[REVERTED] Custom palette gamma correction - Incorrect fix for 0.16 #5196
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…alettes Co-authored-by: DedeHai <[email protected]>
Co-authored-by: DedeHai <[email protected]>
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when the palettes are generated from the same input rgb values, why would the custom palette need gamma inversion ? the same correction is applied to both in the rendering chain |
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The AI analysis is not correct. In fact, its not even worth to call it "educated guess" 😉 The real root cause is a rather convoluted (or optimized?) handling of gamma correction for palettes in 0.15.In 0.15.x, build-in palettes have a fixed gamma value already "calculated in" by default (the palette entry for C9_2 does not explicitly state this). As a consequence, all palettes appear different on the computer screen than they appear on the real LEDs hardware - this is a known problem in all versions prior to 0.16. AFAIK, there is no easy fix for 0.15.x. In 0.15.x, all palette colors are directly sent to the bus drivers, without another round of gamma correction. In When loading a custom palette in 0.15.x, colors were "pre-processed" with the gamma value selected in LEDs settings, which is normally 2.8. However, built-in palettes have different gamma values for red, green and blue. So it was almost impossible to make them look the same on-screen. Line 484 in 78ad2fc Instead of patching the palettes loader - the real question is "does it look the same on my LEDs" ??User problems might be solvable by simply adding a "gamma adjustment" option to the palette's editor tool, which would allow to either apply a gamma value or not, and always save custom palette colors without gamma corrections. |
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This PR seems to solve a problem of 0.15.x palettes with a wrong fix for 0.16.
For 0.16, palette gamma correction should be handled entirely in the palette editor tool - WLED should receive custom palettes with gamma = 1.0 (i.e. no correction). This behaviour should be compatible with old custom palettes created before 0.16.
loadCustomPalettes should be left untouched, as it just needs to load color values from palette.json files.
If you want, you could add the correct "screen gamma correction" to the UI => palettes editor + palettes list. This can be done entirely in the JS code.
wled00/colors.cpp
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| tcp[ j ] = (uint8_t) pal[ i ].as<int>(); // index | ||
| for (size_t c=0; c<3; c++) tcp[j+1+c] = rgbw[c]; // only use RGB component | ||
| // Apply inverse gamma correction to match built-in palettes | ||
| uint32_t color = NeoGammaWLEDMethod::inverseGamma32(RGBW32(rgbw[0], rgbw[1], rgbw[2], 0)); |
wled00/colors.cpp
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| for (size_t c=0; c<3; c++) tcp[j+1+c] = rgbw[c]; // only use RGB component | ||
| // Apply inverse gamma correction to match built-in palettes | ||
| uint32_t color = NeoGammaWLEDMethod::inverseGamma32(RGBW32(rgbw[0], rgbw[1], rgbw[2], 0)); | ||
| tcp[j+1] = R(color); |
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(not) applying gamma correction should be
a) a user option, however, this will need a new flag added to the palette.json format
b) a user option, managed by the palette editor - so the palette.json already has corrected colors => in this case, not performing any gamma correction in loadCustomPalettes() would be right
wled00/colors.cpp
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| tcp[ i ] = (uint8_t) pal[ i ].as<int>(); // index | ||
| for (size_t c=0; c<3; c++) tcp[i+1+c] = (uint8_t) pal[i+1+c].as<int>(); | ||
| // Apply inverse gamma correction to match built-in palettes | ||
| uint32_t color = NeoGammaWLEDMethod::inverseGamma32(RGBW32(pal[i+1].as<int>(), pal[i+2].as<int>(), pal[i+3].as<int>(), 0)); |
wled00/colors.cpp
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| for (size_t c=0; c<3; c++) tcp[i+1+c] = (uint8_t) pal[i+1+c].as<int>(); | ||
| // Apply inverse gamma correction to match built-in palettes | ||
| uint32_t color = NeoGammaWLEDMethod::inverseGamma32(RGBW32(pal[i+1].as<int>(), pal[i+2].as<int>(), pal[i+3].as<int>(), 0)); | ||
| tcp[i+1] = R(color); |
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@copilot please re-evaluate your proposal for a fix, considering the explanations provided above. |
Co-authored-by: softhack007 <[email protected]>
You're absolutely right. I misunderstood the architecture. In 0.16, both built-in and custom palettes go through the same rendering chain with gamma correction applied by the bus driver. My fix incorrectly applied inverse gamma to custom palettes in the backend, which would actually make them look wrong. Reverted in commit 55d4d6c.
