# Reading the Gamma file **URL:** \ **Category:** Uncategorized\ **Created:** [March 26, 2003, 12:50pm UTC](https://psychtoolbox.discourse.group/t/reading-the-gamma-file/1250 "2003-03-26T12:50:42Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![ranibaron](https://avatars.discourse-cdn.com/v4/letter/r/b3f665/32.png) [@ranibaron](https://psychtoolbox.discourse.group/u/ranibaron)\ **Post date:** [March 26, 2003, 12:50pm UTC](https://psychtoolbox.discourse.group/t/reading-the-gamma-file/1250/1 "2003-03-26T12:50:42Z")
Hi all, I can't seem to understand how to use the gamma correction file I create with the VisualGammaDemo (am I wrong to use this file?). Moreover, the VisualGammaDemo should be configured from a distance of 3.2 meters while my experiment room has only about a 1.5 meters from the monitor. I have to test the gamma in the same room I do the experiment. Thanks in advance, Rani Bar-On IIPDM University of Haifa [http://iipdm.haifa.ac.il](http://iipdm.haifa.ac.il) ---
**Author:** ![Allen\_Ingling](https://avatars.discourse-cdn.com/v4/letter/a/a183cd/32.png) [@Allen\_Ingling](https://psychtoolbox.discourse.group/u/Allen_Ingling)\ **Post date:** [March 26, 2003, 11:33pm UTC](https://psychtoolbox.discourse.group/t/reading-the-gamma-file/1250/2 "2003-03-26T23:33:15Z")
I am using PsychSerial to read eye position data from an ASL eyetracker. I am not certain that PsychSerial is functioning properly, but I may also be doing something wrong, or using the wrong settings. The ASL box sends 8 bytes of data in response to a single byte I send to it. Here is an example. According to the serial buffer log I can view with the software that came with my serial-to-USB converter, the ASL box sent the following 8 bytes (in hex): 00 00 28 00 01 38 07 a3. To read the bytes with PsychSerial, I do: tmp=double(PsychSerial('Read',port)); In this case, I get tmp = 0,0,40,0,1,56,7,65443 Everything is fine except for the last "byte". 65443 corresponds to ffa3, so somehow PsychSerial seems to have merged another byte onto the last byte. And in fact this occurs for any byte in the stream greater than or equal to 128. That is, if the most significant bit of the byte is 1, then PsychSerial reports the byte as a 16-bit byte with the first 8 bits set to 1. Does PsychSerial assume that this is a signed byte or something? It is not a problem with Matlab's conversion to double, because this extra ff is also present in the char data. The fix is easy, all I have to do is tmp=bitand(255,tmp), but I am wondering why this is necessary. Also, PsychSerial does not seem to be appending an additional null, as mentioned in the documentation. The ASL box sends 8 bytes, and PsychSerial reports 8 bytes. I am using the (apparently latest) version of PsychSerial that is included with the PTB 2.52 release. Thanks for any help, keith ---
**Author:** ![Allen\_Ingling](https://avatars.discourse-cdn.com/v4/letter/a/a183cd/32.png) [@Allen\_Ingling](https://psychtoolbox.discourse.group/u/Allen_Ingling)\ **Post date:** [April 4, 2003, 3:55pm UTC](https://psychtoolbox.discourse.group/t/reading-the-gamma-file/1250/3 "2003-04-04T15:55:47Z")
I have been considering writing Matlab functions to read form and control and ASL eyetracker and so I was interested to see your query. Did you get any responses or resolve your problem in some other way? Ted Wright ============================================================================== | Charles E. (Ted) Wright: Associate Professor & Chair, Cognitive Sciences | | | | Office: Room 3219 USMail: Dept. of Cognitive Sciences | | Social Science Plaza Social Sciences Plaza, Rm. 3151 | | (949) 824-7589 UC-Irvine | | Fax: (949) 856-2307 Irvine, CA 92697-5100 | | email: [cewright@...](mailto:cewright@...) web: www.socsi.uci.edu/cogsci | ============================================================================== > On Wed, 26 Mar 2003, Keith Schneider wrote: > > \> I am using PsychSerial to read eye position data from an ASL eyetracker. > \> I am not certain that PsychSerial is functioning properly, but I may also > \> be doing something wrong, or using the wrong settings. > \> > \> The ASL box sends 8 bytes of data in response to a single byte I send to > \> it. Here is an example. According to the serial buffer log I can view > \> with the software that came with my serial-to-USB converter, the ASL box > \> sent the following 8 bytes (in hex): 00 00 28 00 01 38 07 a3. > \> > \> To read the bytes with PsychSerial, I do: > \> > \> tmp=double(PsychSerial('Read',port)); > \> > \> In this case, I get tmp = 0,0,40,0,1,56,7,65443 > \> > \> Everything is fine except for the last "byte". 65443 corresponds to > \> ffa3, so somehow PsychSerial seems to have merged another byte onto the > \> last byte. And in fact this occurs for any byte in the stream greater > \> than or equal to 128. That is, if the most significant bit of the byte > \> is 1, then PsychSerial reports the byte as a 16-bit byte with the first 8 > \> bits set to 1. > \> > \> Does PsychSerial assume that this is a signed byte or something? It is > \> not a problem with Matlab's conversion to double, because this extra ff > \> is also present in the char data. > \> > \> The fix is easy, all I have to do is tmp=bitand(255,tmp), but I am > \> wondering why this is necessary. > \> > \> Also, PsychSerial does not seem to be appending an additional null, as > \> mentioned in the documentation. The ASL box sends 8 bytes, and > \> PsychSerial reports 8 bytes. > \> > \> I am using the (apparently latest) version of PsychSerial that is > \> included with the PTB 2.52 release. > \> > \> Thanks for any help, > \> > \> keith > \> > \> > \> [http://psychtoolbox.org](http://psychtoolbox.org) > \> POST a message to: [psychtoolbox@yahoogroups.com](mailto:psychtoolbox@yahoogroups.com) > \> UNSUBSCRIBE by sending a blank message to: > \> [psychtoolbox-unsubscribe@yahoogroups.com](mailto:psychtoolbox-unsubscribe@yahoogroups.com) > \> > \> Your use of Yahoo! Groups is subject to [http://docs.yahoo.com/info/terms/](http://docs.yahoo.com/info/terms/) > \> > \> ---
**Author:** ![Allen\_Ingling](https://avatars.discourse-cdn.com/v4/letter/a/a183cd/32.png) [@Allen\_Ingling](https://psychtoolbox.discourse.group/u/Allen_Ingling)\ **Post date:** [April 4, 2003, 5:39pm UTC](https://psychtoolbox.discourse.group/t/reading-the-gamma-file/1250/4 "2003-04-04T17:39:35Z")
On Fri, 4 Apr 2003, Charles E. (Ted) Wright wrote: > \> I have been considering writing Matlab functions to read form and control > \> and ASL eyetracker and so I was interested to see your query. Did you get > \> any responses or resolve your problem in some other way? Hi, no responses. I don't know why PsychSerial adds those extra bits. But, the solution of bitand(255,output) strips them and seems to work. I'm getting some strange output from the ASL box and I need to talk to one of their engineers. Once I have things ironed out I will post some code. keith