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Element Icon GDI canvas
Category: Visual Stimuli AddIn: Base Layer
Creator: OkazoLab Scope: Parent event
Owns Snippets: None Usage: Stimulus screen


NameDescription                              ConstraintsTypeOn
Visual Appearance
Background Color        Background color for canvas.                stColor         
Alpha Masking        Defines, whether the visual content is used to create an opacity mask on the event surface. If masking is on, the content's pixel luminance (or, its inverse) defines transparency within element's viewport. The alpha masking allows, for example, create transparent holes on the event surface.                Int32         
AntiAliasing        if true, the rendered content will be antialiased for smoother visual appeariance. If false, rendering will be authentic to its source, e.g. a bitmap.                Boolean         
Position        Defines a position of the rendering viewport on the screen                clPoint         
Size        Defines a size of the element's rendering viewport on the screen                clSize         
Z Order        Indicates Z Order of the element on the given event                Int32         
Pivot Point        Defines alignment of the pivot point (or, element's position) relative to element's rendering area. The pivot point sets the center of element's rotation and scale directions.                stAlig..         
Visible        Defines whether the element is visible                Boolean         
Rotation        Rotation angle in degrees                Double         
Run Time
Graphics Class        Returns a core GDI+ class (Graphics type) that provides a set of drawing functions. Drawing operations are invoked via class's methods, e.g. GraphicsClass.DrawEllipse(...);                Graphi..         
Update Canvas Now        Returns a core GDI+ class (Graphics type) that provides a set of drawing functions. Drawing operations are invoked via class's methods, e.g. GraphicsClass.DrawEllipse(...);                Boolean         
Transparent Color        The color of the original pixels of rendered content that will be set as transparent                stColor         
Transparent Tolerance        Tolerance ratio for choosing colors that become transparent. Zero means that effect is not applied, 1 denotes that all colors will become trasparent                Int32         
Color Mask        Produces a multiplicative combination of the original pixels and the selected mask color (except the alpha channel).  The effect emulates a look through a colored glass.  White or transparent mask color dont change the original pixels                stColor         
Opacity        Opacity of the rendered content                Int32         
Contrast        Contrast of the rendered content                Int32         
Brightness        Brightness the rendered content                Int32         
Saturation        Saturation of the rendered content                Double         
Pixelation        Pixelation of the rendered content                Int32         
Blurring        The radius used in the gaussian blur of rendered content, as a pixel-based factor. The default is 0 which means no blurring.                Int32         
Scrambling                        Double         
Scrambling Grid Size        Defines a size of the scrambling grid                clSize         
Positional Jitter
Reset Jitter Now        Runtime command that resets the positional jitter.                Boolean         
Current Jitter        Returns the current jitter                clPoint         
Jitter Range        Defines a possible range for the random jitter. The range is centered to the position of the element.                clSize         
Test Jitter        Click to test the current jitter                Boolean         
Is Enabled        If set to false the element is completely omitted when the experiment is run                Boolean         
Title        Title of the element                String         

GDI Canvas Element provides a rectangular canvas for creating custom vector graphics at runtime. For that purpose, all drawing functions of powerful GDI+ graphics API are directly available in code snippets.


GDI+ is a powerful graphical library for Windows, with thousands functions for drawing graphical primitives and bitmaps, managing colors and geometrical transforms GDI+ tutorial. You can call these functions in your code snippets in order to draw a dynamic 2D graphical content in your experiments.


Example canvas

Example canvas

Example code

Example code

Practical Use

Using the element is very easy. By setting its position and size you define a rectangular area on the screen that represents a rendering canvas for drawing functions. Then you address the Graphics property of the element. This property provides you with the base GDI class which, in turn, possesses all available drawing functions. Now you call these functions directly as in the following C# line:

 Graphics.DrawRectangle(new Pen(Color.Red), 20,20,248,162); 

When a snippet with GDI calls is completed the canvas element redraws its rendering canvas which also reappears on the screen if the parent event is active.

Step by Step: Adding a GDI Canvas element

  1. On an event, click "Add Element", under the "VISUAL STIMULI" category, select the "GDI canvas" element.
  2. Double click on the black square that appears.
  3. Create a proxy variable from the "Graphics" property, e.g "Graphics"
  4. In the bottom 'Snippets' panel, under 'Preparation', click on the 'Start' Snippet.
  5. add the following code:

     Graphics.DrawRectangle(new Pen(Color.Red), 20,20,248,162); 

Note: If unsure about proxy variables, please see the Proxy variables concept page.


  • The drawing coordinates in your code have to be inside of the range defined by the size of the GDI Canvas element in pixels, otherwise the drawn content will be cut off.
  • The GDI canvas element remembers its rendered state even if the parent event is deactivated and activated again. To clear the drawn graphics use the GDI Clear() function.

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