[PATCH v1 1/1] Documentation: Add first version of the tuning-guide
Stefan Klug
stefan.klug at ideasonboard.com
Tue Aug 6 16:44:41 CEST 2024
This patch adds a initial version of the tuning guide for libcamera. The
process is established based on the imx8mp and will be improved when we
do more tuning for different ISPs with our own tooling.
Signed-off-by: Stefan Klug <stefan.klug at ideasonboard.com>
---
Documentation/conf.py | 4 +-
Documentation/guides/tuning/tuning.rst | 276 +++++++++++++++++++++++++
Documentation/index.rst | 1 +
Documentation/meson.build | 1 +
4 files changed, 280 insertions(+), 2 deletions(-)
create mode 100644 Documentation/guides/tuning/tuning.rst
diff --git a/Documentation/conf.py b/Documentation/conf.py
index 7eeea7f3865b..5387942b9af5 100644
--- a/Documentation/conf.py
+++ b/Documentation/conf.py
@@ -21,8 +21,8 @@
# -- Project information -----------------------------------------------------
project = 'libcamera'
-copyright = '2018-2019, The libcamera documentation authors'
-author = u'Kieran Bingham, Jacopo Mondi, Laurent Pinchart, Niklas Söderlund'
+copyright = '2018-2024, The libcamera documentation authors'
+author = u'Kieran Bingham, Jacopo Mondi, Laurent Pinchart, Niklas Söderlund, Stefan Klug'
# Version information is provided by the build environment, through the
# sphinx command line.
diff --git a/Documentation/guides/tuning/tuning.rst b/Documentation/guides/tuning/tuning.rst
new file mode 100644
index 000000000000..1c30f3f20b4e
--- /dev/null
+++ b/Documentation/guides/tuning/tuning.rst
@@ -0,0 +1,276 @@
+.. SPDX-License-Identifier: CC-BY-SA-4.0
+
+Sensor Tuning Guide
+===================
+
+To create visually good images out of the raw data provided by the camera
+sensor, a lot of image processing takes place. This is usually done inside an
+ISP (Image Signal Processor). To be able to do the necessary processing, the
+corresponding algorithms need to be parameterized according to the used hardware
+(typically sensor, light and lens). Creating these parameters is called tuning.
+The tuning process results in a tuning file which is then used by libcamera to
+parameterize the algorithms at runtime.
+
+The processing block of an ISP vary from vendor to vendor and can be
+arbitrarily complex. Never the less a diagram of the most common and important
+blocks is shown below:
+
+ ::
+
+ +--------+
+ | Light |
+ +--------+
+ |
+ v
+ +--------+ +-----+ +-----+ +-----+ +-----+ +-------+
+ | Sensor | -> | BLC | -> | AWB | -> | LSC | -> | CCM | -> | Gamma |
+ +--------+ +-----+ +-----+ +-----+ +-----+ +-------+
+
+**Light** The light used to light the scene has a crucial influence on the
+resulting image. The human eye and brain are specialized in automatically
+adapting to different lights. In a camera this has to be done by an algorithm.
+Light is a complex topic and to correctly describe light sources you need to
+describe the spectrum of the light. To simplify things, lights are categorized
+according to their `color temperature (K)
+<https://en.wikipedia.org/wiki/Color_temperature>`_. This is important to
+keep in mind as it means that calibrations may differ between light sources even
+though they have the same nominal color temperature. For best results the tuning
+images need to be taken for a complete range of light sources.
+
+**Sensor** The sensor captures the incoming light and produces the raw images.
+Data is commonly filtered by a color filter array into red, green and blue
+channels. As these filters are not perfect some postprocessing needs to be done
+to recreate the correct color.
+
+**BLC** Black level correction. Even without incoming light a sensor produces
+pixel values above zero. This is due to a artificially added pedestal value
+(Which is added to get a evenly distributed noise level around zero instead of
+only the upper half) and other effects in the electronics light dark currents.
+To get good images with real black, that black level needs to be subtracted. As
+that level is typically known for a sensor it is hardcoded in libcamera and does
+not need any calibration at the moment.
+
+**AWB** Auto white balance. For a proper image the color channels need to be
+adjusted, to get correct white balance. This means that monochrome objects in
+the scene appear monochrome in the output image (white is white and gray is
+gray). In libcamera this is done based on a gray world model. No tuning is
+necessary for this step.
