Module 1 • Lesson 1📐 Scene-Referred, Display-Referred & Gamma

Master-level color starts with a fact most editors never learn: the numbers in your file are not the light in the scene. Understanding the gamma curve — and when to compute in linear light instead — is what makes your blurs, blends, and composites physically correct rather than merely plausible.

📚 What You'll Learn

By the end of this lesson, you will be able to:

Explain scene-referred vs display-referred color
Read a gamma / transfer curve and why a 50% pixel isn't half the light
Say why encoding is perceptual — where the bits are spent and why
Identify edits that are wrong in gamma space and right in linear light
Switch Photoshop to blend/resize in linear light when it matters

⏱️ Estimated Time: 55 minutes

🎯 Project: A side-by-side proof of one operation (a blur or a defocus composite) done in gamma vs linear light, with a written note on which is correct and why.

In This Lesson

👤 The Goal: The Numbers Aren't the Light

The same tonal ramp, two ways. On the right, the raw linear light values shown directly — the way a sensor records photons, with the shadows crushed together. On the left, the same data gamma-encoded for your eye, so the steps look evenly spaced. Both hold the same information; only one matches human vision. Drag the handle:

Gamma-encoded Linear

A diagram of the principle. Gamma encoding is not a "look" — it is how the same light data is packed so that even steps look even to us, spending more code values in the shadows where our eyes are most sensitive.

🧠 Mental Model: The Transfer Curve

Between the light in a scene and the number in your file sits a transfer function (loosely, "gamma"). sRGB uses roughly a 2.2 power curve. The consequence surprises people: a pixel sitting at 50% on the 0–255 scale is emitting only about 21% of the light of white (0.5^2.2 ≈ 0.21). Middle-grey light (18%) actually lands near pixel value 118, not 128.

Why encode at all? Human vision is roughly logarithmic — we discriminate far more finely in shadows than in highlights. A gamma curve spends more of the limited code values down in the darks, so an 8-bit file avoids visible banding where we'd notice it most. Encoding is a perceptual compression, not decoration.

Figure 1: The gamma curve. A little scene light climbs the encoded value fast (the curve's steep start), which is exactly what packs detail into the shadows. The straight dashed line is what "no encoding" would look like.

🔁 Scene- vs Display-Referred

Two philosophies of what your pixels mean. Display-referred (the everyday JPEG/8-bit world) says a pixel is an instruction to the monitor: 255 = "as bright as this screen goes". Values are bounded 0–1 and already tuned for a display. Scene-referred says a pixel is a measurement of light in the scene, in linear units, unbounded — a bright sky might be 12.0, the sun 10,000. You grade in that space and only map to a display at the very end.

Two pipelines

Figure 2: Scene-referred keeps real, unbounded light until a final display transform — which is why film and HDR pipelines grade there. Display-referred is where most 8-bit editing lives, with the highlights already clipped.

⚠️ Clipped highlights never come back

Once a display-referred file has driven the highlights to 255, the information above "white" is gone. Scene-referred grading (and 32-bit, Lesson 1.3) keeps those values alive, which is why you can pull a blown sky back only when you preserved the scene light in the first place.

🧮 When Linear Light Is Correct

Any operation that simulates physical light mixing is only correct in linear space, because in the real world light adds linearly — gamma-encoded numbers do not. In gamma space these operations produce subtly wrong, usually too-dark results:

Blur & defocus — a bokeh highlight blurred in gamma space dims and greys; in linear it stays bright and glowing, like a real lens.
Downscaling / resizing — averaging pixels in gamma darkens fine detail (thin bright lines vanish); linear averaging preserves brightness.
Blending & opacity — a 50% blend of black and white should be middle-grey light, not middle-grey value.
Adding light — glows, lens flares, and additive composites sum correctly only in linear.

Photoshop can blend and interpolate in linear light for you. In Edit ▸ Color Settings ▸ More Options, tick Blend RGB Colors Using Gamma: 1.00, and enable Image Interpolation awareness on resample. For scene-referred grading proper, work in 32-bit (Lesson 1.3).

✅ You don't do everything in linear

Contrast curves, most color grading, and painting feel right in the perceptual (gamma) space — that's what our eyes expect to push against. Reach for linear specifically for light-simulation math: blur, resize, additive glows, and physically-based composites.

🛠️ Guided Build: Gamma vs Linear

You'll prove the difference on one operation so the theory becomes muscle memory.

Step 1: Make a torture test · 6 min

New document, black background. Add small, pure-white dots and thin white lines (bright highlights on dark — the worst case for gamma math).

Step 2: Blur in gamma · 6 min

With default settings, apply a strong Gaussian Blur. Note how the white dots dim to grey as they spread.

Step 3: Blur in linear · 8 min

Undo. Edit ▸ Color Settings ▸ More Options, tick Blend RGB Colors Using Gamma 1.00 (or convert to 32-bit for true linear).
Blur again with the same radius. The highlights now stay bright and glow — physically correct bokeh.

Step 4: Prove resizing too · 8 min

Downscale the line test by 25% in gamma, then in linear. Compare: the gamma version's thin lines darken and drop out; the linear version holds brightness. 🏆

✅ Project Completion Checklist

☐ A highlights-on-black torture test built
☐ Blur compared gamma vs linear (bokeh stays bright in linear)
☐ Downscale compared gamma vs linear (lines survive in linear)
☐ A written note on which is correct and why
☐ Know where the Blend-Gamma-1.0 switch lives

🧗 Now You: Solo Variation

🌟 Your challenge

Composite a defocused string of fairy lights over a night scene in gamma vs linear. The linear version's bokeh will look like a real lens; the gamma one muddy.
Build a soft glow/bloom on a bright sign both ways and feel which reads as emitted light.
Sample a 128-value grey and confirm with the Info panel (set to a linear readout) that it is not half the light of white. Seeing the number cements the idea.

Going further: read up on OETF/EOTF (the camera's and display's transfer functions) and how HDR standards like PQ differ from sRGB gamma. It's the same idea, extended to far brighter light.

🍳 Recipe Card: Linear Light

Compute light where light lives

Remember: pixel value ≠ light (sRGB ≈ 2.2 gamma; 50% value ≈ 21% light)
Encoding is perceptual — bits spent in the shadows on purpose
Do blur, resize, glows, additive composites in linear light
Do contrast, grading, painting in perceptual (gamma) space
Switch: Color Settings ▸ Blend RGB Using Gamma 1.0, or work in 32-bit

Mantra: the file is an encoding of the light, not the light itself.

📓 Learning Journal

Add to your journal after this lesson:

Key concepts you learned
Techniques that clicked for you
Questions or confusion points to revisit
Ideas you want to try
Your progress and feelings about learning this

✍️ This lesson's prompt: Where in your past work might a "too dark when blurred/resized" result actually have been a gamma-space error? Now you have the vocabulary to name it.

🔭 Where This Leads

You now know what the numbers mean. Next you master the tool that turns real captured light into those numbers on your terms: Lesson 1.2 — Camera Raw / ACR at Depth, the develop stage where a flat capture becomes a graded master.

Is "gamma" the same as "contrast"?

No, though people conflate them. Gamma here is the encoding transfer function that maps light to code values. A contrast curve is an artistic adjustment on top. You can change contrast without changing the underlying encoding, and vice versa.

Do I need to think about this on every edit?

No — most perceptual edits are fine as-is. This matters when an operation simulates light (blur, resize, glows, additive composites) or when you grade in HDR/32-bit. Knowing when is the mastery; the rest of the time, edit normally.