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MobiWorkshop MW3 Curves

FundyBrian

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The use of curves is a natural extension from our MW2 about White Balance & Colour balance. Curves are often used when making corrections to tonality and colour in images.
Before we get into using curves for image correction it makes sense to introduce curves to make sure everyone understands what they represent and what they can do.

I realize this all looks like a bunch of graphs but before we start looking at images we need to explain what the curves are all about.

Introduction to Curves:
B9DFA6A0-5FE2-41FA-856C-7988492D8C88.jpeg

#1 Let’s start here. This is the standard format for curves to be displayed. The two gradients bars are not usually included to save space but their presence is implied and having them there to start with makes it easier to understand what the placement of points means on the graph. You can think of the lower right corner as the starting point.

Across the bottom we have tones becoming darker as we move to the left. Up the side the tones get lighter as we go up. If we wanted to put a point on the graph to represent white it would be in the upper right. And black at the lower left.

The background grid just helps us know the relative position of our curve as we move it.

F8F2BB31-A02B-46F2-B730-FB7FE7DAF983.jpeg

#2 This “curve” is a straight line representing a linear relationship in the placement of tones. When you see this straight line it basically means “no change”. In terms of a response curve this indicates that what you have coming in is exactly the same going out.

Digital cameras have what is called Linear Gamma. Gamma is a description of the slope of the graph. It is not only images and cameras that use this curve but your monitor or screen also has a curve which tells the computer how to place tones on the screen to get a “normal” looking image.

We won’t be doing anything with Monitor gamma but it is probably worthwhile to mention it so you have a greater understanding of what is happening behind the scenes. Monitor Gamma can be quite different from device to device but it is very important to make our images look correct. Monitor calibration involves reading known test values off the screen and creating a new Monitor Gamma curve for your monitor so that the placement of tones and colours is exactly right.

Monitor calibration is necessary to compensate for variations in manufacturing, as well as decline in performance over time. In addition, a general purpose office monitor has no need to be precisely accurate, and they aren’t. It costs more money to make an accurate monitor. A graphics monitor is in the higher price range and what you get is much better colour and tonal accuracy. It is generally not possible to calibrate an office monitor up to graphics standards.


79B7E59F-F155-482E-8FD3-8428AF72458F.jpeg

#3 This curve represents an increase in contrast in the image. Making the graph steeper increases the image contrast. Starting from the black point on the lower left, the assignment of “Black” has been dragged to the right somewhat towards grey. This means that all tones to the left of our new position have been rendered as black. Imagine that your image was made on a foggy day and doesn’t contain any blacks or pure whites. It is a low contrast image. If you wanted to increase the contrast a bit you would do this by pulling the black end inwards.

Looking at the white part of the graph at the top right: If we go back to the foggy day image, it has only weak light grey values with no strong white. Moving the top point of the graph left we are saying we want the light grey values to be brighter. Now anything to the right on the new white position will be pure white. These two adjustments might be good for a low contrast rainy day image but not for a sunny day image. A sunny day image already has a full range of tones from white to black to pulling the white end of the graph inwards will have the effect of bumping your highlight detain into pure white - not desireable at all. Likewise with your black values: pulling the black point inwards means your shadow detail will be rendered as solid black.

When you use the simple contrast slider in many editing apps what it is doing is changing the contrast slope of your “curve”, darkening blacks and lightening whites. However, without seeing some sort of histogram you might not realize that increasing the image contrast is fine for the mid tones but destructive if your image contains delicate highlight values and shadow tones. How can we get around this?

0CD8F165-B09A-49A5-9398-2577AC5C0B6E.jpeg

#4 Changing the curve to look like this lowers contrast. In the upper right, where the white are, the bright whites have been darkened to look light grey. Over at the black end, the blacks have been lightened to be only dark grey. In most images this would look very dull but it is important to understand how the graph works.

When you adjust a simple contrast slider towards lower contrast this is what is happening. Probably not what you wanted. Again, how can we get around this?

C785B8A6-B63E-4B47-B509-E0058B4FCA91.jpeg

#5 Linear gamma (farther above) describes the output of digital cameras. This type of curve represents the typical tonal distribution of film. With digital is is entirely possible to have an linear relationship between the tones. With film we are talking about the threshold in sensitivity of the film not seeing any light to gradually detecting some light at the shadow end and gradually reaching the limit of what can be recorded at the highlight area without all turning white. Aha, you say. This is what makes film look different from digital.

