lutspeak reads a LUT or look file and writes a plain description of what it does to the image. It tells you things like how much contrast it adds, whether it lifts or crushes the blacks, how it shifts colour and hue, and how it treats skin tones. You do not need to load footage. You only need the LUT file itself.
Everything runs in your browser. Your files are never uploaded anywhere. The analysis happens on your own computer.
Opening a LUT
Drag a file onto the page, or click "Open LUT" and choose a file from your computer. The report appears straight away.
Supported file types are .cube, .3dl, .alf4, .alf4c, .aml, and .l16.
Telling it which camera the LUT is for
Many LUTs are built to take log footage from a specific camera and turn it into a normal-looking image. To read the LUT correctly, lutspeak needs to know which camera log the LUT expects as its input.
By default the "Camera" control is set to "auto detect", and lutspeak will try to work this out from the file name. If it cannot tell, or if the file name is misleading, pick your camera from the list yourself. Supported cameras include:
- ARRI ALEXA 35 / 265 (LogC4)
- ARRI ALEXA Mini / LF (LogC3)
- Sony VENICE / FX (S-Log3)
- RED (Log3G10)
- Panasonic (V-Log)
- Canon Cinema EOS (Canon Log 2 or Canon Log 3)
- Blackmagic (Film Gen 5)
- DaVinci Wide Gamut / Intermediate
- Fujifilm (F-Log or F-Log2)
- Nikon (N-Log)
- DJI (D-Log)
- Apple Log
- ACES (ACEScct or ACEScc)
If your LUT is not built from camera log at all, and instead takes a normal video image as its input, choose "Other / display-referred" instead.
Getting this setting right matters. If lutspeak reads the LUT with the wrong camera log, every number in the report will be wrong, because it will be measuring the wrong points on the curve.
Once you have chosen a camera, the "Output" control sets which display the transform targets, for example Rec709 or P3. The Camera control then reads as the whole transform, such as "ARRI LogC4 to Rec709". lutspeak picks a sensible output for you, usually Rec709, so you only need to change it if you want a different display standard.
Choosing what to compare against
lutspeak describes a LUT by comparing it to something else. The "Compare to" control sets what that something is, with four choices:
- standard render (the default): the camera's own official image for the chosen output, so the report describes only what your LUT adds on top of a plain, undoctored image. If no camera is set, this is an unchanged signal.
- another file: load your own reference, for example your show's base LUT or another camera's official render. You can also drop a file straight onto the control.
- another LUT (A/B): load a second LUT and the report describes it relative to the first, for comparing two of your own looks directly.
- nothing: compare against an unchanged signal even when a camera is set, to see the raw transform on its own.
Reading the report
The first line tells you how the LUT was read, for example "Read as ARRI ALEXA 35 (LogC4), detected from the file name". This is the most important line to check: if the input encoding is wrong, every number below it is wrong. When lutspeak cannot tell which camera the LUT is for, it says so and falls back to reading the file as a plain display image, which is flagged so you notice. Use the "change" button next to it to jump to the Camera control and set the right one.
Below that is the verdict. It is a short, direct summary, for example "A punchy, warm look with lifted blacks and a strong shift in the reds."
Below that is a paragraph that explains the verdict in more detail. It covers contrast, blacks and highlights, colour cast, saturation, hue shifts by colour family, and skin tones.
At the fullest Detail setting, a "What this means" section follows the paragraph: a few short points on what the measurements imply for footage going through the LUT. See "Detail" below.
Under the paragraph is a "show numbers" link. Click it to reveal a measurements block with the actual numbers behind the description, such as the exact contrast ratio, the exposure offset in stops, the black floor and peak white levels, and saturation ratios for shadows, mid-tones, and highlights.
Sometimes a note appears above or below the report. These point out things worth knowing, for example that the LUT looks like a technical camera-to-monitor conversion rather than a creative grade, that the file seems to be in legal or video range rather than full range, or that the reference has no colour in it so colour comparisons are not possible. Read these before trusting the rest of the report.
What the words mean
The description uses a fixed vocabulary, so once you have read a few reports the words themselves tell you the size of a move. The same word always means the same band, so you can calibrate against it and stop reading the numbers. Percentages are the change against the reference.
- Contrast. "a touch more" or "a touch softer" from about 4%, "punchy" or "soft" from about 10%, "high-contrast" or "flat" from about 28%.
