[PATCH v4 9/9] libcamera: software_isp: Apply CCM in debayering
Milan Zamazal
mzamazal at redhat.com
Tue Jan 28 16:37:01 CET 2025
Hi Laurent,
thank you for review.
Laurent Pinchart <laurent.pinchart at ideasonboard.com> writes:
> Hi Milan,
>
> Thank you for the patch.
>
> On Mon, Jan 13, 2025 at 02:51:06PM +0100, Milan Zamazal wrote:
>> This patch applies color correction matrix (CCM) in debayering if the
>> CCM is specified. Not using CCM must still be supported for performance
>> reasons.
>>
>> The CCM is applied as follows:
>>
>> [r1 r2 r3]
>> [r g b] * [g1 g2 g3]
>> [b1 b2 b3]
>>
>> The CCM matrix (the right side of the multiplication) is constant during
>> single frame processing, while the input pixel (the left side) changes.
>> Because each of the color channels is only 8-bit in software ISP, we can
>> make 9 lookup tables with 256 input values for multiplications of each
>> of the r_i, g_i, b_i values. This way we don't have to multiply each
>> pixel, we can use table lookups and additions instead. Gamma (which is
>> non-linear and thus cannot be a part of the 9 lookup tables values) is
>> applied on the final values rounded to integers using another lookup
>> table.
>
> What's the speedup compared to using fixed-point integer multiplications
> ?
Just a quick test by replacing the table lookups with multiplications by
arbitrary constant integers + right shift on the final result increases
the measured time per frame by ~25% in my environment. (When on it, I
tried using floats, without the final shift, and the slowdown was close
to 70%.)
>> We use int16_t to store the precomputed multiplications. This seems to
>> be noticeably (>10%) faster than `float' for the price of slightly less
>> accuracy and it covers the range of values that sane CCMs produce. The
>> selection and structure of data is performance critical, for example
>> using bytes would add significant (>10%) speedup but would be too short
>> to cover the value range.
>>
>> The color lookup tables are changed to unions to be able to serve as
>> both simple lookup tables or CCM lookup tables. This is arguable but it
>> keeps the code easier to understand if nothing else. It is important to
>> make the unions on the whole lookup tables rather than their values
>> because the latter might cause a noticeable slowdown on simple lookup
>> due to the sheer fact that the table is not compact. Using unions makes
>> the initialization of the tables dubious because it is done before the
>> decision about using CCM is made but the initial values are dubious
>> anyway.
>
> I have a hard time feeling sympathy towards that argument. It sounds
> like "it's not great already, so let's make it worse".
OK, OK, ... It'll be resolved by not using the union.
>> Using std::variant instead of unions would be even more
>> difficult, consider e.g. mmap allocation and the followup type-casting
>> of the debayer params.
>>
>> The tables are copied (as before), which is not elegant but also not a
>> big problem. There are patches posted that use shared buffers for
>> parameters passing in software ISP (see software ISP TODO #5) and they
>> can be adjusted for the new parameter format.
>>
>> Color gains from white balance are supposed not to be a part of the
>> specified CCM. They are applied on it using matrix multiplication,
>> which is simple and in correspondence with future additions in the form
>> of matrix multiplication, like saturation adjustment.
>>
>> With this patch, the reported per-frame slowdown when applying CCM is
>> about 45% on Debix Model A and about 75% on TI AM69 SK.
>>
>> Using std::clamp in debayering adds some performance penalty (a few
>> percent). The clamping is necessary to eliminate out of range values
>> possibly produced by the CCM. If it could be avoided by adjusting the
>> precomputed tables some way then performance could be improved a bit.