If you think about the effect of this curve on the image it means the highlights (the shoulder area of the graph) are somewhat compressed, and becoming increasingly hard to differentiate. At the “toe” region, we have a stretched out area of shadow separation. The more or less straight midtone area has higher contrast.

Every film has it’s own distinctive response curve. Some were nowhere near linear in the mid tones. Some had a soft shoulder and others more sharp. Some films had a “long toe” while others were closer to linear. Here we are only looking at the overall tonality, as if it were a B&W film, but don’t forget that each RGB layer in a colour film also has it’s own response curve and they don’t necessarily lay on atop the other with perfect accuracy. This is where we get the differences in colour response between different types of films. Manufacturers struggled hard to get closer to linear gamma but is was impossible. So now we have linear gamma in digital cameras and for some reason some people want to go back to the nonlinear response curves.

So here it is: If you want to make your digital images look like film it is all in the curves. Maybe this will be some incentive for some people to pay attention to this part.

So I don’t leave you hanging until I post the next part,
185E231B-C5BA-4FB2-A6C3-AA6138AA4020.jpeg

#6 This curve represents brightening the mid tones without changing the whites or blacks in the image. This doesn’t mean there have not been slight changes in the upper and lower regions but we will look at the mid tone values first. You can see the curve has been pushed out about the width of one square in the background grid. The middle tones will be noticeably brighter. The upper light tones are also lightened so some highlight details may be at risk. (Pay attention).

Looking at the upper half of the curve you can see that the contrast slope is lowered for the upper tones. The upper half of the curve looks like the low contrast curve from diagram #4 above.
The contrast slope is increased in the lower tones. Like diagram #3 above. So it has become easier to differentiate shadow tones but harder in the bright tones. In the shadow area, even though black remains where it was the lower mid tone values have been lightened. There is a bigger increase in tone brightness as we leave black.

910B3683-4D02-4FC2-9969-6BAB43600592.jpeg

#7 This curve represents darkening the middle image values without changing the blacks or whites. Very useful. Also notice the contrast slope in the lower tones is decreased and the upper mid tone slope is increased. So we have greater separation in the upper middle tones but a bit less in the lower part.

Let’s suppose you wanted to increase the mid tone contrast without moving your whites and blacks. Look at diagram #5.

Continued next message. That may be tonight or tomorrow due to other commitments.
 
Last edited:

ImageArt

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The use of curves is a natural extension from our MW2 about White Balance & Colour balance. Curves are often used when making corrections to tonality and colour in images.
Before we get into using curves for image correction it makes sense to introduce curves to make sure everyone understands what they represent and what they can do.

I realize this all looks like a bunch of graphs but before we start looking at images we need to explain what the curves are all about.

Introduction to Curves:
View attachment 119851
#1 Let’s start here. This is the standard format for curves to be displayed. The two gradients bars are not usually included to save space but their presence is implied and having them there to start with makes it easier to understand what the placement of points means on the graph. You can think of the lower right corner as the starting point.

Across the bottom we have tones becoming darker as we move to the left. Up the side the tones get lighter as we go up. If we wanted to put a point on the graph to represent white it would be in the upper right. And black at the lower left.

The background grid just helps us know the relative position of our curve as we move it.

View attachment 119846
#2 This “curve” is a straight line representing a linear relationship in the placement of tones. When you see this straight line it basically beans “no change”. In terms of a response curve this indicates that what you have coming in is exactly the same going out.

Digital cameras have what is called Linear Gamma. Gamma is a description of the slope of the graph. It is not only images and cameras that use this curve but your monitor or screen also has a curve which tells the computer how to place tones on the screen to get a “normal” looking image.

We won’t be doing anything with Monitor gamma but it is probably worthwhile to mention it so you have a greater understanding of what is happening behind the scenes. Monitor Gamma can be quite different from device to device but it is very important to make our images look correct. Monitor calibration involves reading known test values off the screen and creating a new Monitor Gamma curve for your monitor so that the placement of tones and colours is exactly right.

