pango-Cairo-Rendering: Cairo Rendering
In RGtk2: R Bindings for Gtk 2.8.0 and Above

Description Methods and Functions Hierarchy Detailed Description Structures User Functions Author(s) References

Rendering with the Cairo backend

pangoCairoFontMapGetDefault()
pangoCairoFontMapSetDefault(fontmap)
pangoCairoFontMapNew()
pangoCairoFontMapNewForFontType(fonttype)
pangoCairoFontMapGetFontType(object)
pangoCairoFontMapSetResolution(object, dpi)
pangoCairoFontMapGetResolution(object)
pangoCairoFontMapCreateContext(object)
pangoCairoFontMapCreateContext(object)
pangoCairoFontGetScaledFont(object)
pangoCairoContextSetResolution(context, dpi)
pangoCairoContextGetResolution(context)
pangoCairoContextSetFontOptions(context, options)
pangoCairoContextGetFontOptions(context)
pangoCairoContextSetShapeRenderer(object, func, data)
pangoCairoContextGetShapeRenderer(object)
pangoCairoCreateContext(cr)
pangoCairoUpdateContext(cr, context)
pangoCairoCreateLayout(cr)
pangoCairoUpdateLayout(cr, layout)
pangoCairoShowGlyphString(cr, font, glyphs)
pangoCairoShowGlyphItem(cr, text, glyph.item)
pangoCairoShowLayoutLine(cr, line)
pangoCairoShowLayout(cr, layout)
pangoCairoShowErrorUnderline(cr, x, y, width, height)
pangoCairoGlyphStringPath(cr, font, glyphs)
pangoCairoLayoutLinePath(cr, line)
pangoCairoLayoutPath(cr, layout)
pangoCairoErrorUnderlinePath(cr, x, y, width, height)

GInterface
   +----PangoCairoFont
GInterface
   +----PangoCairoFontMap

The Cairo library (http://cairographics.org) is a vector graphics library with a powerful rendering model. It has such features as anti-aliased primitives, alpha-compositing, and gradients. Multiple backends for Cairo are available, to allow rendering to images, to PDF files, and to the screen on X and on other windowing systems. The functions in this section allow using Pango to render to Cairo surfaces.

Using Pango with Cairo is straightforward. A PangoContext created with pangoCairoFontMapCreateContext can be used on any Cairo context (cairo_t), but needs to be updated to match the current transformation matrix and target surface of the Cairo context using pangoCairoUpdateContext. The convenience functions pangoCairoCreateLayout and pangoCairoUpdateLayout handle the common case where the program doesn't need to manipulate the properties of the PangoContext.

When you get the metrics of a layout or of a piece of a layout using functions such as pangoLayoutGetExtents, the reported metrics are in user-space coordinates. If a piece of text is 10 units long, and you call cairo_scale (cr, 2.0), it still is more-or-less 10 units long. However, the results will be affected by hinting (that is, the process of adjusting the text to look good on the pixel grid), so you shouldn't assume they are completely independent of the current transformation matrix. Note that the basic metrics functions in Pango report results in integer Pango units. To get to the floating point units used in Cairo divide by PANGO_SCALE.

Using Pango with Cairo

RADIUS <- 150
N.WORDS <- 10
FONT <- "Sans Bold 27"

draw.text <- function(widget, event, data)
{
  width <- widget[["allocation"]][["width"]]
  height <- widget[["allocation"]][["height"]]
  
  device.radius <- min(width, height) / 2.
  
  cr <- gdkCairoCreate(widget[["window"]])
  
  ## Center coordinates on the middle of the region we are drawing
  cr$translate(device.radius + (width - 2 * device.radius) / 2,
               device.radius + (height - 2 * device.radius) / 2)
  cr$scale(device.radius / RADIUS, device.radius / RADIUS)
  
  ## Create a PangoLayout, set the font and text
  layout <- pangoCairoCreateLayout(cr)
  
  layout$setText("Text")
  desc <- pangoFontDescriptionFromString(FONT)
  layout$setFontDescription(desc)
  
  ## Draw the layout N.WORDS times in a circle
  for (i in 1:N.WORDS) {
    angle <- (360 * i) / N.WORDS
    
    cr$save()
    
    ## Gradient from red at angle 60 to blue at angle 300
    red <- (1 + cos((angle - 60) * pi / 180)) / 2
    cr$setSourceRgb(red, 0, 1.0 - red)
    
    cr$rotate(angle * pi / 180)
    
    ## Inform Pango to re-layout the text with the new transformation
    pangoCairoUpdateLayout(cr, layout)
    
    size <- layout$getSize()
    cr$moveTo(- (size$width / .PangoScale) / 2, - RADIUS)
    pangoCairoShowLayout(cr, layout)
    
    cr$restore()
  }
  return(FALSE)
}

white <- c( 0, "0xffff", "0xffff", "0xffff" )

window <- gtkWindow("toplevel", show = F)
window$setTitle("Rotated Text")
drawing.area <- gtkDrawingArea()
window$add(drawing.area)

# This overrides the background color from the theme
drawing.area$modifyBg("normal", white)

gSignalConnect(drawing.area, "expose-event", draw.text)

window$showAll()

PangoCairoFont: PangoCairoFont is an interface exported by fonts for use with Cairo. The actual type of the font will depend on the particular font technology Cairo was compiled to use. Since 1.18
PangoCairoFontMap: PangoCairoFontMap is an interface exported by font maps for use with Cairo. The actual type of the font map will depend on the particular font technology Cairo was compiled to use. Since 1.10

PangoCairoShapeRendererFunc(cr, attr, do.path, data)

Function type for rendering attributes of type PANGO_ATTR_SHAPE with Pango's Cairo renderer.

cr: [Cairo] a Cairo context with current point set to where the shape should be rendered
attr: [PangoAttrShape] the PANGO_ATTR_SHAPE to render
do.path: [logical] whether only the shape path should be appended to current path of cr and no filling/stroking done. This will be set to TRUE when called from pangoCairoLayoutPath and pangoCairoLayoutLinePath rendering functions.
data: [R object] user data passed to pangoCairoContextSetShapeRenderer