>>
>> Signed-off-by: Milan Zamazal <mzamazal at redhat.com>
>> ---
>> .../internal/software_isp/debayer_params.h | 20 +++++-
>> src/ipa/simple/algorithms/lut.cpp | 63 ++++++++++++++-----
>> src/ipa/simple/algorithms/lut.h | 1 +
>> src/libcamera/software_isp/debayer.cpp | 50 +++++++++++++++
>> src/libcamera/software_isp/debayer_cpu.cpp | 34 +++++++---
>> src/libcamera/software_isp/debayer_cpu.h | 7 ++-
>> src/libcamera/software_isp/software_isp.cpp | 6 +-
>> 7 files changed, 147 insertions(+), 34 deletions(-)
>>
>> diff --git a/include/libcamera/internal/software_isp/debayer_params.h b/include/libcamera/internal/software_isp/debayer_params.h
>> index 7d8fdd48..f8731eae 100644
>> --- a/include/libcamera/internal/software_isp/debayer_params.h
>> +++ b/include/libcamera/internal/software_isp/debayer_params.h
>> @@ -18,11 +18,25 @@ namespace libcamera {
>> struct DebayerParams {
>> static constexpr unsigned int kRGBLookupSize = 256;
>>
>> + struct CcmColumn {
>> + int16_t r;
>> + int16_t g;
>> + int16_t b;
>> + };
>> +
>> using ColorLookupTable = std::array<uint8_t, kRGBLookupSize>;
>> + using CcmLookupTable = std::array<CcmColumn, kRGBLookupSize>;
>> + using GammaLookupTable = std::array<uint8_t, kRGBLookupSize>;
>> +
>> + union LookupTable {
>> + ColorLookupTable simple;
>> + CcmLookupTable ccm;
>> + };
>>
>> - ColorLookupTable red;
>> - ColorLookupTable green;
>> - ColorLookupTable blue;
>> + LookupTable red;
>> + LookupTable green;
>> + LookupTable blue;
>> + GammaLookupTable gammaLut;
>
> This is difficult to understand, especially without comments. If I'm not
> mistaken, the idea is that the CPU implementation of the software ISP
> will use either <color>.simple, or <color>.ccm + gammaLut. The former is
> used when CCM is disabled, and combines colour gains and gamma in a
> single LUT. The latter is used when CCM is enabled, and combines colour
> gains and CCM in the CCM LUT, with the gamma LUT applying gamma only.
Yes.
> I think it would be simpler to understand if you specified all LUTs for
> all colour components, unconditionally.
>
> struct ColorLookupTable {
> CcmLookupTable ccm;
> GammaLookupTable gamma;
> }
>
> ColorLookupTable red;
> ColorLookupTable green;
> ColorLookupTable blue;
Something like this was the other option I originally considered, so I'm
fine with such a change. Gamma is applied only on the final result
(with CCM), so it must be kept separately. And non-CCM and CCM lookup
tables should be separated. I'll do it in v5.
>> };
>>
>> } /* namespace libcamera */
>> diff --git a/src/ipa/simple/algorithms/lut.cpp b/src/ipa/simple/algorithms/lut.cpp
>> index 243f0818..ddb2adf1 100644
>> --- a/src/ipa/simple/algorithms/lut.cpp
>> +++ b/src/ipa/simple/algorithms/lut.cpp
>> @@ -80,6 +80,11 @@ void Lut::updateGammaTable(IPAContext &context)
>> context.activeState.gamma.contrast = contrast;
>> }
>>
>> +int16_t Lut::ccmValue(unsigned int i, float ccm) const
>> +{
>> + return std::round(i * ccm);
>> +}
>> +
>> void Lut::prepare(IPAContext &context,
>> [[maybe_unused]] const uint32_t frame,
>> [[maybe_unused]] IPAFrameContext &frameContext,
>> @@ -91,28 +96,52 @@ void Lut::prepare(IPAContext &context,
>> * observed, it's not permanently prone to minor fluctuations or
>> * rounding errors.