Monitor calibration is necessary to compensate for variations in manufacturing, as well as decline in performance over time. In addition, a general purpose office monitor has no need to be precisely accurate, and they aren’t. It costs more money to make an accurate monitor. A graphics monitor is in the higher price range and what you get is much better colour and tonal accuracy. It is generally not possible to calibrate an office monitor up to graphics standards.


View attachment 119849
#3 This curve represents an increase in contrast in the image. Making the graph steeper increases the image contrast. Starting from the black point on the lower left, the assignment of “Black” has been dragged to the right somewhat towards grey. This means that all tones to the left of our new position have been rendered as black. Imagine that your image was made on a foggy day and doesn’t contain any blacks or pure whites. It is a low contrast image. If you wanted to increase the contrast a bit you would do this by pulling the black end inwards.

Looking at the white part of the graph at the top right: If we go back to the foggy day image, it has only weak light grey values with no strong white. Moving the top point of the graph left we are saying we want the light grey values to be brighter. Now anything to the right on the new white position will be pure white. These two adjustments might be good for a low contrast rainy day image but not for a sunny day image. A sunny day image already has a full range of tones from white to black to pulling the white end of the graph inwards will have the effect of bumping your highlight detain into pure white - not desireable at all. Likewise with your black values: pulling the black point inwards means your shadow detail will be rendered as solid black.

When you use the simple contrast slider in many editing apps what it is doing is changing the contrast slope of your “curve”, darkening blacks and lightening whites. However, without seeing some sort of histogram you might not realize that increasing the image contrast is fine for the mid tones but destructive if your image contains delicate highlight values and shadow tones. How can we get around this?

View attachment 119848
#4 Changing the curve to look like this lowers contrast. In the upper right, where the white are, the bright whites have been darkened to look light grey. Over at the black end, the blacks have been lightened to be only dark grey. In most images this would look very dull but it is important to understand how the graph works.

When you adjust a simple contrast slider towards lower contrast this is what is happening. Probably not what you wanted. Again, how can we get around this?

View attachment 119847
#5 Linear gamma (farther above) describes the output of digital cameras. This type of curve represents the typical tonal distribution of film. With digital is is entirely possible to have an linear relationship between the tones. With film we are talking about the threshold in sensitivity of the film not seeing any light to gradually detecting some light at the shadow end and gradually reaching the limit of what can be recorded at the highlight area without all turning white. Aha, you say. This is what makes film look different from digital.

If you think about the effect of this curve on the image it means the highlights (the shoulder area of the graph) are somewhat compressed, and becoming increasingly hard to differentiate. At the “toe” region, we have a stretched out area of shadow separation. The more or less straight midtone area has higher contrast.

Every film has it’s own distinctive response curve. Some were nowhere near linear in the mid tones. Some had a soft shoulder and others more sharp. Some films had a “long toe” while others were closer to linear. Here we are only looking at the overall tonality, as if it were a B&W film, but don’t forget that each RGB layer in a colour film also has it’s own response curve and they don’t necessarily lay on atop the other with perfect accuracy. This is where we get the differences in colour response between different types of films. Manufacturers struggled hard to get closer to linear gamma but is was impossible. So now we have linear gamma in digital cameras and for some reason some people want to go back to the nonlinear response curves.

So here it is: If you want to make your digital images look like film it is all in the curves. Maybe this will be some incentive for some people to pay attention to this part.

So I don’t leave you hanging until I post the next part,
View attachment 119855
#6 This curve represents brightening the mid tones without changing the whites or blacks in the image. This doesn’t mean there have not been slight changes in the upper and lower regions but we will look at the mid tone values first. You can see the curve has been pushed out about the width of one square in the background grid. The middle tones will be noticeably brighter. The upper light tones are also lightened so some highlight details may be at risk. (Pay attention).

Looking at the upper half of the curve you can see that the contrast slope is lowered for the upper tones. The upper half of the curve looks like the low contrast curve from diagram #4 above.
The contrast slope is increased in the lower tones. Like diagram #3 above. So it has become easier to differentiate shadow tones but harder in the bright tones. In the shadow area, even though black remains where it was the lower mid tone values have been lightened. There is a bigger increase in tone brightness as we leave black.

