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()