# How to use backBuffer instead of frontBuffer GetImage

**URL:** https://psychtoolbox.discourse.group/t/how-to-use-backbuffer-instead-of-frontbuffer-getimage/474
**Category:** Uncategorized
**Created:** [November 2, 2016, 12:59pm UTC](https://psychtoolbox.discourse.group/t/how-to-use-backbuffer-instead-of-frontbuffer-getimage/474 "2016-11-02T12:59:00Z")
**Posts on this page:** 2
**Page:** 1

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### Author: ![aytemur\_a](https://avatars.discourse-cdn.com/v4/letter/a/da6949/32.png) [@aytemur\_a](https://psychtoolbox.discourse.group/u/aytemur_a)
#### Post date: [November 2, 2016, 12:59pm UTC](https://psychtoolbox.discourse.group/t/how-to-use-backbuffer-instead-of-frontbuffer-getimage/474/1 "2016-11-02T12:59:00Z")

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Hi everybody,

I have been trying to use GetImage but it is crashing my code&nbsp; I think because it is using frontBuffer by default. Can anyone help me to change this to backBuffer? This might be very basic question but I can't find answer to this anywhere.

How can I define backBuffer?&nbsp; Apparently just saying backBuffer is not enough, I need to define it but I don't know how.

Here is the error I am getting.

Undefined function or variable 'backBuffer'.

Error in&nbsp;myExp (line 306)  
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; imageArray = Screen('GetImage', window, [940 520 980 560], backBuffer);

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### Author: ![xrli2002](https://avatars.discourse-cdn.com/v4/letter/x/48db29/32.png) [@xrli2002](https://psychtoolbox.discourse.group/u/xrli2002)
#### Post date: [November 2, 2016, 2:46pm UTC](https://psychtoolbox.discourse.group/t/how-to-use-backbuffer-instead-of-frontbuffer-getimage/474/2 "2016-11-02T14:46:54Z")

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imageArray = Screen('GetImage', window, [940 520 980 560], 'backBuffer');