View attachment 119856
#7 This curve represents darkening the middle image values without changing the blacks or whites. Very useful. Also notice the contrast slope in the lower tones is decreased and the upper mid tone slope is increased. So we have greater separation in the upper middle tones but a bit less in the lower part.

Let’s suppose you wanted to increase the mid tone contrast without moving your whites and blacks. Look at diagram #5.

Continued next message. That may be tonight or tomorrow due to other commitments.
This was very useful to me. I usuallly play around with a curve to get something outlandish not really knowing what I was doing. Now that I know what each axis represents the curve makes much more sense. Thanks!
 

rizole

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This was very useful to me. I usuallly play around with a curve to get something outlandish not really knowing what I was doing. Now that I know what each axis represents the curve makes much more sense. Thanks!
That's completed the gestalt for me now. What with last weeks photographic colour wheel, the write up above and cracking open photoshop to play with curves, I have it. I think I need to solidify where yellow, cyan and magenta fall between RGB now and I have a useful bit of working knowledge. Thanks FundyBrian.
 

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Here’s another look at the curves and the changes in values that occurs when dragging points on the curve.

CC5902F5-C335-4CFA-8D4F-4DD365022276.jpeg

#1 - When you tap on the curve a control point (or node) appears. They work just the same as nodes on a vector drawing. Here I have tapped on the curve (still linear) in 3 places, and now I have 3 new control points (in addition to the start and end points).
89FC61FD-7879-420B-895D-49E3D991CFAA.jpeg

#2 - Starting with the bottom control point: so far I have not moved anything. I’m just getting ready to make an adjustment. There is a vertical line leading down to the gradient bar showing the tone of the selected control point. I added a coloured dot on top of the control point to show exactly what colour is at that location. The horizontal line goes to the right on the vertical gradient bar. You can think of this as “what is coming in is the same as what is going out”.

Likewise with the other 2 control points. Exactly the same in both places on the gradient bars.

CE2EFDD3-073E-4E12-9B78-3563E6531C0F.jpeg

#3 - Here I have dragged the bottom control point straight downward by the amount indicated by the 2 horizontal lines beside the darkest dot. Now, the starting tone as indicated by the line on the bottom gradient bar has been darkened by the distance I pulled the control point downwards, as shown on the vertical gradient bar. The upper of the 2 lines represents the starting tone and the lower line represents the new tone. On the bottom bar we have the source tones and the vertical bar can be thought of as the new output tones. Input and output.

In this example the middle tone remains unchanged.

The top control point has been raised by the amount indicated by the horizontal lines beside it. The lower of the 2 lines shows the starting point and the upper line shows the new position. Looking over at the vertical gradient bar the difference between the old tone and the new one is shown by the distance between the two horizontal lines. Now if we look down following the vertical line below the dot to the bottom gradient bar, that is the source tone. Back over to the vertical gradient bar for the same dot we see the new output tone.

This way you can visualize the difference between the original input tone and the new output tones.

Looking at the curve overall, the dark tones were darkened, the light tones were lightened, the mid tones remain the same, and looking at the slope of the new curve it shows the midtone contrast has been increased. The black and the white values were not changed.
 

FundyBrian

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Now we will move to working with curves in an app to edit colour. I’ll use snapseed because most people have it. The operation of the curves is basically the same in every app.

Here is a photo I made with my iPhone 5 using TrueHDR and I made the mistake of adjusting it in the sunshine where I couldn’t see preoperly. I’m embarrassed to even admit I made this mess. The result was truly hideous and I never was able to fix the colour with the typical slider adjustments.
0937F9C4-FF13-4B80-A002-83866E6AA556.jpeg

You can see a lot of blue in the black shadow areas, especially the foreground rock. The sky went way cyan. You might want to download this top image to try the curves adjustments along with me.

Here we are in Snapseed. From the Tools menu select Curves and this is where you end up. The curves preset panel opens right up. Instead of opening a no-curve adjustment it defaults to a contrast boost so the first thing you have to do is select Neutral at the far left of the presets.

