>Psychtoolbox>PsychDemos

MovieDemo

Show a movie, easy as pie.

The comments below review the difference between the OS 9 version of
the toolbox and PTB-3.

New and different on OS Psychtoolbox:

The OS 9 Psychtoolbox is based on QuickTime and PTB-3 is
based on OpenGL. Differences between these underlying graphics libraries
have unavoidably resulted in differences between the OS 9 and PTB-3
versions of Screen; Some subfunctions of OS 9 Screen are replaced by
similar, but not identical functions in PTB-3.

In OS 9: In PTB-3

WaitBlanking Flip
PutImage MakeTexture
CopyWindow DrawTexture

WaitVBL vs. Flip

In OS 9 an onscreen window consisted of only one buffer: the front
buffer. Any pixel drawn to an onscreen window would automatically
appear on the display as soon as the monitor scan passed that pixel,
the maximum delay between drawing a pixel and its appearnce on the
monitor being one frame period. To prevent vertical shearing of
animated displays, Psyhctoolbox scripts called WaitBlanking before
updating the display. WaitBlanking synchonrized drawing into vide RAM
wih the monitor scan by delaying the onset of drawing until the scan
reached the bottom right corener of the display, at which point a
vertical blank would occur while the CRT beam retraced to the upper
left corner of the display.

In contrast, PTB-3 onscreen windows by default have not one but two
buffers: the front buffer and the back buffer. In double-buffered
mode, all Screen drawing commands are issued to the back buffer, where
what is drawn is hidden from view. The contents of the back buffer
remain hidden from view until The Screen(‘Flip’, …) command is
issued. Flip waits until the vertical retrace and then interchages the
contents of the front and back buffers, bringining into view on the
display what had previosly been hidden in the back buffer.

OpenOffscrenWindow + PutImage + CopyWindow vs. MakeTexture + DrawTexture

In OS 9, PutImage copied a MATLAB matrix into an offsceen window. The
offscreen Window could then be quickly copied to the display during an
animation. Thus PutImage was one part of three for quickly
displaying MATLAB matrices during an animation:

  Steps to quickly display an image matrix in OS 9:  
    1. Create an offscreen window by calling [OpenOffscreenWindow](OpenOffscreenWindow)  
    2. Copy the matrix to an offsceen window using [PutImage](PutImage)  
    3. Copy the offscreen window to an onscreen window using  
    [CopyWindow](CopyWindow)  

Instead of using Offscreen Windows to quickly display MATLAB Matrices
the OSX Psychtoolbox uses Textures.

  Steps to quickly display an image matrix in PTB-3:    
    1. Create a texture and copy into it a matrix by using [MakeTexture](MakeTexture)  
    2. Copy the texture quickly to the onscreen window using  
    [DrawTexure](DrawTexure)  

What is the difference betwen offscreen windows and textures? The
difference is that Drawing commands such as FillOval may be issued to
offscreen windows but not to Textures; You may only fill textures by
using matrices.

How do we draw ovals during an animation if we can not store them
within offscren or textures? Unlike QuickDraw, OpenGL is fast enought
to render all drawing commands directly to the display during an
animation loop. Except for MATLAB matrices stored in textures, there
should be no need to prerender and buffer what is displayed during
animation.

Animation loops: OS 9 VS PTB-3

To understand the differences between OS 9 and PTB-3 Screen its helpful
to compare animation loops:

OS 9:

  %create and fill offscreen windows here...   
  for i=1:numberOfMovieFrames  
      [Screen](Screen)(window, 'WaitBlanking');  
      [Screen](Screen)('CopyWindow', offscreenWindow(i), window);  
  end  

PTB-3:

  %generate textures here...  
  for i=1:numberOfMovieFrames  
      [Screen](Screen)('DrawTexture', texture(i), window);  
      [Screen](Screen)(window, '[Flip](Flip)');  
  end  

See also: DriftDemo, PsychDemos.

HISTORY

7/3/04 awi Wrote it. Based on Denis Pelli’s MovieDemo for OS 9.
7/19/04 awi restored Priority, and time tests, added option to plot results.
7/24/04 awi Cosmetic
9/8/04 awi Added Try/Catch.
4/23/05 mk Added Priority(0) to Catch-Section.
11/19/06 dhb Remove OSX from name.