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OpenGL and Rendering

All rendering performed by PsychoPy uses hardware-accelerated OpenGL rendering where possible. This means that, as much as possible, the necessary processing to calculate pixel values is performed by the graphics card GPU rather than by the CPU. For example, when an image is rotated the calculations to determine what pixel values should result, and any interpolation that is needed, are determined by the graphics card automatically.

In the double-buffered system, stimuli are initially drawn into a piece of memory on the graphics card called the ‘back buffer’, while the screen presents the ‘front buffer’. The back buffer initially starts blank (all pixels are set to the window’s defined color) and as stimuli are ‘rendered’ they are gradually added to this back buffer. The way in which stimuli are combined according to transparency rules is determined by the blend mode of the window. At some point in time, when we have rendered to this buffer all the objects that we wish to be presented, the buffers are ‘flipped’ such that the stimuli we have been drawing are presented simultaneously. The monitor updates at a very precise fixed rate and the flipping of the window will be synchronised to this monitor update if possible (see Sync to VBL and wait for VBL).

Each update of the window is referred to as a ‘frame’ and this ultimately determines the temporal resolution with which stimuli can be presented (you cannot present your stimulus for any duration other than a multiple of the frame duration). In addition to synchronising flips to the frame refresh rate, PsychoPy can optionally go a further step of not allowing the code to continue until a screen flip has occurred on the screen, which is useful in ascertaining exactly when the frame refresh occurred (and, thus, when your stimulus actually appeared to the subject). These timestamps are very precise on most computers. For further information about synchronising and waiting for the refresh see Sync to VBL and wait for VBL.

If the code/processing required to render all you stimuli to the screen takes longer to complete than one screen refresh then you will ‘drop/skip a frame’. In this case the previous frame will be left on screen for a further frame period and the flip will only take effect on the following screen update. As a result, time-consuming operations such as disk accesses or execution of many lines of code, should be avoided while stimuli are being dynamically updated (if you care about the precise timing of your stimuli). For further information see the sections on Detecting dropped frames and Reducing dropped frames.

Fast and slow functions

The fact that modern graphics processors are extremely powerful; they can carry out a great deal of processing from a very small number of commands. Consider, for instance, the PsychoPy Coder demo elementArrayStim in which several hundred Gabor patches are updated frame by frame. The graphics card has to blend a sinusoidal grating with a grey background, using a Gaussian profile, several hundred times each at a different orientation and location and it does this in less than one screen refresh on a good graphics card.

There are three things that are relatively slow and should be avoided at critical points in time (e.g. when rendering a dynamic or brief stimulus). These are:

  1. disk accesses
  2. passing large amounts of data to the graphics card
  3. making large numbers of python calls.

Functions that are very fast:

  1. Calls that move, resize, rotate your stimuli are likely to carry almost no overhead
  2. Calls that alter the color, contrast or opacity of your stimulus will also have no overhead IF your graphics card supports OpenGL Shaders
  3. Updating of stimulus parameters for psychopy.visual.ElementArrayStim is also surprisingly fast BUT you should try to update your stimuli using numpy arrays for the maths rather than for… loops

Notable slow functions in PsychoPy calls:

  1. Calls to set the image or set the mask of a stimulus. This involves having to transfer large amounts of data between the computer’s main processor and the graphics card, which is a relatively time-consuming process.
  2. Any of your own code that uses a Python for… loop is likely to be slow if you have a large number of cycles through the loop. Try to ‘vectorise’ your code using a numpy array instead.

Tips to render stimuli faster

  1. Keep images as small as possible. This is meant in terms of number of pixels, not in terms of Mb on your disk. Reducing the size of the image on your disk might have been achieved by image compression such as using jpeg images but these introduce artefacts and do nothing to reduce the problem of send large amounts of data from the CPU to the graphics card. Keep in mind the size that the image will appear on your monitor and how many pixels it will occupy there. If you took your photo using a 10 megapixel camera that means the image is represented by 30 million numbers (a red, green and blue) but your computer monitor will have, at most, around 2 megapixels (1960x1080).
  2. Try to use square powers of two for your image sizes. This is efficient because computer memory is organised according to powers of two (did you notice how often numbers like 128, 512, 1024 seem to come up when you buy your computer?). Also several mathematical routines (anything involving Fourier maths, which is used a lot in graphics processing) are faster with power-of-two sequences. For the psychopy.visual.GratingStim a texture/mask of this size is required and if you don’t provide one then your texture will be ‘upsampled’ to the next larger square-power-of-2, so you can save this interpolation step by providing it in the right shape initially.
  3. Get a faster graphics card. Upgrading to a more recent card will cost around £30. If you’re currently using an integrated Intel graphics chip then almost any graphics card will be an advantage. Try to get an nVidia or an ATI Radeon card.

