SBHelper app for iPhone and iPad

This is a tool for you to get an alarm when an edge of a platform is close to you on a subway station. This does not replace your white cane or a guide dog. It is an attempt to add another layer of alarm with your iPhones camera to sense a change of brightness because a platform is usually brighter than its railroad at most, if not all, subway stations.

When the iPhone is held in the portrait position, with its back camera facing slightly down, this camera compares the top half brightness and bottom half brightness of the view every two seconds. If the bottom half is twice as much brighter or more than the top half, this tool sounds an alarm with vibration. To compensate a bright wall, the camera also divide the view into three regions: top one third, middle one third, and bottom one third. If the bottom one third is twice as brighter or more than the middle one third, it also sounds a different alarm with vibration. And if the middle one third and bottom one third are both too dark to compare, it sounds yet another alarm with vibration.

To also get magnetic North, when you hold your iPhone on a face up or down position, it tries to find the magnetic North direction and sounds an alarm when found. This alarm continues as long as your iPhone is heading to North within plus or minus 30 degrees. If it cannot find any magnetic direction, you will get a different alarm regardless of the iPhones direction. Because the brightness comparison is turned off in this mode, do not move around when you hold your iPhone face up or down. The magnetic North can get a large error when there is a magnetic device around you.

This app does not record anything. It uses the back camera as merely a brightness sensor. This app claims no responsibility for users safety and suitability of alarms that it generates for your use. You, as a user, is fully responsible to determine whether or not this app works for your situation. The author of this app shall not be held liable for the decision you made. Because of the brightness assumption, this app does not work for a bright railroad or on a train station above the ground with lots of ambient light. Be sure to try this app with someone you trust first on your subway stations to learn its behavior and limitations. Because each camera has a different light sensitivity, you must test this app with your own iPhone to determine its suitability.

For the first time only, this app asks for your permissions to use the camera and location service (compass) of your iPhone. You must grant the two permissions for this app to work. You dont get any vibration on iPad. GPS is not used.