Psychtoolbox>Screen.{mex*} subfunction

Set or return the maximum color component value that PTB should allow for
provided color values when drawing into a window ‘windowPtr’ or its associated
Offscreen windows. ‘maximumvalue’ is the optional new setting for the maximum
allowed color component value: PTB expects the values of provided color
components (red, green, blue, alpha or intensity) to be in the range 0 to
maximumvalue. 0 is mapped to minimum output intensity, maximumvalue is mapped to
maximum output intensity, values outside the range 0-maximumvalue are
automatically saturated (clamped) at zero or maximumvalue if clamping is
enabled. Initially color range clamping is enabled and the maximumvalue defaults
to the biggest integral number displayable by your video hardware, e.g., 255 for
a standard 8 bit per color component framebuffer as present on most consumer
graphics hardware.
A maximumvalue == 1.0 has special meaning: A maximumvalue of 1.0 will enable PTB
to pass color values in OpenGL’s native floating point color range of 0.0 to
1.0: This has two advantages: First, your color values are independent of
display device depth, i.e. no need to rewrite your code when running it on
higher resolution hardware. Second, PTB can skip any color range remapping
operations - this can speed up drawing significantly in some cases.
‘clampcolors’: By default, OpenGL clamps colors to the range 0-maximumvalue.
Negative colors are clamped to zero, color values greater than maximumvalue are
clamped to maximumvalue. If you set the optional flag ‘clampcolors’ to 0, PTB
will disable color clamping on hardware that supports unclamped colors. This
allows to pass arbitrary (even negative) floating point numbers as color values.
This is useful in conjunction with special high dynamic range display devices
and for certain image processing operations when using PTB’s image processing
pipeline. If your hardware/operating system does support shaders, but not
unclamped colors, Screen will try to use a shader-based workaround to enable
unclamped color processing despite missing hardware capabilities - This comes at
some performance penalty. You can also force Screen to always use its own
unclamped color implementation by setting ‘clampcolors’ to a value of -1. On
some graphics hardware, this may increase the precision with which the color of
drawn objects is handled, but again at some speed penalty.
CAUTION: If you change the color range or clamping of an onscreen window with
this function, the change will only affect textures and offscreen windows
created *after* this function call, not ones created before. It’s therefore
recommended to execute this function immediately after creating an onscreen
window to guarantee consistent behaviour of your code. Color values provided as
uint8 arrays or as textures, e.g., from video capture, movies or
Screen(‘MakeTexture’) are not rescaled, asthey are (expected) to be in a proper
format for the given color depth already. However, the Screen(‘MakeTexture’)
command has an optional flag ‘floatprecision’ that allows you to pass image
matrices unclamped and with either 16 bpc or 32 bpc floating point color
precision if you want.
Additionally you can force Screen(‘MakeTexture’) to apply the ‘maximumvalue’
setting to regular textures which are provided as Matlab double type matrices by
setting the optional parameter ‘applyToDoubleInputMakeTexture’ to 1. This will
allow an input range of double values between zero and ‘maximumvalue’ to make
your code more consistent, but it will still limit the range of valid values to
that range and will represent that range only with 8 bit for 256 different
levels, ie., the description of the ‘clampcolors’ setting do not apply to such
uint8 low-precision textures! For full precision unconstrained textures, you’ll
still need to set the ‘floatprecision’ flag accordingly, as such textures
require more memory and processing resources.

###See also: OpenWindow OpenOffscreenWindow