DF172912-3088-4119-A58C-ACEECEFDFDC0.jpeg


F7612A1A-B72F-4F61-860A-9C809C01BBD8.jpeg

Actually, before you leave the curves preset panel go along to the right and look at some of the presets. This one is S02 I think. Look at all the curve adjustments. Every layer has some twist to it. This is how presets are made and you could easily enough make your own.

B3082DDA-832A-4937-B555-EF8B6764D4FC.jpeg

Tapping on the second icon from the left at the bottom brings up the colour layer selection panel. One pain about Snapseed is that every panel pops up and covers part of the image. The first thing I’m going to attack is the excessive blue in the shadows. Tap on the blue dot.
My general thinking is I want to reduce the blues in the shadows but I also want to increase blue in the sky. If I increase the blue in the sky it will just get lighter so that won’t work. I will have to reduce the other two colours in the sky instead.

0E56639F-A1DC-4533-A12F-B0FCB9127D5A.jpeg

Light colours at the top right of the curve, dark tones at the bottom left. Unfortunately you can’t slide the photo up into that unused space at the top so the graph is always on top of the photo. Tapping the eye icon in the middle bottom will make the curves graph disappear. Then you can zoom in and move the image but not upwards (!).


56C2DBEF-86F0-48C4-AAD0-66751D312913.jpeg

I want to reduce blue in the dark shadows. Tap on the curve to set a new adjustment point and drag it to the right. Bear in mind this photo is pretty extreme so you won’t likely ever make such drastic adjustments. I pulled the adjustment point to the right until the blue in the dark shadows disappeared. I kept the adjustment point low down in the darker tones. I also tried dragging the bottom left point to the right but this didn’t work as well in this case.
Already the picture has improved. No more blues in the shadows.
At any time during your adjustments you can press and hold on the screen to see the unedited version of the image. Tapping on the eye icon makes the curves disappear so you can get a better look at your image.

E13DD30B-6A41-48AE-97C2-B358E45D46EC.jpeg

Just for interest here is the horizontal orientation in Snapseed. Bigger picture but smaller curves. Just like on the vertical orientation there is a lot of empty space on the screen that could be used for the photo. Why can’t you move the picture over to the blank space on the left? And why doesn’t it do that automatically? You can zoom in a bit and that makes it bigger on the left but you can’t get the full image to move left.


2D5B8125-F591-4679-AC09-CF046A9056C1.jpeg

Cyan is blue and green. If I can’t fix the sky by adding blue I should reduce the green. Here I tapped the top right curve end point to select it and dragged it straight down to reduce green in the bright tones. You might think, Egads! That’s way off! But don’t forget this is only half of this maneuver The blue sky is looking a lot better in spite of the lovely pink that replaced the white. The white was just overexposed sky and maybe some haze.

BB3F9ED4-17A5-4F0C-8E09-9ECBB1C2A3DC.jpeg

This time select the Red layer and tap the top right control point to make it active and just drag it straight down until the pink area looks more neutral.
I often had trouble moving the control points. I would tap and tap and it wouldn’t work. Eventually if I persisted I got it.
Overall the colour is getting more balanced. After reducing so much in the red & green the beach rocks area is darker.

Here’s a friendly warning. If you accidentally make a bad control point you will find you can’t get rid of it. In some apps you can get rid of an extra control point by dragging it to one of the end points. For some reason I couldn’t make that work in Snapseed. If you tap the “X” at the bottom left all your adjustments revert to zero. For the fun of it sometime, add a couple of control points to a curve and crank them tight up or down and you might get inspired by the totally wacky results. Hmmm. Special effects.

Tip: If you find an adjustment you are making at the lower end does something undesirable at the top end you can add additional control points to keep the upper part of the curve to a more normal shape. This is when you will appreciate the background grid to help you know where “normal” would be.


I’m going to continue this in the next post since I’m getting close to maxing out this one.

Next will come an overall adjustment to lighten the beach areas that got too dark, and a saturation reduction. Also, curves in another app. This evening.
 

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With a picture this wrecked I doubt if it can ever be made as good as new again. But it is a good example.

