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Low-end Android Cameras and the Exposure Triangle Martin Cerman

Low-end Android cameras and the exposure triangle

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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