Manage Screen processing hook chains. Hook chains are a way to extend PTBs
behaviour with plugins for generic processing or fast image processing. They
should allow to interface PTB seamlessly with 3rd party special response
collection devices or stimulus display devices. They also allow to perform high
performance image processing, utilizing modern graphics hardwares huge
computational bandwidth. Read ‘help PsychGLImageProcessing’ for more infos.
Subsubcommands and their syntax:
Screen(‘HookFunction’, [windowPtr], ‘ListAll’);
Print out a listing of all supported ‘HookName’ hook points, including a short
synopsis on what they do, to the Matlab console in a human readable format -
Useful as a reference for coding. The ‘windowPtr’ argument is optional. If left
out (replaced by empty  brackets) it prints all implemented chains.
[slot idstring blittercfg voidptr glslid luttexid insertString] =
Screen(‘HookFunction’, windowPtr, ‘Query’, hookname, slotnameOrIndex);
Query information about a specific command slot in a specific hook processing
chain: ‘hookname’ is the name of the chain to query, e.g.,
‘StereoCompositingBlit’ for the stereo processing chain. Use the subcommand
‘ListAll’ for a printout of all available processing hooks and a short help on
them. ‘slotnameOrIndex’, either the symbolic name of the requested slot or the
index in the hook-chain: Indices start with 0. Names can be assigned to slots
when you add a processing slot with the ‘Append’ or ‘Prepend’ command, or they
are system assigned names like , e.g., ‘StereoCompositingShader’ for builtin
shaders that are needed for stereo processing.
Return arguments, all optional: ‘slot’ is the index of the named slot in the
chain to which the queried subfunction is assigned, or -1 if no such slot exists
in the chain. ‘glslid’ numeric GLSL handle of the OpenGL GLSL shader object if
the slot contains a GLSL shader for image processing on the GPU, 0 otherwise.
‘luttexid’ OpenGL texture handle of the first assigned lookup texture, 0 if none
assigned. ‘voidptr’ Memory pointer (encoded as double) to a C callback function,
if one is assigned to this slot, 0 otherwise. ‘blittercfg’ either a parameter
string with a meaning dependent of slot type, or the string ‘NONE’ if none
assigned. ‘idstring’ The symbolic name of this slot if any assigned, ‘NONE’
otherwise. ‘insertString’ the subcommand to use for reinserting this slot at the
place it was after a deletion.
Screen(‘HookFunction’, windowPtr, ‘AppendShader’, hookname, idString, glslid [,
Append a new instruction slot to the end of hook processing chain ‘hookname’,
assign the symbolic name ‘idString’ for later query by the ‘Query’ command.
‘glslid’ must be the name of a valid GLSL program object which defines the
algorithm to apply on the GPU. ‘blittercfg’ optional string with configuration
commands for the shader. ‘luttexid’ Optional handle to an OpenGL texture which
is used to encode lookup tables for the shader.
Screen(‘HookFunction’, windowPtr, ‘PrependShader’, hookname idString, glslid [,
Same as ‘AppendShader’ but add shader slot to beginning of the hook chain. It’s
recommended that you prepend slots instead of appending them, because PTB itself
may add special slots at the end of a chain.
Screen(‘HookFunction’, windowPtr, ‘AppendCFunction’, hookname, idString,
Screen(‘HookFunction’, windowPtr, ‘PrependCFunction’, hookname, idString,
Attach a C callable function to the chain. voidfunctionptr is a double value
which encodes a memory pointer to the function in memory. Encoding of the
pointer and requirements to the function are non-trivial, so this is for expert
Screen(‘HookFunction’, windowPtr, ‘AppendMFunction’, hookname, idString,
Screen(‘HookFunction’, windowPtr, ‘PrependMFunction’, hookname, idString,
Add a Matlab callable function to the chain. ‘fevalstring’ is the function call
string: It will be passed to and evaluated by Matlabs or Octaves feval()
function, so it has to work with that function. It is not allowed to return any
return arguments. Some special names in the string will be replaced by PTB with
internal settings, think of it as a macro replacement. Caution: The called
function is not allowed to call any Screen() commands or you’ll likely get
undefined behaviour or a crash – Screen is not reentrant!
Screen(‘HookFunction’, windowPtr, ‘AppendBuiltin’, hookname, idString,
Screen(‘HookFunction’, windowPtr, ‘PrependBuiltin’, hookname, idString,
Add a call to a PTB built-in function ‘builtincmd’.
You can also insert a hook function somewhere at a specific slot index via:
Screen(‘HookFunction’, windowPtr, ‘InsertAtXXXYYY’, …);
where XXX is a numeric slot id and YYY is the type specifier. Example: Insert a
at index 4 in hook ‘UserDefinedBlit’:
Screen(‘HookFunction’, windowPtr, ‘InsertAt4Builtin’, ‘UserDefinedBlit’, …);
Screen(‘Hookfunction’, windowPtr, ‘Remove’, hookname, slotindex);
Remove slot at index ‘slotindex’ in hookchain ‘hookname’. The slot after this
slot will move up by one.
Screen(‘HookFunction’, windowPtr, ‘Enable’, hookname);
Screen(‘HookFunction’, windowPtr, ‘Disable’, hookname);
Enable or disable a specific hook chain. Chains are disabled until you enable
them, with the exception of a few internal chains that get initialized and
enabled by PTB itself, e.g., stereo algorithm chain. Disabled chains are not
Screen(‘HookFunction’, windowPtr, ‘Reset’, hookname);
Reset a processing hook chain: All slots are deleted, resetting the chain to its
startup state. Seldomly needed.
Screen(‘HookFunction’, windowPtr, ‘Dump’, hookname);
Print out the full chain for hook ‘hookname’ to the Matlab console in a human
readable format - Useful for debugging.
Screen(‘HookFunction’, windowPtr, ‘DumpAll’);
Print out all chains for the given onscreen window ‘windowPtr’ to the Matlab
console in a human readable format - Useful for debugging.
oldImagingMode = Screen(‘HookFunction’, proxyPtr, ‘ImagingMode’ [,
Change or query imagingMode flags of provided proxy window ‘proxyPtr’ to
‘imagingMode’. Proxy windows are used to define image processing operations,
mostly for Screen(‘TransformTexture’). Returns old imaging mode.
* Hook chains are per onscreen window, so each window can have a different
configuration and enable state.
* Read all available documentation on the Psychtoolbox imaging pipeline in ‘help
PsychGLImageprocessing’, the PsychDocumentation folder and on the Wiki before
you make use of this function. Its way to complex to use it by guessing ;)