While working with curves in different apps, basically doing the same thing, I encountered one issue worth mentioning. I have always found Snapseed to be a very useful app for many types of problems. Working on this project I realized one important drawback. At each step you have to commit to your changes. If you move on and do something else, such as reduce saturation, and then decide to revisit your curves settings, you cant. You just get a new blank curves window. Not so with several other apps. At any time you can go back and fiddle with previously made adjustments. With some apps you can step back through your edit history, redo a step and carry on forward from there. But that’s not the same as being able to revisit any of the settings you have made using adjustments layers, such as in Affinity Photo. Darkroom and MaxCurve also allow you to revisit previous adjustments.

Here I am back in Snapseed and using the Luminance channel to brighten the beach area that got darker after the RBG curves adjustments.
0A1E233A-6147-4DBD-AE94-391B705FD99F.jpeg

You can see here I still need to go back and reduce the red in the brighter band of the sky.

E987E546-19AA-4D3A-9E0F-9D03A6E33F0E.jpeg

Here is about the same step in Fine - Photo Editor. A very clean layout. You don’t have to keep bringing up the layers panel to select the different RGB curves to work on. Also the histogram is in the top corner of the curves area.

After brightening the beach area I reduced the saturation. I think to do more repairs will require localized work with masking or dodging. This is not bad especially considering all my adjustments so far have been global using curves, with no masking or local retouching. This looks al lot closer to the way I remember the beach from numerous visits.
546CDA64-AE67-4084-A64A-EF91AE08FB11.jpeg

I have not straightened the images as yet but this is much more normal looking than what I started with. To be sure there is no way I could have got this close using the regular sliders for colour temperature and tint. You can see how using curves gave me nonlinear control to manipulate colour and exposure.

180F8F90-6485-436F-9BFE-B6C6D7C4D9E1.jpeg

Here is a view of Snapseed on iPad.

F5F149BE-F53A-4A69-A306-16B9AFE5BDD8.jpeg

An iPad view using MaxCurve. It has the best selection of curves I have seen as well as a nice interface. More about MaxCurve later. One feature I like in the Darkroom curves inset is having all colour channel histograms overlaid at once. That really helps understand what is happening on other channels. Also, you have the mini views of the other curves below.

577949C5-401C-456B-86A8-E58E34D7EF2A.jpeg

Darkroom on iPad. Another excellent choice. Darkroom uses a different take on the curves panel. There are 5 areas of adjustment, each with a vertical slider. This makes it much more obvious that you are raising or lowering a tone in a particular area rather than just dragging the curve any which way. This fits with a logical understanding with how curves work.

331DC78B-9F23-44FA-BE04-F6759FFF30F3.jpeg

This photo shows localized exposure adjustment to brighten the beach rocks.
Affinity Photo on iPad. I’m getting quite familiar with how everything works although at first it felt quite illogical after being used to Photoshop. Affinity Photo is a good example of an app using non-destructive editing through adjustment layers. You can go back and revisit previous adjustments to fine tune them based on more recent edits. Affinity Photo doesn’t do everything but before long it probably will.

I’m seriously over-apped these days. I can’t keep track of every feature in every app, especially when new features are added in updates. But you never know when something new will be indispensable. Still, I’m considering eliminating apps that only offer a sub-set of features already in bigger apps. Focusing on those fewer apps can only lead to knowing them better and not being distracted by the junior players.
 

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With a picture this wrecked I doubt if it can ever be made as good as new again. But it is a good example.

While working with curves in different apps, basically doing the same thing, I encountered one issue worth mentioning. I have always found Snapseed to be a very useful app for many types of problems. Working on this project I realized one important drawback. At each step you have to commit to your changes. If you move on and do something else, such as reduce saturation, and then decide to revisit your curves settings, you cant. You just get a new blank curves window. Not so with several other apps. At any time you can go back and fiddle with previously made adjustments. With some apps you can step back through your edit history, redo a step and carry on forward from there. But that’s not the same as being able to revisit any of the settings you have made using adjustments layers, such as in Affinity Photo. Darkroom and MaxCurve also allow you to revisit previous adjustments.

Here I am back in Snapseed and using the Luminance channel to brighten the beach area that got darker after the RBG curves adjustments.
View attachment 120003
You can see here I still need to go back and reduce the red in the brighter band of the sky.

View attachment 120004
Here is about the same step in Fine - Photo Editor. A very clean layout. You don’t have to keep bringing up the layers panel to select the different RGB curves to work on. Also the histogram is in the top corner of the curves area.