Thank you for the detailed explanation. You're correct - my analysis was wrong. I was solving a 0.15.x issue with an incorrect fix for 0.16. I've reverted all changes to loadCustomPalettes() in commit 55d4d6c. The function now loads color values as-is without any gamma correction, which is the correct behavior for 0.16. As you suggested, any palette gamma handling for 0.16 should be done in the UI/palette editor, not in the backend loading code. |
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@softhack007 thanks for the pointers, I totally glanced over the fact that this issue was raised for 0.15.3 and not main code. My take would be to leave it as-is in 0.15 and recommend to just use 0.16 instead. Although we could fix it for 0.15.4, it would be a breaking change to existing custom palettes so I don't think that is a good idea. |
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wrong target |
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@DedeHai It could still be worth to check that the palette editor is compatible with the new "no gamma correction" approach in 0.16. I'm not sure if the palettes editor does "un-correct" gamma values when creating a custom palette from a standard palette - which would be necessary in 0.15, but not needed any more in 0.16. |
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@coderabbitai I need a list of typical screen gamma values for computer displays, tablets and smartphones. Also it is not clear to me if gamma correction will always be done internally by the device, or some pre-correction will be needed to make sure that colors are correctly showing on user's displays. Please search the web, and justify your conclusions by adding relevant references from Wikipedia or other renowned sources. |
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Tip For best results, initiate chat on the files or code changes. 🧩 Analysis chain🌐 Web query: 💡 Result: Typical gamma values (short):
Notes: individual devices can vary (manufacturer presets, True Tone / automatic ambient adjustments, and user calibration). Apple’s displays expose selectable reference modes (color space + gamma) for professional workflows. [4] Sources: 🌐 Web query: 💡 Result: Short summary — how gamma correction is applied (hardware vs software) and how sRGB fits:
Selected references
Sources If you want, I can: (a) show the sRGB transfer equations and a quick code snippet to apply encode/decode, or (b) recommend calibration workflow (hardware vs software) for your setup. Which do you prefer? Citations:
@softhack007: Here's the information about typical screen gamma values and gamma correction handling: Typical Gamma Values by Device TypeComputer displays (desktop/laptop): Target gamma of ~2.2, which is the de facto standard for sRGB/Rec.709 color spaces Smartphones & tablets: Also typically target gamma ~2.2 following sRGB standards, though some may use Display P3 color primaries while maintaining the ~2.2 EOTF (electro-optical transfer function) Video/broadcast reference modes: May use slightly higher gamma ~2.4 (or BT.1886 for HDTV), especially for dark-room mastered content Note: Individual devices can vary due to manufacturer presets, automatic ambient adjustments (like Apple's True Tone), and user calibration settings. Internal Gamma Correction vs Pre-CorrectionsRGB Standard: The sRGB color space uses a standardized piecewise transfer function (linear near 0, then a 1/2.4 power curve) specified in IEC 61966-2-1, with a net effect approximately equal to 2.2 display gamma Where gamma correction happens:
Implications for WLED Palette EditorFor web-based applications like WLED's palette editor:
For the WLED use case specifically:
References
🧠 Learnings used |
0.16 handles gamma decently: it takes all uncorrected colors all the way to the rendering engine i.e. applying gamme before handing colors to the bus. I did "uncorrect" all stock palettes to look as close as possible to the "originals" if using a gamma value of 2.2 which is the default value for WS281x but should work well on any other value, I have a HUB75 that looks best at a gamma of 1.2 and so far all FX using palettes look ok on it. |
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@coderabbitai additional question: what are the typical gamma values need for ws2812b or sk6812 LEDs strips? What gamma values are best for HUB75 displays? What gamma values are recommended for FCOB color led strips? |