- Saturation. "muted" from about 11% down and "heavily muted" from about 28% down; a boost is quoted as a number until about 28% up, where it becomes "richly saturated". The strong word lands at the same 28% as contrast.
- Warmth and tint. "a touch warm" or "cool" for a slight cast, "warm" or "cool" for a clear one, "heavily" for a strong one; the same three steps apply to a green or magenta tint. "split-toned" means the warm and cool ends pull opposite ways, the orange-and-teal arrangement.
- Exposure. Given in the stops a cinematographer says out loud, a third at a time, for example "a print-down of about 1⅓ stops".
- Blacks. "lifted" then "heavily lifted" as the floor rises off true black, "crushed" when shadow detail is lost, "a deeper black floor" when blacks sit denser than the reference.
- Highlights. "clip" when highlights hit one level, "a plateau" when they level off early below peak white, "capped" when the brightest the look reaches is well under white. "flat" is reserved for contrast, so it is never used of highlights.
- Hue. A colour family "leans", then "pushes", then "swings" toward another hue as the rotation grows.
The panels
Below the report are three visual panels.
The tone curve panel plots how input brightness maps to output brightness, compared to the reference. It also marks the points where shadows crush to black and where highlights clip to white, if any.
The skin spheres panel shows two rendered spheres side by side, one lit as the reference would render it and one lit as your LUT would render it, so you can see the effect on skin tone directly.
The hue and saturation shape panel is a wheel that shows how each colour is rotated and how much more or less saturated it becomes, compared to a perfect circle which would mean no change at all.
Checking several files at once
You can drop more than one file, or a whole folder of files, onto the page at once. Instead of a single report, you get a table listing every file with its contrast, saturation, and warmth, sorted however you like. Click a row to open the full report for that file, and use the "Back to list" button above the report to return to the table.
Data levels
Some LUTs output what is called legal or video range, meaning black and white sit slightly inside the normal 0 to 100 percent range rather than at the very ends. If lutspeak is not told about this, the LUT can look like it has lifted blacks and capped whites when it does not.
The "Data levels" control has three settings. "Auto" detects this automatically and is correct almost all the time. "Full range" and "Legal / video" let you force the setting by hand if you know better than the automatic detection. On a phone or small screen, this control only appears when there is genuine doubt about which setting applies, so most of the time you will not need to touch it at all.
Detail
Under "View" is a "Detail" control that sets how much the report says in words. It has three settings, and each one keeps everything the setting below it said:
- Terse gives you the verdict on its own: one sentence naming what the LUT does.
- Normal adds the readout paragraph, which walks through what was measured: contrast, black floor, highlights, cast, saturation and hue.
- Full adds a "What this means" section: what follows from those measurements for footage going through this LUT, such as which detail can no longer be graded back, whether a black floor will fail a deliverable, or whether a saturation reading is really the contrast curve at work. It is written as consequences rather than instructions, because the report knows what the LUT does but not what your grade is for. It says at most four things, and on a LUT with nothing notable about it, nothing.
The setting is remembered between visits. The measurements are not part of this ladder: they sit behind the "show numbers" link at every setting, so you can read a one-line verdict and still check a number.
Theme
Under "View" you can also switch between a dark and a light theme.
Copying or printing the report
The copy and print buttons sit at the top of the report itself. Copy report always puts the complete text on your clipboard, including the numbers and the "What this means" section, whatever "Detail" is set to on screen, so a pasted report stands on its own in an email or a document. Print gives you the report as it appears on screen, so it follows the Detail setting; it opens your browser's print dialogue, which you can also use to save a PDF.
If a warning appears
lutspeak will sometimes warn you rather than just describe the LUT. Common warnings include:
- A LUT and its reference using different camera logs, which means the comparison is not meaningful.
- A reference that is itself a technical camera-to-monitor conversion rather than a plain image, in which case the right move is usually to set that file as your reference rather than treat it as a look to describe.
- A reference with no colour in it, which means saturation and hue cannot be measured, only tone.
- An HDR output (PQ or HLG), which lutspeak describes in nits or relative to reference white rather than as a percentage, since ordinary percentages do not mean the same thing in HDR.
If you see a warning, read it before trusting the report below it. It is telling you something the plain description cannot capture on its own.