve can be edited from 16 individual points, the main ones being 5: black point, shadows, mid-tones, highlights and white point.

You can also use the ON IMAGE tool in the curve to alter an specific range within the image

In the curve we can also find 3 eyedroppers that can be used to select the 3 main points: whites, mids and black point respectively.

These can be used for automatic color/contrast corrections because when we select the darkest/lightest point of the image, the curve will automatically make the whites pure white and the blacks pure black, getting rid of any tint or color contamination.
There´s a tool you can use to accurately find the darkest/lightest points of your image: Threshold

Pushing the threshold slider to the left, it will make the whole image white, except for the darkest point

Pushing the threshold slider to the right, it will make the whole image black, except for the lightest point

We can use it as a reference when using the curves eyedropper to select the white/black points more efficiently.

The curves will automatically make the necessary adjustments to the image, correcting the tint.
But lets understand how to make adjustments ourselves, from simple changes to more advance color grading.
Lets say you have red and you want the red to be orange.
First, lets learn what red is

And what orange is

So the difference between that red and that orange is 128 points of green.
Take the curve, pull up the green from the input 0 take it to input 128 and you get orange.

Input and output are how curves work.
Lets look at that into more detail:
Initially, the tonal range of an image is represented with a straight diagonal line over a graphic. When working in RGB the top right represents lights while the bottom right represents shadows.
Horizontal axis represents the input values (the current values in the image)
The vertical axis represents the output, the values after you adjust them.
Moving the curve up increases the values and moving the curve down decreases them
The same happens with the color curves here´s a visual representation of what happens when you move the curves.

And you can even see how it affects an image

In each of these examples the color curves were pulled up from input 128 (midtowns) for the top/left triangle and pulled down from input 128 for the bottom right part.

In each of these examples the color curves were pulled up from input 128 (midtowns) for the top/left triangle and pulled down from input 128 for the bottom right part.

In each of these examples the color curves were pulled up from input 128 (midtowns) for the top/left triangle and pulled down from input 128 for the bottom right part.
Using curves you can also change the saturation of colors not only the hue/luminosity.
Lets go back to my red square. I turned it orange but it´s too saturated for, lets say, skin-tones.

This part is kind of logic, if you want white you need the 3 tones in their pure state 255 -255- 255 so if you want grey then you need 128-128-128
You had orange 255-128-0
You need that orange to be brighter (closer to white): there for more even in values, yet keeping the proportions so it´s still “orange"
Something more like 220 - 190 - 150 (Approximately, this is not exact, but I´m sure it will work)
So RED is starting in 255 - we need to go to the input white point (255) and pull it down to 220
The GREEN was at 128 - So we go to the input 128 (mid-tones) and move them up to 190
The blue was in 0 -So we need to go to the input 0 (black point) and pull it up to 150
Result:

See? Told you it would work.

So do you need to use numbers or know these numbers to understand curves? No, not really. Knowing the numbers is gonna make it exact and mathematical but you can also do it by eye and use the color wheel as a base.

We only have 3 Channels (RED, GREEN, BLUE), there for if I want YELLOW, there´s no yellow curve to move, I will need to remove blue if I want yellow. Skin is yellow and red.
So If I want to turn something grey into skin-tones looking tone I need to pull down the blue and add some red. Probably pretty dark, so I add some light using the RGB.
Always starting from the input of 128 because it´s grey.

You can see this as you go without knowing any of the numbers.
You do need to know your color wheel.
But… didn´t we just used another curve to do the skin-tones with the even values and all that? Yup! And that´s the beauty of curves.
It also works! Differen´t curve, Same result

This last one has a mathematical base because it was thought out from color numbers, while the previous one is pure color knowledge “well… if I want to do skinniness then I need red, yellow and light” - Both ways work as long as the theory behind the thought process is correct.
Lets change more colors and forget the numbers a bit.
Lets go from black to Skintone - Evidently we start from the input of 0 now because we are trying to change black
Also considering I´m starting from black - Complete lack of color - 0 0 0 - My idea of putting the blue down, does´t help me much.
I need to pull the green up

When changing whites, again, I need to change the input from where I´m starting.
I can´t alter the midtowns or black point since that would´t change the image at all.
Now I need to start from the white point, 255-255-255
I can´t add light because it´s already the brightest it can be so I need to reduce the tones, keeping the proportion.

