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Low-end Android Cameras and the Exposure Triangle
Low-end Android Cameras and the Exposure TriangleMartin Cerman
I assume some of you have already tried developing a camera app on android. It sucks.
Why? Support of low-end devices with bad cameras.
Explain theory behind camera and show couple of ways how to improve them.1
Approx. 45% of Android devices are running ICS or Jelly Bean (Versions 4.0.3 to 4.3)
Partially very bad cameras on low-end devicesLow resolutionAuto focus problemsNot completely standardized camera settingsExposure lockingBad FPS priorities
Real-time image processing
=> We need the best possible image quality at highest possible frame-rate
Motivation
2
Company that does real-time image processing on mobile devices
Support iOS, Android, Windows Phone coming soon
iOS simple couple of devices with good camera and good processing power
Android literally hundreds of devices that need to be supported high variability in camera quality and processing power
API level >=15 Ice Cream Sandwich
5 year old devices from which many were designed as low cost devices
Hardware Software
Not a single supported focus area
Map with key value pairs . iso=200, camera_iso=2002
Take picture and process it offlineProvides higher quality images (separate preview and capture settings)More callback options (shutter callback, raw picture callback,)Take picture and forget, or switch view and process
Get byte array from onPreviewFramefrom Camera.PreviewCallbackLower quality images (WYSIWYG)Only a single callback functionProvides constant stream of images with variable FPS
Image Capturing
3
Variable FPS rather bad More time passes between frames, R and T may be higher3
Exposure Triangle
ExposureISOShutter speedApertureExposure amount of light per unit area reaching the image sensor4
Visual representation of which factors contribute to the exposure of a camera4
Aperture
A hole or an opening through whichlight travelsDefines from what angle and how much light reaches the sensorInfluences Depth of Field5
Aperture Depth of Field
Small aperture - f/32Large aperture - f/5.66
Narrow vs. wide DOF6
Aperture Depth of Field
7
Moving image plane equal to moving object in terms of rays through lens
Point projected onto image plane as circle the larger the more blurry the point7
Shutter SpeedLength of time the image sensor is exposed to lightDepends on opening size of mechanical shutterInfluences motion blur
8
8
Shutter Speed9
*video on the next slide
Mechanical shutter may pass vertically or horizontally
Look out of moving car and take picture, image may seem displaced
Shutter simulated in mobile devices9
Shutter Speed
10
ISO
Measure of a photographic films sensitivity to lightDescribes also sensitivity of a camera sensorInfluences noise in the image11
ISO
12
High ISO is also termed fast film requires less exposure12
Exposure Triangle Revisited
Low-end Android cameras have a fixed aperture!Exposure is thus defined only by ISO and shutter speed13
Aperture generally fixed at 2.8
Move along red line and select ISO and shutter speed13
Camera Priority ModesGood DSLR cameras allow to set all factors manually or define a priority
Shutter priority S mode
Aperture priority A mode
Less common Sensitivity Priority or ISO Priority Sv mode
Low-end Android cameras use by default ISO Priority with lowest possible ISO for less noisy images => lower FPS
14
Low ISO, Slow Shutter Speed
15*video on the next slide
High ISO, Fast Shutter Speed
16*video on the next slide
Android SettingsAndroid has Auto mode or Scene modesAuto mode permits change of ISO settings, but supports shutter speed very poorlyScene modes have set ISO and shutter speed settingsScene modes cannot be obtained by manually changing settings in auto modeAndroid has ISO priority mode
Solution: use Sports scene modeHas locked 30 FPSSacrifices ISO in favor of constant FPS, while noise is acceptable in most lighting conditions
17
Auto-focus ProblemsSome devices lock the exposure after focusingLock needs to be removed manually or lock is removed when camera cannot focusCauses Over-/Under-exposure when switching between well and badly lit scenesTwo solutions at the moment:
1)2)18
Auto-focus Problems Over-exposure
19*video on the next slide
Auto-focus Problems Under-exposure
20*video on the next slide
Thank you!Martin Cerman