GraphicsContext

The GraphicsContext object provides methods for drawing images, text and shapes on the canvas.

local dlg = Dialog()
dlg:canvas{
  onpaint = function(ev)
    local gc = ev.context
    -- gc is a GraphicsContext
  end
}

GraphicsContext.width

local width = gc.width

Gets the width of the visible area in pixels. Canvas dimensions change when the dialog is resized by the user.

GraphicsContext.height

local height = gc.height

Gets the height of the visible area in pixels. Canvas dimensions change when the dialog is resized by the user.

GraphicsContext.antialias

local aa = gc.antialias
gc.antialias = true

Gets or sets whether paths and shapes are painted on using antialiasing.

Comparison between a line drawn on canvas without and with the antialiasing

GraphicsContext.color

local color = gc.color
gc.color = Color(255)

Gets or sets the color to paint with the path functions.

GraphicsContext.strokeWidth

local sw = gc.strokeWidth
gc.strokeWidth = 10

Gets or sets the width of lines painted when calling GraphicsContext:stroke() or GraphicsContext:strokeRect().

GraphicsContext.blendMode

local bm = gc.blendMode
gc.blendMode = BlendMode.NORMAL

Blend mode used in stroke(), fill()), drawImage(), etc.

GraphicsContext.opacity

local opacity = gc.opacity
assert(opacity >= 0 and opacity <= 255)
gc.opacity = newOpacity

Opacity used in stroke(), fill()), drawImage(), etc.

  • opacity: 0 completelly transparenty, 255 completelly opaque.

GraphicsContext:save()

gc:save()

Saves the current state of the canvas to restore it later, including:

GraphicsContext:restore()

gc:restore()

Restores the last saved canvas state.

GraphicsContext:clip()

gc:clip()

Sets the current path as a clipping area for following drawing operations.

Warning: The only option to draw paths not clipped after calling GraphicsContext:clip() is restoring a previously saved state.

-- Save state before clipping
gc:save()

-- Create a rectangle and use it as a clipping region
gc:beginPath()
gc:rect(Rectangle(0, 30, 100, 40))
gc:clip()

-- Stroke a circle with width of 4
gc.strokeWidth = 4

gc:beginPath()
gc:roundedRect(Rectangle(10, 10, 80, 80), 50)
gc:stroke()

-- Restore the state
gc:restore()

-- Fill a smaller circle in white
gc.color = Color {r = 255, g = 255, b = 255, a = 255}

gc:beginPath()
gc:roundedRect(Rectangle(20, 20, 60, 60), 30)
gc:fill()

Clipping region

GraphicsContext:strokeRect()

gc:strokeRect(rectangle)

Paints the edges of the given rectangle with the current color and current width.

GraphicsContext:fillRect()

gc:fillRect(rectangle)

Fills the given rectangle with the current color.

GraphicsContext:fillText()

gc:fillText(string, x, y)

Draws on the canvas the given text string, at a position specified by the xy-coordinates.

GraphicsContext:measureText()

local size = gc:measureText(string)

Returns the size of the given text using the current font.

GraphicsContext:drawImage()

gc:drawImage(image, x, y)
gc:drawImage(image, srcRect, dstRect)
gc:drawImage(image, srcX, srcY, srcW, srcH, dstX, dstY, dstW, dstH)

Draws on the canvas the given image.

If given xy-coordinates, the full image will be drawn at the specified position, in it's original scale.

Image drawn on a canvas

If given source and destination bounds (or their respective values), a part of the image is drawn (specified by the srcRect, or srcX/srcY/srcW/srcH, relative to the image) on the canvas (specified by the dstRect or dstX/dstY/dstW/dstH). This allows drawing only a part of and/or scaling the image.

Image part drawn a on canvas

GraphicsContext:drawThemeImage()

gc:drawThemeImage(partId, point)
gc:drawThemeImage(partId, x, y)

Draws on the canvas a theme part specified by the given partId, at a given Point or at specified xy-coordinates.

List of possible parts can be found in theme.xml.

GraphicsContext:drawThemeRect()

gc:drawThemeRect(partId, rectangle)
gc:drawThemeRect(partId, x, y, w, h)

Draws on the canvas a theme part specified by the given partId, filling a given Rectangle or at specified xy-coordinates, with given width and height. This method uses nine-slice scaling for parts that have their Slice's center defined.

List of possible parts can be found in theme.xml.

GraphicsContext:beginPath()

gc:beginPath()

Starts a new path, emptying the list of tracked sub-paths. This is the first method to call when drawing paths.

GraphicsContext:closePath()

gc:closePath()

Closes the current sub-path by connecting the current point with the first point of the current sub-path.

GraphicsContext:moveTo()

gc:moveTo(x, y)

Starts a new sub-path at the specified xy-coordinates. This is the second method to call (after GraphicsContext:beginPath()) when drawing paths.

GraphicsContext:lineTo()

gc:lineTo(x, y)

Adds a line to the current sub-path, from the last point to the specified xy-coordinates.

Note: This method doesn't draw on the canvas, to draw the path onto the canvas call GraphicsContext:stroke() or GraphicsContext:fill().

GraphicsContext:cubicTo()

gc:cubicTo(cp1x, cp1y, cp2x, cp2y, x, y)

Adds a cubic B├ęzier curve to the current sub-path, from the last point to the specified xy-coordinates, with two control points (specified by cp1x/cp1y and cp2x/cp2y coordinates).

Note: This method doesn't draw on the canvas, to draw the path onto the canvas call GraphicsContext:stroke() or GraphicsContext:fill().

GraphicsContext:oval()

gc:oval(rectangle)

Adds an oval enclosed by the given Rectangle to the current sub-path.

Note: This method doesn't draw on the canvas, to draw the path onto the canvas call GraphicsContext:stroke() or GraphicsContext:fill().

GraphicsContext:rect()

gc:rect(rectangle)

Adds a given Rectangle to the current sub-path.

Note: This method doesn't draw on the canvas, to draw the path onto the canvas call GraphicsContext:stroke() or GraphicsContext:fill().

GraphicsContext:roundedRect()

gc:roundedRect(rectangle, r)
gc:roundedRect(rectangle, rx, ry)

Adds a given Rectangle to the current sub-path with rounded corners.

If a single radius is specified, a rectangle with circular corners is created. This method can be used to easily draw circles.

Rounded rectangle with a single radius specified

If two radii are specified, a rectangle with elliptical corners is created.

Rounded rectangle with two radii specified

Note: This method doesn't draw on the canvas, to draw the path onto the canvas call GraphicsContext:stroke() or GraphicsContext:fill().

GraphicsContext:stroke()

gc:stroke()

Paints the edges of the current path with the current color and current width.

GraphicsContext:fill()

gc:fill()

Fills the current path with the current color.