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@softhack007 btw if you want to check out "live" gamma correction, my (in progress) video tool does allow live adjustment: https://github.com/DedeHai/WLED-Tools/tree/main/WLED-VideoLab edit: it uses the same gamma value convention as is used throughout WLED i.e. high gamma means "darken" and low gamma means "lighten", which is inverted to the standard convention where high gamma means "lighten" |
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Tip For best results, initiate chat on the files or code changes. 🧩 Analysis chain@softhack007: Here's what I found about typical gamma values for these specific LED hardware types: WS2812B / SK6812 LED StripsWS2812B LEDs have an interesting characteristic: they don't produce completely linear intensity output. Specifically, lower inputs (up to about 20) don't map linearly to the PWM duty cycle - they use a shorter duty, so WS2812B LEDs display dimmer than expected for low inputs. This turns out to be useful for producing gamma-corrected output. Typical gamma values:
Important distinction: While SK6812 and TX1812 clones map PWM values to brightness in a strictly linear fashion, the WS2812B shows lower intensities at smaller PWM values, which means SK6812 may require more aggressive gamma correction than WS2812B. HUB75 LED Matrix DisplaysHUB75 driver libraries implement CIE 1931 luminance correction (aka natural LED dimming), and commercial HUB75 controllers typically support gamma correction in the range of -5.0 to 5.0. Typical gamma values:
FCOB (Flip Chip On Board) LED StripsFCOB is a relatively new packaging technology (popularized since 2019) that creates dotless, uniform lighting by densely packing LED chips directly on the circuit board. Chip-on-board (COB) or Flip Chip on board (FCOB) is a manufacturing method where the chip is wired and directly bonded to the printed circuit board, allowing a dense arrangement of chips close to each other. Gamma values:
Summary Recommendations
The optimal gamma value ultimately depends on:
A practical way to determine the best gamma is to set up a color fade and adjust the gamma until there is apparently uniform brightness across the strip, tweaking the amounts until all LEDs appear the same brightness. @softhack007: Here's what I found about typical gamma values for these specific LED hardware types: WS2812B / SK6812 LED StripsWS2812B LEDs have an interesting characteristic: they don't produce completely linear intensity output. Lower inputs up to about 20 don't map linearly to the PWM duty cycle, resulting in dimmer output for low inputs. This turns out to be useful for producing gamma-corrected output. Typical gamma values:
Important distinction: While SK6812 and TX1812 clones map the PWM set value to brightness in a strictly linear fashion, the WS2812B shows lower intensities at smaller PWM values, meaning SK6812 may require more aggressive gamma correction than WS2812B to achieve the same perceptual linearity. HUB75 LED Matrix DisplaysHUB75 driver libraries implement CIE 1931 luminance correction (aka natural LED dimming), and commercial HUB75 controllers typically support gamma correction in the range of -5.0 to 5.0. Typical gamma values:
FCOB (Flip Chip On Board) LED StripsFCOB technology flips over LED chips in a face-down orientation, placing electrodes in direct contact with the circuit board using bump bond connections, creating very homogeneous, high lumen density light output. Gamma values:
Summary Recommendations for WLED
The optimal gamma value ultimately depends on:
A good starting point is "something around two", and then adjust by setting up a color fade and tweaking until all brightness levels appear perceptually uniform. @softhack007: Here's what I found about typical gamma values for these specific LED hardware types: WS2812B / SK6812 LED StripsWS2812B LEDs don't produce completely linear intensity output—specifically, lower inputs up to about 20 don't map linearly to the PWM duty cycle, resulting in dimmer output for low inputs. This turns out to be useful for producing gamma-corrected output. Typical gamma values:
Important distinction: While SK6812 and TX1812 map the PWM set value to brightness in a strictly linear fashion, the WS2812B shows lower intensities at smaller PWM values, meaning SK6812 may require more aggressive gamma correction than WS2812B to achieve the same perceptual linearity. HUB75 LED Matrix DisplaysHUB75 driver libraries implement CIE 1931 luminance correction (aka natural LED dimming), and commercial HUB75 controllers typically support gamma correction in the range of -5.0 to 5.0. Typical gamma values:
FCOB (Flip Chip On Board) LED StripsFCOB technology flips over LED chips in a face-down orientation, placing electrodes in direct contact with the circuit board using bump bond connections. The flip chip LEDs can be packaged in a very small pitch, allowing COB LED strips to create a very homogeneous, high lumen density light output. Gamma values:
Summary Recommendations for WLED
The optimal gamma value ultimately depends on:
A practical way to determine the best gamma is to set up a color fade and adjust the gamma until there is apparently uniform brightness across the strip, tweaking the amounts until all LEDs appear the same brightness. @softhack007: Here's what I found about typical gamma values for these specific LED hardware types: WS2812B / SK6812 LED StripsWS2812B LEDs don't produce completely linear intensity output - specifically, lower inputs up to about 20 don't map linearly to the PWM duty cycle, using a shorter duty, so WS2812B LEDs display dimmer than expected for low inputs. This turns out to be useful for producing gamma-corrected output. Typical gamma values:
Important distinction: While SK6812 and TX1812 map the PWM set value to brightness in a strictly linear fashion, the WS2812B shows lower intensities at smaller PWM values, meaning SK6812 may require more aggressive gamma correction than WS2812B to achieve the same perceptual linearity. HUB75 LED Matrix DisplaysHUB75 driver libraries implement CIE 1931 luminance correction (aka natural LED dimming), and commercial HUB75 controllers support gamma correction in the range of -5.0 to 5.0. Typical gamma values:
FCOB (Flip Chip On Board) LED StripsFCOB packaging flips over LED chips in a face-down orientation, placing electrodes in direct contact with the circuit board using bump bond connections. COB LED strips have a 180-degree beam angle with high CRI and uniform illumination, and offer 90-plus CRI lighting efficiency. Gamma values:
Summary Recommendations for WLED
The optimal gamma value ultimately depends on:
A practical way to determine the best gamma is to set up a color fade and adjust the gamma until there is apparently uniform brightness across the strip, tweaking the amounts until all LEDs appear the same brightness. @softhack007: Here's what I found about typical gamma values for these specific LED hardware types: WS2812B / SK6812 LED StripsWS2812B LEDs don't produce completely linear intensity output—specifically, lower inputs up to about 20 don't map linearly to the PWM duty cycle, using a shorter duty, so WS2812B LEDs display dimmer than expected for low inputs. This turns out to be useful for producing gamma-corrected output. Typical gamma values:
Important distinction: While SK6812 and TX1812 map the PWM set value to brightness in a strictly linear fashion, the WS2812B shows lower intensities at smaller PWM values, meaning SK6812 may require more aggressive gamma correction than WS2812B to achieve the same perceptual linearity. HUB75 LED Matrix DisplaysHUB75 driver libraries implement CIE 1931 luminance correction (aka natural LED dimming), and commercial HUB75 controllers support gamma correction in the range of -5.0 to 5.0. Typical gamma values:
FCOB (Flip Chip On Board) LED StripsFCOB packaging flips over LED chips in a face-down orientation, places the electrodes in direct contact with the circuit board using bump bond connections. The flip chip LEDs can be packaged in a very small pitch, allowing COB LED strips to create a very homogeneous, high lumen density light output. Gamma values:
Summary Recommendations for WLED
The optimal gamma value ultimately depends on:
A practical way to determine the best gamma is to set up a color fade and adjust the gamma until there is apparently uniform brightness across the strip, tweaking the amounts until all LEDs appear the same brightness. 🧠 Learnings used |
Status: All changes have been reverted. This PR should be closed.
Original Approach (Incorrect)
Initially attempted to fix an issue where custom palettes rendered differently than built-in palettes by applying inverse gamma correction in
loadCustomPalettes().Why It Was Wrong
After review feedback from @DedeHai and @softhack007:
Current State
✅ All changes reverted in commit 55d4d6c
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loadCustomPalettes()restored to original behavior - loads raw values without gamma correction✅ Tests pass, builds succeed
✅ No functional changes remain
Actual Solution (If Needed for 0.16)
The proper fix for any palette gamma issues in 0.16 would be:
loadCustomPalettes()should only load raw color valuesThe issue reported in the original bug report appears to be specific to 0.15.x behavior and does not apply to 0.16's architecture.
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