1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
|
/* SPDX-License-Identifier: LGPL-2.1-or-later */
/*
* Copyright (C) 2024, Paul Elder <paul.elder@ideasonboard.com>
*
* exposure_mode_helper.cpp - Helper class that performs computations relating to exposure
*/
#include "exposure_mode_helper.h"
#include <algorithm>
#include <libcamera/base/log.h>
/**
* \file exposure_mode_helper.h
* \brief Helper class that performs computations relating to exposure
*
* AEGC algorithms have a need to split exposure between shutter time, analogue
* and digital gain. Multiple implementations do so based on paired stages of
* shutter time and gain limits; provide a helper to avoid duplicating the code.
*/
namespace libcamera {
using namespace std::literals::chrono_literals;
LOG_DEFINE_CATEGORY(ExposureModeHelper)
namespace ipa {
/**
* \class ExposureModeHelper
* \brief Class for splitting exposure into shutter time and total gain
*
* The ExposureModeHelper class provides a standard interface through which an
* AEGC algorithm can divide exposure between shutter time and gain. It is
* configured with a set of shutter time and gain pairs and works by initially
* fixing gain at 1.0 and increasing shutter time up to the shutter time value
* from the first pair in the set in an attempt to meet the required exposure
* value.
*
* If the required exposure is not achievable by the first shutter time value
* alone it ramps gain up to the value from the first pair in the set. If the
* required exposure is still not met it then allows shutter time to ramp up to
* the shutter time value from the second pair in the set, and continues in this
* vein until either the required exposure time is met, or else the hardware's
* shutter time or gain limits are reached.
*
* This method allows users to strike a balance between a well-exposed image and
* an acceptable frame-rate, as opposed to simply maximising shutter time
* followed by gain. The same helpers can be used to perform the latter
* operation if needed by passing an empty set of pairs to the initialisation
* function.
*
* The gain values may exceed a camera sensor's analogue gain limits if either
* it or the IPA is also capable of digital gain. The configure() function must
* be called with the hardware's limits to inform the helper of those
* constraints. Any gain that is needed will be applied as analogue gain first
* until the hardware's limit is reached, following which digital gain will be
* used.
*/
/**
* \brief Construct an ExposureModeHelper instance
* \param[in] stages The vector of paired shutter time and gain limits
*
* The input stages are shutter time and _total_ gain pairs; the gain
* encompasses both analogue and digital gain.
*
* The vector of stages may be empty. In that case, the helper will simply use
* the runtime limits set through setShutterGainLimits() instead.
*/
ExposureModeHelper::ExposureModeHelper(const Span<std::pair<utils::Duration, double>> stages)
{
minShutter_ = 0us;
maxShutter_ = 0us;
minGain_ = 0;
maxGain_ = 0;
for (const auto &[s, g] : stages) {
shutters_.push_back(s);
gains_.push_back(g);
}
}
/**
* \brief Set the shutter time and gain limits
* \param[in] minShutter The minimum shutter time supported
* \param[in] maxShutter The maximum shutter time supported
* \param[in] minGain The minimum analogue gain supported
* \param[in] maxGain The maximum analogue gain supported
*
* This function configures the shutter time and analogue gain limits that need
* to be adhered to as the helper divides up exposure. Note that this function
* *must* be called whenever those limits change and before splitExposure() is
* used.
*
* If the algorithm using the helpers needs to indicate that either shutter time
* or analogue gain or both should be fixed it can do so by setting both the
* minima and maxima to the same value.
*/
void ExposureModeHelper::setLimits(utils::Duration minShutter,
utils::Duration maxShutter,
double minGain, double maxGain)
{
minShutter_ = minShutter;
maxShutter_ = maxShutter;
minGain_ = minGain;
maxGain_ = maxGain;
}
utils::Duration ExposureModeHelper::clampShutter(utils::Duration shutter) const
{
return std::clamp(shutter, minShutter_, maxShutter_);
}
double ExposureModeHelper::clampGain(double gain) const
{
return std::clamp(gain, minGain_, maxGain_);
}
/**
* \brief Split exposure time into shutter time and gain
* \param[in] exposure Exposure time
*
* This function divides a given exposure time into shutter time, analogue and
* digital gain by iterating through stages of shutter time and gain limits. At
* each stage the current stage's shutter time limit is multiplied by the
* previous stage's gain limit (or 1.0 initially) to see if the combination of
* the two can meet the required exposure time. If they cannot then the current
* stage's shutter time limit is multiplied by the same stage's gain limit to
* see if that combination can meet the required exposure time. If they cannot
* then the function moves to consider the next stage.