+
+**LSC** Lens shading correction. The lens in use has a big influence on the
+resulting image. The typical effects on the image are lens-shading (also called
+vignetting). This means that due to the physical properties of a lens the
+transmission of light falls of to the corners of the lens. To make things even
+harder, this falloff can be different for different colors/wave lengths. LSC is
+therefore tuned for a set of light sources and for each color channel
+individually.
+
+**CCM** Color correction matrix. After the previous processing blocks the grays
+are preserved, but colors are still not correct. This is mostly due to the
+color temperature of the light source and the imperfections in the color filter
+array of the sensor. To correct for this a so called color correction matrix is
+calculated. This is a 3x3 matrix, that is used to optimize the captured colors
+regarding the perceived color error. To do this a chart of known and precisely
+measured colors (macbeth chart) is captured. Then the matrix is calculated
+using linear optimization to best map each measured colour of the chart to it's
+known value. The color error is measured in `deltaE
+<https://en.wikipedia.org/wiki/Color_difference>`_.
+
+**Gamma** Gamma correction. Today's display usually apply a gamma of 2.2 for
+display. For images to be perceived correctly by the human eye, they need to be
+encoded with the corresponding inverse gamma. See also
+<https://en.wikipedia.org/wiki/Gamma_correction>. This block doesn't need
+tuning, but is crucial for correct visual display.
+
+Materials needed for the tuning process
+---------------------------------------
+
+Precisely calibrated optical equipment is very expensive and out of the scope of
+this document. Still it is possible to get reasonably good calibration results
+at little costs. The most important devices needed are:
+
+ - A light box with the ability to produce defined light of different color
+ temperatures. Typical temperatures used for calibration are 2400K
+ (incandescent), 2700K (fluorescent), 5000K (daylight fluorescent), 6500K
+ (daylight). As a first test, keylights for webcam streaming can be used.
+ These are available with support for color temperatures ranging from 2500K
+ to 9000K. For better results professional light boxes are needed.
+ - A ColorChecker chart. These are sold from calibrite and it makes sense to
+ get the original one.
+ - A integration sphere. This is used to create completely homogenious light
+ for the lens shading calibration. We had good results with the use of
+ a homogeneous area light (the above mentioned keylight).
+ - An environment without external light sources. Ideally calibration is done
+ without any external lights in a fully dark room. A black curtain is one
+ solution, working by night is the cheap alternative.
+
+
+Overview over the process
+-------------------------
+
+The tuning process of libcamera consists of the following steps:
+
+ 1. Take images for the tuning steps with different color temperatures
+ 2. Configure the tuning process
+ 3. Run the tuning script to create a corresponding tuning file
+
+
+Taking raw images with defined exposure/gain
+--------------------------------------------
+
+For all the tuning files you need to capture a raw image with a defined exposure
+time and gain. It is crucial that no pixels are saturated on any channel. At the
+same time the images should not be too dark. Strive for a exposure time, so that
+the brightest pixel is roughly 90% saturated. Finding the correct exposure time
+is currently a manual process. We are working on solutions to aid in that
+process. After finding the correct exposure time settings, the easiest way to
+capture a raw image is with the ``cam`` application that gets installed with the
+regular libcamera install.
+
+Create a ``constant-exposure.yaml`` file with the following content:
+
+.. code:: yaml
+
+ frames:
+ - 0:
+ AeEnable: false
+ ExposureTime: 1000
+
+Then capture one or more images using the following command:
+ ``cam -c 1 --script constant-exposure.yaml -C -s role=raw --file=image_#.dng``
+
+
+Capture images for lens shading correction
+------------------------------------------
+
+To be able to correct lens shading artifacts, images of a homogeneously lit
+neutral gray background are needed. Ideally a integration sphere is used for
+that task.
+
+As a fallback images of a area light or a white wall can be used. If there are
+multiple images for the same color temperature, the tuning scripts will average
+the tuning results which can further improve the tuning quality.
+
+.. Note:: Currently lsc is ignored in the ccm calculation. This can lead to
+ slight color deviations and will be improved in the future.
+
+Images shall be taken for multiple color temperatures and named
+``alsc_<temperature>k_#.dng``. ``#`` can be any number, if multiple images are
+taken for the same color temperature.
+
+.. figure:: img/setup_lens_shading.jpg
+ :width: 50%
+
+ Sample setup using an area light. Due to the wide angle lens the camera has
+ to moved closely in front of the light. This is not a perfect solution as
+ angle to the light might get way too large.
+
+.. figure:: img/camshark-alsc.png
+ :width: 100%
+
+ A calibration image for lens shading correction.