>> */
>> - if (context.activeState.gamma.blackLevel != context.activeState.blc.level ||
>> - context.activeState.gamma.contrast != context.activeState.knobs.contrast)
>> + const bool gammaUpdateNeeded =
>> + context.activeState.gamma.blackLevel != context.activeState.blc.level ||
>> + context.activeState.gamma.contrast != context.activeState.knobs.contrast;
>> + if (gammaUpdateNeeded)
>> updateGammaTable(context);
>>
>> auto &gains = context.activeState.awb.gains;
>> auto &gammaTable = context.activeState.gamma.gammaTable;
>> const unsigned int gammaTableSize = gammaTable.size();
>> -
>> - for (unsigned int i = 0; i < DebayerParams::kRGBLookupSize; i++) {
>> - const double div = static_cast<double>(DebayerParams::kRGBLookupSize) /
>> - gammaTableSize;
>> - /* Apply gamma after gain! */
>> - unsigned int idx;
>> - idx = std::min({ static_cast<unsigned int>(i * gains.red / div),
>> - gammaTableSize - 1 });
>> - params->red[i] = gammaTable[idx];
>> - idx = std::min({ static_cast<unsigned int>(i * gains.green / div),
>> - gammaTableSize - 1 });
>> - params->green[i] = gammaTable[idx];
>> - idx = std::min({ static_cast<unsigned int>(i * gains.blue / div),
>> - gammaTableSize - 1 });
>> - params->blue[i] = gammaTable[idx];
>> + const double div = static_cast<double>(DebayerParams::kRGBLookupSize) /
>> + gammaTableSize;
>> +
>> + if (!context.activeState.ccm.enabled) {
>> + for (unsigned int i = 0; i < DebayerParams::kRGBLookupSize; i++) {
>> + /* Apply gamma after gain! */
>> + unsigned int idx;
>> + idx = std::min({ static_cast<unsigned int>(i * gains.red / div),
>> + gammaTableSize - 1 });
>> + params->red.simple[i] = gammaTable[idx];
>> + idx = std::min({ static_cast<unsigned int>(i * gains.green / div),
>> + gammaTableSize - 1 });
>> + params->green.simple[i] = gammaTable[idx];
>> + idx = std::min({ static_cast<unsigned int>(i * gains.blue / div),
>> + gammaTableSize - 1 });
>> + params->blue.simple[i] = gammaTable[idx];
>> + }
>> + } else if (context.activeState.ccm.changed || gammaUpdateNeeded) {
>> + Matrix<double, 3, 3> gainCcm = { { gains.red, 0, 0,
>> + 0, gains.green, 0,
>> + 0, 0, gains.blue } };
>> + auto ccm = gainCcm * context.activeState.ccm.ccm;
>> + auto &red = params->red.ccm;
>> + auto &green = params->green.ccm;
>> + auto &blue = params->blue.ccm;
>> + for (unsigned int i = 0; i < DebayerParams::kRGBLookupSize; i++) {
>> + red[i].r = ccmValue(i, ccm[0][0]);
>> + red[i].g = ccmValue(i, ccm[1][0]);
>> + red[i].b = ccmValue(i, ccm[2][0]);
>> + green[i].r = ccmValue(i, ccm[0][1]);
>> + green[i].g = ccmValue(i, ccm[1][1]);
>> + green[i].b = ccmValue(i, ccm[2][1]);
>> + blue[i].r = ccmValue(i, ccm[0][2]);
>> + blue[i].g = ccmValue(i, ccm[1][2]);
>> + blue[i].b = ccmValue(i, ccm[2][2]);
>> + params->gammaLut[i] = gammaTable[i / div];
>> + }
>> }
>> }
>>
>> diff --git a/src/ipa/simple/algorithms/lut.h b/src/ipa/simple/algorithms/lut.h
>> index 889f864b..77324800 100644
>> --- a/src/ipa/simple/algorithms/lut.h
>> +++ b/src/ipa/simple/algorithms/lut.h
>> @@ -33,6 +33,7 @@ public:
>>
>> private:
>> void updateGammaTable(IPAContext &context);
>> + int16_t ccmValue(unsigned int i, float ccm) const;
>> };
>>
>> } /* namespace ipa::soft::algorithms */
>> diff --git a/src/libcamera/software_isp/debayer.cpp b/src/libcamera/software_isp/debayer.cpp
>> index 45fe6960..559de3e6 100644
>> --- a/src/libcamera/software_isp/debayer.cpp
>> +++ b/src/libcamera/software_isp/debayer.cpp
