Distance measuring apparatus of a target tracking type

An alarm apparatus incorporated in a first vehicle for preventing a rear-end collision with a second vehicle, comprising:

• a plurality of frame memories for receiving quantized density data of an image in successive image frames obtained at fixed intervals of time by imaging a road along which said first vehicle is travelling and for selectively storing the received quantized density data in a matrix of columns and rows;
• comparison means for determining a difference between the quantized density data stored in said frame memories at corresponding coordinate points and for outputting density difference data;
• column accumulation means for accumulating the density difference data output by said comparison means for each of the columns;
• row accumulation means for accumulating the density difference data output by said comparison means for each of the rows;
• tracking means for tracking said second vehicle travelling on said road along a path in front of said first vehicle in accordance with an output of said column accumulation means;
• distance measurement means for measuring the distance between the first and second vehicles in accordance with an output of said row accumulation means;
• velocity measurement means for measuring a velocity of the first vehicle;
• alarm determination means for determining, from the measured distance and the measured velocity, whether to set an alarm for preventing a collision between said first and second vehicles;
• and
• alarming means for generating an alarm when said alarm determination means determines that an alarm should be set.