After brightening the beach area I reduced the saturation. I think to do more repairs will require localized work with masking or dodging. This is not bad especially considering all my adjustments so far have been global using curves, with no masking or local retouching. This looks al lot closer to the way I remember the beach from numerous visits.
View attachment 120024
I have not straightened the images as yet but this is much more normal looking than what I started with. To be sure there is no way I could have got this close using the regular sliders for colour temperature and tint. You can see how using curves gave me nonlinear control to manipulate colour and exposure.

View attachment 120020
Here is a view of Snapseed on iPad.

View attachment 120021
An iPad view using MaxCurve. It has the best selection of curves I have seen as well as a nice interface. More about MaxCurve later. One feature I like in the Darkroom curves inset is having all colour channel histograms overlaid at once. That really helps understand what is happening on other channels. Also, you have the mini views of the other curves below.

View attachment 120022
Darkroom on iPad. Another excellent choice. Darkroom uses a different take on the curves panel. There are 5 areas of adjustment, each with a vertical slider. This makes it much more obvious that you are raising or lowering a tone in a particular area rather than just dragging the curve any which way. This fits with a logical understanding with how curves work.

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This photo shows localized exposure adjustment to brighten the beach rocks.
Affinity Photo on iPad. I’m getting quite familiar with how everything works although at first it felt quite illogical after being used to Photoshop. Affinity Photo is a good example of an app using non-destructive editing through adjustment layers. You can go back and revisit previous adjustments to fine tune them based on more recent edits. Affinity Photo doesn’t do everything but before long it probably will.

I’m seriously over-apped these days. I can’t keep track of every feature in every app, especially when new features are added in updates. But you never know when something new will be indispensable. Still, I’m considering eliminating apps that only offer a sub-set of features already in bigger apps. Focusing on those fewer apps can only lead to knowing them better and not being distracted by the junior players.
I too have way to many apps. I keep too many ‘just-in-case’ apps and feel a strong urge to do some serious culling.
 

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I too have way to many apps. I keep too many ‘just-in-case’ apps and feel a strong urge to do some serious culling.
Me too. But you just never know when an update may transform an app from a wannabe to a contender. I think the main thing will be to keep apps that have at least one unique feature.
 

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This will be my last entry to the Curves workshop unless there are further questions to respond to.

This time I want to encourage you to explore curves in two ways. One is to try some extreme curves just to see what happens.
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#1 - In this case I was just doing a fairly reasonable brightness adjustment when I decided to make a small tweak to the topmost curve control point. I tried to tap the top control point but accidentally created a new control point very close to the top point and moving that point a small amount caused the curve to take off at a sharp angle and oscillate between the rest of the existing control points. If you follow the dots fron the lower left going towards the upper right you can see what the basic curve shape was until that one new point caused the curve to go wild. It serves to point out that some very wild special effects can be created by abusing the curves.

Second - For your continuing edification open up MaxCurve and check out some of the presets with the purpose of looking at the curves to see how that effect was made.
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#2 - So here we are in MaxCurve on iPad. I have tapped the Presets icon at the far right of the bottom tool bar and this selection has appeared. Load an image to see what presets do to your image. I almost never find any presets that come close to what I want but I find it interesting to see how they do it. As it turns out I didn’t choose the ideal image to evaluate the presets but it is mostly the curves I want you to see.

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#3 - I can’t remember which preset was chosen. Start by selecting each of the curve sets to see what is going on. It helps to look at them all before any preset has been applied so you are familiar with what the neutral state is for each curve set. In this case the RGB set didn’t look like much but the CMYK set has some fairly wild humps and hollows in the curves.

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#4 - In this case the HSL curve set held the secret recipe.

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#5 - Here is another preset but it has only one curve adjustment - a fairly wild kink made to the cyan curve.

Once you explore this with several presets you will soon understand that you can make your own unique presets by adjusting some curves and saving the result as a preset. You will probably have the greatest luck by thinking about specific situations you are likely to revisit and call up your preset again for that purpose. For instance, people photos, rainy day photos, overcast day street photography, etc. You know how to emulate the curve shape of old analog film photography and combining that with adjusted colour response with the colour channels you can create the look you like.

Happy creative curving!
 
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