>> @@ -23,11 +23,56 @@ namespace libcamera {
>> * \brief Size of a color lookup table
>> */
>>
>> +/**
>> + * \struct DebayerParams::CcmColumn
>> + * \brief Type of a single column of a color correction matrix
>> + */
>> +
>> +/**
>> + * \var DebayerParams::CcmColumn::r
>> + * \brief Red (first) component of a CCM column
>> + */
>> +
>> +/**
>> + * \var DebayerParams::CcmColumn::g
>> + * \brief Green (second) component of a CCM column
>> + */
>> +
>> +/**
>> + * \var DebayerParams::CcmColumn::b
>> + * \brief Blue (third) component of a CCM column
>> + */
>> +
>> /**
>> * \typedef DebayerParams::ColorLookupTable
>> + * \brief Type of the simple lookup tables for red, green, blue values
>> + */
>> +
>> +/**
>> + * \typedef DebayerParams::CcmLookupTable
>> + * \brief Type of the CCM lookup tables for red, green, blue values
>> + */
>> +
>> +/**
>> + * \typedef DebayerParams::GammaLookupTable
>> + * \brief Type of the gamma lookup tables for CCM
>> + */
>> +
>> +/**
>> + * \union DebayerParams::LookupTable
>> * \brief Type of the lookup tables for red, green, blue values
>> */
>>
>> +/**
>> + * \var DebayerParams::LookupTable::simple
>> + * \brief Simple lookup table for red, green, blue values
>> + */
>> +
>> +/**
>> + * \var DebayerParams::LookupTable::ccm
>> + * \brief CCM lookup table for red, green, blue values
>> + */
>> +
>> /**
>> * \var DebayerParams::red
>> * \brief Lookup table for red color, mapping input values to output values
>> @@ -43,6 +88,11 @@ namespace libcamera {
>> * \brief Lookup table for blue color, mapping input values to output values
>> */
>>
>> +/**
>> + * \var DebayerParams::gammaLut
>> + * \brief Gamma lookup table used with color correction matrix
>> + */
>> +
>> /**
>> * \class Debayer
>> * \brief Base debayering class
>> diff --git a/src/libcamera/software_isp/debayer_cpu.cpp b/src/libcamera/software_isp/debayer_cpu.cpp
>> index 3c6597f9..38715b72 100644
>> --- a/src/libcamera/software_isp/debayer_cpu.cpp
>> +++ b/src/libcamera/software_isp/debayer_cpu.cpp
>> @@ -11,6 +11,7 @@
>>
>> #include "debayer_cpu.h"
>>
>> +#include <algorithm>
>> #include <stdlib.h>
>> #include <sys/ioctl.h>
>> #include <time.h>
>> @@ -51,8 +52,12 @@ DebayerCpu::DebayerCpu(std::unique_ptr<SwStatsCpu> stats)
>> enableInputMemcpy_ = true;
>>
>> /* Initialize color lookup tables */
>> - for (unsigned int i = 0; i < DebayerParams::kRGBLookupSize; i++)
>> - red_[i] = green_[i] = blue_[i] = i;
>> + for (unsigned int i = 0; i < DebayerParams::kRGBLookupSize; i++) {
>> + red_.simple[i] = green_.simple[i] = blue_.simple[i] = i;
>> + red_.ccm[i].r = red_.ccm[i].g = red_.ccm[i].b = 0;
>> + green_.ccm[i].r = green_.ccm[i].g = green_.ccm[i].b = 0;
>> + blue_.ccm[i].r = blue_.ccm[i].g = blue_.ccm[i].b = 0;
>> + }
>> }
>>
>> DebayerCpu::~DebayerCpu() = default;
>> @@ -62,12 +67,24 @@ DebayerCpu::~DebayerCpu() = default;
>> const pixel_t *curr = (const pixel_t *)src[1] + xShift_; \
>> const pixel_t *next = (const pixel_t *)src[2] + xShift_;
>>
>> -#define STORE_PIXEL(b, g, r) \
>> - *dst++ = blue_[b]; \
>> - *dst++ = green_[g]; \
>> - *dst++ = red_[r]; \
>> - if constexpr (addAlphaByte) \
>> - *dst++ = 255; \
>> +#define GAMMA(value) \
>> + *dst++ = gammaLut_[std::clamp(value, 0, static_cast<int>(gammaLut_.size()) - 1)]
>
> What's the point in specifying the CCM lookup table values as 16-bit
> integers if you clamp to 8 bits anyway ? Is that because you need signed
> values ?
