[FFmpeg-user] Constant 't' not working inside filter chain

[Murthy avsn]

Hi all,
I suing a filter chain to zoom in  overlay video slowly .I am using the following command
ffplay -i f1.mp4 -vf  "[in]split[one][two];[one]scale=iw-t:ih-t[scaled];[two][scaled]overlay=(W-w)/2:(H-h)/2" 
Problem : I am getting an error  undefined constant t.
Can some one please help me.
Here is the output.
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'f1.mp4':  Metadata:    major_brand     : mp42    minor_version   : 0    compatible_brands: isommp42    creation_time   : 2011-12-08 18:04:22  Duration: 00:02:59.95, start: 0.000000, bitrate: 456 kb/s    Stream #0:0(und): Video: h264 (Constrained Baseline) (avc1 / 0x31637661), yuv420p, 640x360, 358 kb/s, 24 fps, 24 tbr, 48 tbn, 48 tbc    Metadata:      creation_time   : 1970-01-01 00:00:00      handler_name    : VideoHandler    Stream #0:1(und): Audio: aac (mp4a / 0x6134706D), 44100 Hz, stereo, s16, 95 kb/s    Metadata:      creation_time   : 2011-12-08 18:04:22      handler_name    : IsoMedia File Produced by Google, 5-11-2011[src @ 0xb6600700] Flat options syntax is deprecated, use key=value pairs.[src @ 0xb6600700] w:640 h:360 pixfmt:yuv420p tb:1/48 fr:0/0 sar:0/1 sws_param:[Parsed_scale_1 @ 0xb6601820] [Eval @ 0xad5fcc88] Undefined constant or missing '(' in 't'    Last message repeated 1 timesError when evaluating the expression 'ih-t'.Maybe the expression for out_w:'iw-t' or for out_h:'ih-t' is self-referencing.[Parsed_scale_1 @ 0xb6601820] Failed to configure output pad on Parsed_scale_1   0.00 A-V:-1340355013.840 fd=   0 aq=    0KB vq=   10KB sq=    0B f=0/0