+
+- Ensure the full sensor is lit. Especially with wide angle lenses it may be
+ difficult to get the full field of view lit homogeneously.
+- Ensure no color channel is saturated (can be checked with camshark)
+
+Take a raw dng image for multiple color temperatures:
+ ``cam -c 1 --script constant-exposure.yaml -C -s role=raw --file=alsc_3200k_#.dng``
+
+
+
+Capture images for color calibration
+------------------------------------
+
+To do the color calibration, raw images of the color checker need to be taken
+for different light sources. These need to be named
+``<sensor_name>_<lux-level>l_<temperature>k_0.dng``.
+
+For best results the following hints should be taken into account:
+ - Ensure that the 18% gray patch (third from the right in bottom line) is
+ roughly at 18% saturation for all channels (a mean of 16-20% should be fine)
+ - Ensure the color checker is homogeneously lit (ideally from 45 degree above)
+ - No straylight from other light sources is present
+ - The color checker is not too small and not too big (so that neither lens
+ shading artifacts nor lens distortions are prevailing in the area of the
+ color chart)
+
+If no lux meter is at hand to precisely measure the lux level, a lux meter app
+on a mobile phone can provide a sufficient estimation.
+
+.. figure:: img/setup_calibration2.jpg
+ :width: 50%
+
+
+Run the tuning scripts
+----------------------
+
+After taking the calibration images, you should have a directory with all the
+tuning files. It should look something like this:
+
+ ::
+
+ ../tuning-data/
+ ├── alsc_2500k_0.dng
+ ├── alsc_2500k_1.dng
+ ├── alsc_2500k_2.dng
+ ├── alsc_6500k_0.dng
+ ├── imx335_1000l_2500k_0.dng
+ ├── imx335_1200l_4000k_0.dng
+ ├── imx335_1600l_6000k_0.dng
+
+
+The tuning scripts are part of the libcamera source tree. After cloning the
+libcamera sources the necessary steps to create a tuning file are:
+
+ ::
+
+ # install the necessary python packages
+ cd libcamera
+ python -m venv venv
+ source ./venv/bin/activate
+ pip install -r utils/tuning/requirements.txt
+
+ # run the tuning script
+ utils/tuning/rkisp1.py -c config.yaml -i ../tuning-data/ -o tuning-file.yaml
+
+After the tuning script has run, the tuning file can be tested with any
+libcamera based application like `qcam`. To quickly switch to a specific tuning
+file, the environment variable ``LIBCAMERA_<pipeline>_TUNING_FILE`` is helpful.
+E.g.:
+
+ ::
+
+ LIBCAMERA_RKISP1_TUNING_FILE=/path/to/tuning-file.yaml qcam -c1
+
+
+Sample images
+-------------
+
+
+.. figure:: img/image-no-blc.png
+ :width: 800
+
+ Image without black level correction (and completely invalid color estimation).
+
+.. figure:: img/image-no-lsc-3000.png
+ :width: 800
+
+ Image without lens shading correction @ 3000k. The vignetting artifacts can
+ be clearly seen
+
+.. figure:: img/image-no-lsc-6000.png
+ :width: 800
+
+ Image without lens shading correction @ 6000k. The vignetting artifacts can
+ be clearly seen
+
+.. figure:: img/image-fully-tuned-3000.png
+ :width: 800
+
+ Fully tuned image @ 3000k
+
+.. figure:: img/image-fully-tuned-6000.png
+ :width: 800
+
+ Fully tuned image @ 6000k
+
diff --git a/Documentation/index.rst b/Documentation/index.rst
index 5442ae75dde7..991dcf2b66fb 100644
--- a/Documentation/index.rst
+++ b/Documentation/index.rst
@@ -16,6 +16,7 @@
Developer Guide <guides/introduction>
Application Writer's Guide <guides/application-developer>
+ Sensor Tuning Guide <guides/tuning/tuning>
Pipeline Handler Writer's Guide <guides/pipeline-handler>
IPA Writer's guide <guides/ipa>
Tracing guide <guides/tracing>
diff --git a/Documentation/meson.build b/Documentation/meson.build
index 30d395234952..471eabcac344 100644
--- a/Documentation/meson.build
+++ b/Documentation/meson.build
@@ -77,6 +77,7 @@ if sphinx.found()
'guides/ipa.rst',
'guides/pipeline-handler.rst',
'guides/tracing.rst',
+ 'guides/tuning/tuning.rst',
'index.rst',
'lens_driver_requirements.rst',
'python-bindings.rst',
--
2.43.0
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