*
* When a combination of shutter time and gain _stage_ limits are found that are
* sufficient to meet the required exposure time, the function attempts to
* reduce shutter time as much as possible whilst fixing gain and still meeting
* the exposure time. If a _runtime_ limit prevents shutter time from being
* lowered enough to meet the exposure time with gain fixed at the stage limit,
* gain is also lowered to compensate.
*
* Once the shutter time and gain values are ascertained, gain is assigned as
* analogue gain as much as possible, with digital gain only in use if the
* maximum analogue gain runtime limit is unable to accommodate the exposure
* value.
*
* If no combination of shutter time and gain limits is found that meets the
* required exposure time, the helper falls-back to simply maximising the
* shutter time first, followed by analogue gain, followed by digital gain.
*
* \return Tuple of shutter time, analogue gain, and digital gain
*/
std::tuple<utils::Duration, double, double>
ExposureModeHelper::splitExposure(utils::Duration exposure) const
{
ASSERT(maxShutter_);
ASSERT(maxGain_);
bool gainFixed = minGain_ == maxGain_;
bool shutterFixed = minShutter_ == maxShutter_;
/*
* There's no point entering the loop if we cannot change either gain
* nor shutter anyway.
*/
if (shutterFixed && gainFixed)
return { minShutter_, minGain_, exposure / (minShutter_ * minGain_) };
utils::Duration shutter;
double stageGain;
double gain;
for (unsigned int stage = 0; stage < gains_.size(); stage++) {
double lastStageGain = stage == 0 ? 1.0 : clampGain(gains_[stage - 1]);
utils::Duration stageShutter = clampShutter(shutters_[stage]);
stageGain = clampGain(gains_[stage]);
/*
* We perform the clamping on both shutter and gain in case the
* helper has had limits set that prevent those values being
* lowered beyond a certain minimum...this can happen at runtime
* for various reasons and so would not be known when the stage
* limits are initialised.
*/
if (stageShutter * lastStageGain >= exposure) {
shutter = clampShutter(exposure / clampGain(lastStageGain));
gain = clampGain(exposure / shutter);
return { shutter, gain, exposure / (shutter * gain) };
}
if (stageShutter * stageGain >= exposure) {
shutter = clampShutter(exposure / clampGain(stageGain));
gain = clampGain(exposure / shutter);
return { shutter, gain, exposure / (shutter * gain) };
}
}
/*
* From here on all we can do is max out the shutter time, followed by
* the analogue gain. If we still haven't achieved the target we send
* the rest of the exposure time to digital gain. If we were given no
* stages to use then set stageGain to 1.0 so that shutter time is maxed
* before gain touched at all.
*/
if (gains_.empty())
stageGain = 1.0;
shutter = clampShutter(exposure / clampGain(stageGain));
gain = clampGain(exposure / shutter);
return { shutter, gain, exposure / (shutter * gain) };
}
/**
* \fn ExposureModeHelper::minShutter()
* \brief Retrieve the configured minimum shutter time limit set through
* setShutterGainLimits()
* \return The minShutter_ value
*/
/**
* \fn ExposureModeHelper::maxShutter()
* \brief Retrieve the configured maximum shutter time set through
* setShutterGainLimits()
* \return The maxShutter_ value
*/
/**
* \fn ExposureModeHelper::minGain()
* \brief Retrieve the configured minimum gain set through
* setShutterGainLimits()
* \return The minGain_ value
*/
/**
* \fn ExposureModeHelper::maxGain()
* \brief Retrieve the configured maximum gain set through
* setShutterGainLimits()
* \return The maxGain_ value
*/
} /* namespace ipa */
} /* namespace libcamera */
|
