Effects on pixel when changes are made, such as exposure

Processing
1

Suppose you have a patch of color with all the pixels at rgb of 128,0,0. What happens to those numbers when you:

Increase saturation
Increase exposure
Lighten
Brighten
Increase luma
Increase chroma

The answer to the first is, I believe, the 128 is increased. Yes, no? The others?

2

Hi @Underexposed,

Erā€¦ are you asking generally, or regarding a certain piece of software?
If the latter, it must be dead easy to check?
Or have I misunderstood something?

Have fun!
Claes in Lund, Sweden

3

Iā€™m on my phone, canā€™t write a complete answer, but exposure is a multiplication, so the zeros wonā€™t moveā€¦

In general, Iā€™d surmise that all of the operators in your list have an arithmetic multiplication as their foundation, so that dynamic is in play. However, under the hood things are never that simple. In gimp, make an artificial image of red, lighten it up, see what comes outā€¦

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4

It depends on the definition of each term.

For example, using ImageMagickā€™s HSL colorspace, (128,0,0) is at maximum saturation, 100%. But saturation can be increased by increasing red while decreasing green and blue. Yes, this gives negative G and B values.

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5

The problem is not the problem. The problem is what you mean by ANY value youā€™ve mention or operation you mention.

First - what does 128 mean at all? and why 128? Do you mean integer encoding between 0 and 256? Soā€¦ 0.5? and what 0.5 means?

the operations youā€™ve mention are just math. what actual representations are decoded/operated on/encodedā€¦ thatā€™s a different matter and it depends.

I know itā€™s easy to skip to software but I really find trying to understand things from ground up helps in general.

My recommendation: start from https://hg2dc.com/ (question 1) and work your way up. And for processing - Aurelienā€™s videos apply usually to all processing :slight_smile:

6

Good points, all. Iā€™m using GIMP and putting together a table of results. Will post soon.

7

Actually, values are less important than what you mean by the terms. Each term can have any number of interpretations. You mentioned that you are using GIMP. You would have to tell us what your document settings are and which tools you are using. That way someone could look into the code and tell you exactly what is going on.

PS I will add the GIMP tag to your OP for clarification.

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8

I was really just interested in how a pixelā€™s values change when these tools are applied, not really the theory. Just what does it do to a pixel to add saturation, and so on.

Tools Used: Colors/
Exposure
Color Temperature
Saturation
Brightness-Contrast
Hue-Chroma

R orig 128 G orig 0 B orig 0
Exposure + 245 0 0
Exposure - 97 0 0
Color Temperature + 132 0 0
Color Temperature - 109 0 0
Saturation + 140 0 0
Saturation - 108 38 38
Contrast + 128 0 0
Contrast - 128 65 65
Chroma + 160 0 0
Chroma - 95 48 38
Brighten + 139 22 22
Brighten - 105 0 0
Lighten + 185 65 44
Lighten - 62 0 0
R orig 128 G orig 40 B orig 50
Exposure + 245 84 103
Exposure - 69 18 23
Color Temperature + 131 40 47
Color Temperature - 118 41 60
Saturation + 137 28 43
Saturation - 115 52 57
Contrast + 128 2 16
Contrast - 128 67 74
Chroma + 161 0 36
Chroma - 93 65 65
Brighten + 147 63 81
Brighten - 103 32 40
Lighten + 199 103 108
Lighten - 67 0 0
9

Could you please order it by tool?

10

The ordering is by similarity of effect, e.g., both exposure and color temperature only change R regardless of whether you increase or decrease the effect from the starting point. Edit: thatā€™s only true when g and b are 0. How each tool calculates the changes is a complicated affair involving equations that give me nightmares. :laughing:

Tool: Colors/ R orig 128 G orig 0 B orig 0
Exposure Exposure + 245 0 0
Exposure Exposure - 97 0 0
Color Temperature Color Temperature + 132 0 0
Color Temperature Color Temperature - 109 0 0
Saturation Saturation + 140 0 0
Saturation Saturation - 108 38 38
Brightness-Contrast Contrast + 128 0 0
Brightness-Contrast Contrast - 128 65 65
Hue-Chroma Chroma + 160 0 0
Hue-Chroma Chroma - 95 48 38
Brightness-Contrast Brighten + 139 22 22
Brightness-Contrast Brighten - 105 0 0
Hue-Chroma Lighten + 185 65 44
Hue-Chroma Lighten - 62 0 0
R orig 128 G orig 40 B orig 50
Exposure Exposure + 245 84 103
Exposure Exposure - 69 18 23
Color Temperature Color Temperature + 131 40 47
Color Temperature Color Temperature - 118 41 60
Saturation Saturation + 137 28 43
Saturation Saturation - 115 52 57
Brightness-Contrast Contrast + 128 2 16
Brightness-Contrast Contrast - 128 67 74
Hue-Chroma Chroma + 161 0 36
Hue-Chroma Chroma - 93 65 65
Brightness-Contrast Brighten + 147 63 81
Brightness-Contrast Brighten - 103 32 40
Hue-Chroma Lighten + 199 103 108
Hue-Chroma Lighten - 67 0 0

Contrast is interesting to me because it doesnā€™t change the original R value, just G and B.

