Controlling Display Gamma on the Synaptics DRM Driver

gamma_set is a userspace application to display BMP images and control gamma correction on DRM CRTCs using the Synaptics display driver.

Features

• Loads and displays BMP images (24-bit and 32-bit)

• Direct CRTC gamma control with specified CRTC ID

• Interactive gamma value switching (-i option)

• Dynamic gamma table generation

• Dumps gamma tables with formula (-d option)

• BMP images are centered on the display

Gamma Formula

The gamma lookup table is generated using:

output = 255 * (input / 255) ^ gamma

Where:

• input = 0, 1, 2, …, 255

• gamma = 1.8, 2.2, 2.4, 2.6

Supported Gamma Values

Value	Effect
0	Clear gamma (disable)
1.8	Lighter image
2.2	Standard (sRGB)
2.4	Slightly darker
2.6	Darker image

Note

Gamma change is applicable only for SL1620 and SL26XX series platforms.

Source Code

https://github.com/synaptics-astra/application/blob/scarthgap_6.12_v2.3.0/display/gamma_set.c

Compilation

1. Setup Cross-Compilation Environment

. /data/jjogj/repos/snapshots/poly/toolchain/syna_image/poky_voip/environment-setup-cortexa55-poky-linux

2. Compile

$CC gamma_set.c -o gamma_set -ldrm -lm \
    -I /data/jjogj/repos/snapshots/poly/toolchain/syna_image/poky_voip/sysroots/cortexa55-poky-linux/usr/include/drm/

Note

-lm is required for math functions (pow, round).

Usage

gamma_set [-d] [-b <bmp_file>] [-i] <crtc_id> <gamma>

Arguments:
  crtc_id         DRM CRTC identifier (e.g., 36)
  gamma           Gamma value: 0 (clear), 1.8, 2.2, 2.4, 2.6

Options:
  -d              Dump all gamma tables before applying
  -b <bmp_file>   Display BMP file on CRTC before applying gamma
  -i              Interactive mode for changing gamma continuously

Examples

# Set gamma 2.2 on CRTC 36 (exits immediately)
gamma_set 36 2.2

# Dump tables, then set gamma
gamma_set -d 36 2.2

# Interactive mode on CRTC 36
gamma_set -i 36 2.2

# Display BMP and set gamma (waits for 'q' to quit)
gamma_set -b image.bmp 36 2.2

# Dump tables, display BMP, set gamma
gamma_set -d -b image.bmp 36 2.2

# Display BMP with interactive gamma switching
gamma_set -b image.bmp -i 36 2.2

Execution

Prerequisites

• The application requires DRM master access (run as root)

• Stop any display server that may hold DRM master (for -b mode)

• For BMP mode: BMP image must be smaller than or equal to the display resolution

Mode 1: Direct CRTC Gamma Mode

Set gamma directly on a CRTC. Exits immediately after applying:

./gamma_set 36 2.2

Output:

Gamma 2.2 applied

Mode 2: BMP Display Mode

Display a BMP image and apply gamma. Waits for q to quit:

# Stop weston if running
systemctl stop weston

# Run gamma_set with BMP
./gamma_set -b /usr/share/grey.bmp 36 2.2

Output:

Loading BMP: /usr/share/grey.bmp
BMP loaded: 1920x1080
CRTC 36 current mode: 1920x1080
Found display: connector 33, CRTC 36, 1920x1080@60Hz
BMP displayed on CRTC 36
Gamma 2.2 applied
Press 'q' to quit: q

Mode 3: Interactive Mode

Change gamma values interactively without restarting:

./gamma_set -i 36 2.2

Output:

Gamma 2.2 applied

Interactive mode. Available gamma values: 0 (clear), 1.8, 2.2, 2.4, 2.6
Press 'q' to quit (or enter gamma value): 1.8
Gamma 1.8 applied
Press 'q' to quit (or enter gamma value): 0
Gamma cleared
Press 'q' to quit (or enter gamma value): q

Mode 4: BMP with Interactive Mode

Display a BMP and interactively change gamma:

./gamma_set -b image.bmp -i 36 2.2

Mode 5: With modetest

Use modetest to hold the display active while gamma_set controls gamma.

Terminal 1 – Start display with modetest:

systemctl stop weston
modetest -M synaptics -s 33@36:#0 -d

Terminal 2 – Set gamma:

./gamma_set 36 2.2
./gamma_set 36 1.8
./gamma_set 36 0

Note

The CRTC ID and connector ID can be found using:

modetest -M synaptics

Mode 6: Dump Gamma Tables

./gamma_set -d 36 2.2

Output (truncated):

Gamma Table Generation Formula:
================================

  output = 255 * (input / 255) ^ gamma

  where: input = 0, 1, 2, ..., 255
         gamma = 1.8, 2.2, 2.4, 2.6

Gamma 1.8:  output = 255 * (input/255)^1.8
---------
  0,  0,  0,  0,  0,  0,  0,  0,  1,  1,  1,  1,  1,  1,  1,  2
  ...

Gamma 2.2 applied

Expected Results

Gamma	Visual Effect
1.8	Image appears lighter / washed out
2.2	Standard appearance
2.4	Image appears darker
2.6	Image appears significantly darker
0	Gamma disabled (linear)

BMP Image Support

• Supported formats: 24-bit and 32-bit BMP

• BMP must be smaller than or equal to the display resolution

• Images smaller than the display are centered on screen

• Images larger than the display will cause an error:

  Error: BMP image (2560x1440) is larger than display (1920x1080)
  

Display Support

• Works with any connected display resolution

• Automatically selects the preferred mode from EDID

• Falls back to the first available mode if no preferred mode is reported

Error Messages

Error	Cause
Failed to load BMP	Invalid or unsupported BMP file
BMP image (WxH) is larger than display (WxH)	BMP resolution exceeds display
No connector found for CRTC	No display connected to specified CRTC
Invalid gamma value	Gamma value not in the supported list
GAMMA_LUT property not found for CRTC	Driver does not support GAMMA_LUT on this CRTC

Notes

• Direct mode (gamma_set 36 2.2) exits immediately after applying gamma.

• BMP mode (-b) waits for q to quit, keeping the display active.

• Interactive mode (-i) allows changing gamma values without restarting.

• Options can be combined: -d -b image.bmp -i.

• Gamma changes take effect immediately during the next vertical blanking interval.

• Use modetest -M synaptics to find connector and CRTC IDs.

Gamma Curve

The formula below is used to calculate the gamma table; these values are used to plot the gamma curve:

output = 255 * (input / 255) ^ gamma

where: input = 0, 1, 2, ..., 255
       gamma = 1.8, 2.2, 2.4, 2.6

Example 1

Gamma disable

Gamma 1.8

Gamma 2.2

Gamma 2.4

Gamma 2.6

Example 2

Gamma disable

Gamma 1.8

Gamma 2.2

Gamma 2.4

Gamma 2.6.

Please find the attached original test images