The table values are uint8_t values multiplied by matrix elements, which
may be arbitrary numbers. Each multiplication result can be within
uint8_t range or negative or bigger. Three multiplication results are
summed so they may return back to uint8_t range and only then clamping
can be applied.
>> +
>> +#define STORE_PIXEL(b_, g_, r_) \
>> + if constexpr (ccmEnabled) { \
>> + DebayerParams::CcmColumn &blue = blue_.ccm[b_]; \
>> + DebayerParams::CcmColumn &green = green_.ccm[g_]; \
>> + DebayerParams::CcmColumn &red = red_.ccm[r_]; \
>
> const
Ack.
>> + GAMMA(blue.r + blue.g + blue.b); \
>> + GAMMA(green.r + green.g + green.b); \
>> + GAMMA(red.r + red.g + red.b); \
>> + } else { \
>> + *dst++ = blue_.simple[b_]; \
>> + *dst++ = green_.simple[g_]; \
>> + *dst++ = red_.simple[r_]; \
>> + } \
>> + if constexpr (addAlphaByte) \
>> + *dst++ = 255; \
>> x++;
>>
>> /*
>> @@ -752,6 +769,7 @@ void DebayerCpu::process(uint32_t frame, FrameBuffer *input, FrameBuffer *output
>> green_ = params.green;
>> red_ = swapRedBlueGains_ ? params.blue : params.red;
>> blue_ = swapRedBlueGains_ ? params.red : params.blue;
>> + gammaLut_ = params.gammaLut;
>>
>> /* Copy metadata from the input buffer */
>> FrameMetadata &metadata = output->_d()->metadata();
>> diff --git a/src/libcamera/software_isp/debayer_cpu.h b/src/libcamera/software_isp/debayer_cpu.h
>> index b2ec8f1b..35c51e4f 100644
>> --- a/src/libcamera/software_isp/debayer_cpu.h
>> +++ b/src/libcamera/software_isp/debayer_cpu.h
>> @@ -137,9 +137,10 @@ private:
>> /* Max. supported Bayer pattern height is 4, debayering this requires 5 lines */
>> static constexpr unsigned int kMaxLineBuffers = 5;
>>
>> - DebayerParams::ColorLookupTable red_;
>> - DebayerParams::ColorLookupTable green_;
>> - DebayerParams::ColorLookupTable blue_;
>> + DebayerParams::LookupTable red_;
>> + DebayerParams::LookupTable green_;
>> + DebayerParams::LookupTable blue_;
>> + DebayerParams::GammaLookupTable gammaLut_;
>> debayerFn debayer0_;
>> debayerFn debayer1_;
>> debayerFn debayer2_;
>> diff --git a/src/libcamera/software_isp/software_isp.cpp b/src/libcamera/software_isp/software_isp.cpp
>> index 6a07de85..40e11b9e 100644
>> --- a/src/libcamera/software_isp/software_isp.cpp
>> +++ b/src/libcamera/software_isp/software_isp.cpp
>> @@ -82,9 +82,9 @@ SoftwareIsp::SoftwareIsp(PipelineHandler *pipe, const CameraSensor *sensor,
>> for (unsigned int i = 0; i < 256; i++)
>> gammaTable[i] = UINT8_MAX * std::pow(i / 256.0, 0.5);
>> for (unsigned int i = 0; i < DebayerParams::kRGBLookupSize; i++) {
>> - debayerParams_.red[i] = gammaTable[i];
>> - debayerParams_.green[i] = gammaTable[i];
>> - debayerParams_.blue[i] = gammaTable[i];
>> + debayerParams_.red.simple[i] = gammaTable[i];
>> + debayerParams_.green.simple[i] = gammaTable[i];
>> + debayerParams_.blue.simple[i] = gammaTable[i];
>> }
>>
>> if (!dmaHeap_.isValid()) {
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