OpenGL Shaders

You may have heard mention of ‘shaders’ on the users mailing list and wondered what that meant (or maybe you didn’t wonder at all and just went for a donut!). OpenGL shader programs allow modern graphics cards to make changes to things during the rendering process (i.e. while the image is being drawn). To use this you need a graphics card that supports OpenGL 2.1 and PsychoPy will only make use of shaders if a specific OpenGL extension that allows floating point textures is also supported. Nowadays nearly all graphics cards support these features - even Intel chips from Intel!

One example of how such shaders are used is the way that PsychoPy colors greyscale images. If you provide a greyscale image as a 128x128 pixel texture and set its color to be red then, without shaders, PsychoPy needs to create a texture that contains the 3x128x128 values where each of the 3 planes is scaled according to the RGB values you require. If you change the color of the stimulus a new texture has to be generated with the new weightings for the 3 planes. However, with a shader program, that final step of scaling the texture value according to the appropriate RGB value can be done by the graphics card. That means we can upload just the 128x128 texture (taking 1/3 as much time to upload to the graphics card) and then we each time we change the color of the stimulus we just a new RGB triplet (only 3 numbers) without having to recalculate the texture. As a result, on graphics cards that support shaders, changing colors, contrasts and opacities etc. has almost zero overhead.

Blend Mode

A ‘blend function’ determines how the values of new pixels being drawn should be combined with existing pixels in the ‘frame buffer’.

blendMode = ‘avg’

This mode is exactly akin to the real-world scenario of objects with varying degrees of transparency being placed in front of each other; increasingly transparent objects allow increasing amounts of the underlying stimuli to show through. Opaque stimuli will simply occlude previously drawn objects. With each increasing semi-transparent object to be added, the visibility of the first object becomes increasingly weak. The order in which stimuli are rendered is very important since it determines the ordering of the layers. Mathematically, each pixel colour is constructed from opacity*stimRGB + (1-opacity)*backgroundRGB. This was the only mode available before PsychoPy version 1.80 and remains the default for the sake of backwards compatibility.

blendMode = ‘add’

If the window blendMode is set to ‘add’ then the value of the new stimulus does not in any way replace that of the existing stimuli that have been drawn; it is added to it. In this case the value of opacity still affects the weighting of the new stimulus being drawn but the first stimulus to be drawn is never ‘occluded’ as such. The sum is performed using the signed values of the color representation in PsychoPy, with the mean grey being represented by zero. So a dark patch added to a dark background will get even darker. For grating stimuli this means that contrast is summed correctly.

This blend mode is ideal if you want to test, for example, the way that subjects perceive the sum of two potentially overlapping stimuli. It is also needed for rendering stereo/dichoptic stimuli to be viewed through colored anaglyph glasses.

If stimuli are combined in such a way that an impossible luminance value is requested of any of the monitor guns then that pixel will be out of bounds. In this case the pixel can either be clipped to provide the nearest possible colour, or can be artificially colored with noise, highlighting the problem if the user would prefer to know that this has happened.

Sync to VBL and wait for VBL

PsychoPy will always, if the graphics card allows it, synchronise the flipping of the window with the vertical blank interval (VBL aka VBI) of the screen. This prevents visual artefacts such as ‘tearing’ of moving stimuli. This does not, itself, indicate that the script also waits for the physical frame flip to occur before continuing. If the waitBlanking window argument is set to False then, although the window refreshes themselves will only occur in sync with the screen VBL, the win.flip() call will not actually wait for this to occur, such that preparations can continue immediately for the next frame. For rendering purposes this is actually optimal and will reduce the likelihood of frames being dropped during rendering.

By default the PsychoPy Window will also wait for the VBL (waitBlanking=True) . Although this is slightly less efficient for rendering purposes it is necessary if we need to know exactly when a frame flip occurred (e.g. to timestamp when the stimulus was physically presented). On most systems this will provide a very accurate measure of when the stimulus was presented (with a variance typically well below 1ms but this should be tested on your system).