11

Your choice of input actually set that up. ā€˜Contrastā€™ is about the rate at which tone changes, which can be considered with a graph of a line with a certain slope. Steeper the slope, higher the contrast, and vice versa. A simple contrast toolā€™s graph will ā€˜pivotā€™ about the center as contrast is changed, so a value of 128 on a 0-255 scale will sit on that pivot, and not change as the slope is changed.

You can demonstrate such with a tone curve. To increase contrast with a curve, take the lower-left control point and drag it right, and then drag the upper-right point left by an equal amount. Should look like this:

rawproc-contrast1

Note the midpoint of the line still intersects the center of the graph. X=128, Y=128. Now, a decrease in contrast looks like this:

rawproc-contrast2

Still, the midpoint of the line intersects the graph center. X=128, Y=128ā€¦

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12

Thanks for pointing that out. Iā€™ll pick some new points and test them.

13

I would add that ā€œcontrastā€ is usually a measure of comparison between pixels. Changing the contrast of a single pixel has no meaning. If we have a mid-gray pixel surrounded by black pixels then increasing the contrast of the image will lighten the gray. But a mid-gray pixel surrounded by white pixels then increasing the contrast of the image will darken the gray.

But a ā€œcontrastā€-increasing control might do something different, and simply lighten all values above some middle, and darken all values below that middle.

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14
Tool: Colors/ R orig 128 G orig 0 B orig 0
Exposure Exposure + 245 0 0
Exposure Exposure - 97 0 0
Color Temperature Color Temperature + 132 0 0
Color Temperature Color Temperature - 109 0 0
Saturation Saturation + 140 0 0
Saturation Saturation - 108 38 38
Brightness-Contrast Contrast + 128 0 0
Brightness-Contrast Contrast - 128 65 65
Hue-Chroma Chroma + 160 0 0
Hue-Chroma Chroma - 95 48 38
Brightness-Contrast Brighten + 139 22 22
Brightness-Contrast Brighten - 105 0 0
Hue-Chroma Lighten + 185 65 44
Hue-Chroma Lighten - 62 0 0
R orig 128 G orig 40 B orig 50
Exposure Exposure + 245 84 103
Exposure Exposure - 69 18 23
Color Temperature Color Temperature + 131 40 47
Color Temperature Color Temperature - 118 41 60
Saturation Saturation + 137 28 43
Saturation Saturation - 115 52 57
Brightness-Contrast Contrast + 128 2 16
Brightness-Contrast Contrast - 128 67 74
Hue-Chroma Chroma + 161 0 36
Hue-Chroma Chroma - 93 65 65
Brightness-Contrast Brighten + 147 63 81
Brightness-Contrast Brighten - 103 32 40
Hue-Chroma Lighten + 199 103 108
Hue-Chroma Lighten - 67 0 0
R orig 170 G orig 40 B orig 50
Exposure Exposure + 255 75 92 1.742
Exposure Exposure - 97 19 25 -1.742
Color Temperature Color Temperature + 176 40 46 7600
Color Temperature Color Temperature - 162 40 56 5400
Saturation Saturation + 189 0 13 1.371
Saturation Saturation - 147 65 69 0.629
Brightness-Contrast Contrast + 187 5 19 0.106
Brightness-Contrast Contrast - 158 65 72 -0.106
Hue-Chroma Chroma + 188 0 39 17.42
Hue-Chroma Chroma - 150 63 62 -17.42
Brightness-Contrast Brighten + 179 63 72 0.106
Brightness-Contrast Brighten - 152 36 45 -0.106
Hue-Chroma Lighten + 233 90 90 17.42
Hue-Chroma Lighten - 118 0 14 -17.42
R orig 170 G orig 40 B orig 50
Exposure Exposure + 85 35 42 1.742
Exposure Exposure - -73 -21 -25 -1.742
Color Temperature Color Temperature + 6 0 -4 7600
Color Temperature Color Temperature - -8 0 6 5400
Saturation Saturation + 19 -40 -37 1.371
Saturation Saturation - -23 25 19 0.629
Brightness-Contrast Contrast + 17 -35 -31 0.106
Brightness-Contrast Contrast - -12 25 22 -0.106
Hue-Chroma Chroma + 18 -40 -11 17.42
Hue-Chroma Chroma - -20 23 12 -17.42
Brightness-Contrast Brighten + 9 23 22 0.106
Brightness-Contrast Brighten - -18 -4 -5 -0.106
Hue-Chroma Lighten + 63 50 40 17.42
Hue-Chroma Lighten - -52 -40 -36 -17.42
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