Well… Now you know how to change whites and blacks and mitones.
Now that we know what happens to white, grey and black, lets move on to colors.
An image is regularly conformed by neutral tones.

In this visual everything that is not grey (lights and shadows) is black and everything that is midtown has been filled with 50% grey. A good way to select midtowns is to use “color range” in the select menu, it has an option to select only midtones.
It´s also common that images have mostly even channels, usually within the midtown range.

This means the image will change (be affected) when making movements on curves from the midpoint (128 input value)
This is not always so, some times we have too dark/too light or too saturated parts and we need to understand how the color curves work then.

These colors were purposely chosen to have 100% sat/lum so they would create uneven channels.
Lets see how the channels look in these mostly pure tones.
RED

GREEN

BLUE
The part of the image with the pure colors is mostly extreme (black orwhite) in the channels- with little mid-tone
It is important to understand that since each color is mostly white or black , they will not be affected by a movement of a curve from the middle tone. Because of being mostly pure colors , they will always have 255 of one or two of the 3 colors and 0 of the other one - so the channels generally will be almost black/white with little mid-tone range

Looking at channels will help us know from where in the curve you need to make the moves. This is only necessary at first, because after a while you get used to literally "seeing in channels" and you no longer have to look at them to know how they look like.

To understand this visually

If I apply a blue curve from the mid-tones it will only affect areas of the image containing gray in the blue channel

So, also, most of the image itself

Another thing to take into account is that, in an image, when we change the colors from the ends (white or black input) we will be changing the apparent brightness of the image and depth, so we have to keep that in mind when making changes. Not only is will be more blue, but also brighter.
If we make the changes from the black or white point, the image will gain or lose contrast as well.
Lets see how we can use what we have learnt to actually work on an image.
The sample image that I´m using has a lot of redness around the eyes

But i´s not really red, it´s more like pink

And I want that pink, to be orange. So starting from the mid-tones (remember our image had mostly grey tones in the channels) I can take some blue out and add a bit of green, then use a mask to constrain the correction and, with this curve neutralise the “redness” without loosing saturation and keeping the area with the original tonal variations.
When you use saturation or paint on an empty layer or any other correction that implies one flat color, you lose naturality.
With curves, it´s a lot better.

The mask was created exaggerating the pink with a saturation adjustment in conjunction with a black and white adjustment to enhance the difference further and then using channels to extract it.

Like this you can do all sort of corrections, always taking into account the RGB wheel for the relation between colors.
With curves you can also do creative changes.
Very popular examples being the split tones. Since the human brain is very keen on complimentaries.

But most of the times, we have a reference in mind.
Sometimes is a clear one, like another image.
I´ve decided to use a non traditional reference that came up in my retouching group.

http://www.bijoux-components.cz/
The way to match references is to learn to see. Understand what to look for in references. If the thought process is organized then it should be easy and fast (after years of doing it, of course)

The main issue is to know where to look and what to look for.
My brain is already fully programmed to think in "curve” format, there for my mind immediately asks a few questions.

1- How broad is the range between the highlights, lights, midtones, shadows and black point? How much contrast does the image have? - In the case of this reference, the contrast is quite low, with the exception of a few highlight.
1- The highlights: Have a tint? Which? Are they pure white or greyish? - The lights are definitely not white (with the exception of those few highlights) and has a green tint for sure.
2- Blacks have a tint? Which? Are they pure black? Blacks are not even close to being pure black, and have a lot of red.
3- What happens in the midtones? Having little contrast and complementary colors in highlights and shadows in this reference generatesa neutralised appearance of midtones.
4- Saturation? A Lot? A Little? Where? In this case, there is not much saturation in general while both LIGHTS and BLACKS are very saturated.

Result: