Cairo context 和持久性

Cairo context 和持久性

Cairo 是一个广泛使用的 2D 图形库，支持多种输出设备，包括屏幕、PDF、SVG等。Cairo 的核心是它的“context”（上下文）概念，这是进行所有绘制操作的中心点。在 Cairo 中，context 通常与特定的图形目标（或称作“surface”）关联，如窗口或图像文件。

1、问题背景

在使用 pycairo 绘制时，用户遇到了一个有趣的错误。该程序创建了一个简单的 GTK+ 窗口，在上面绘制一个矩形，然后有一个回调函数在键盘输入时绘制一条随机线条。但是，每次键盘输入时，用户都需要创建一个新的上下文，否则在程序收到第一个键盘输入时就会出现错误（特别是在 .stroke() 行上）。错误如下，如有必要。'BadDrawable (invalid Pixmap or Window parameter)'。

以下是代码的详细信息：

#! /usr/bin/env python
import pygtk
pygtk.require('2.0')
import gtk, gobject, cairo, math, random
# Create a GTK+ widget on which we will draw using Cairo
class Screen(gtk.DrawingArea):
# Draw in respe to an expose-event
  __gsignals__ = { "expose-event": "override" }
​
  # Handle the expose-event by drawing
  def do_expose_event(self, event):
    # Create the cairo context
     = self._create()
    # Restrict Cairo to the exposed area; avoid extra work
    .rectangle(event.area.x, event.area.y, event.area.width, event.area.height)
    .clip()
​
    self.draw(*self.window.get_size())
​
  def key_press_event(self, *args):
    # print args
     = self._create() # This is the line I have to add
    # in order to make this function not throw the error. ote that cr is only
    # given as attribute of self in order to stop it going out of scope when this line
    # doesn't exist
    .set_source_rgb(random.random(), random.random(), random.random())
    .move_to(*[z/2.0 for z in self.window.get_size()])
    .line_to(*[z*random.random() for z in self.window.get_size()])
    .stroke()
​
  def draw(self, width, height):
    # Fill the background with gray
    .set_source_rgb(.5,.5,.5)
    .rectangle(0, 0, width,height)
    .fill()
​
    .set_source_rgb(1,0,0)
    .arc(width/2.0, height/2.0, min(width,height)/2.0 - 20.0, 0.0, 2.0*math.pi)
    .stroke()
​
#create a gtk window, attach to exit button, and whatever is passed as arg becomes the body of the window. AWESOME
def run(Widget):
  window = gtk.Window()
  widget = Widget()
  ("delete-event", _quit)
  ('key-press-event',widget.key_press_event)
  widget.show()
  window.add(widget)
  window.present()
  ()
​
if __name__ == "__main__":
  run(Screen)

2、解决方案一：

不使用双缓冲

Cairo 图形无法持久保存。因此，您需要在曝光处理程序中完成所有绘制，否则，正如您已经发现的那样，它将在每次窗口重绘时消失。 Cairo 上下文不会持久存在，因为使用了双缓冲：请参阅 C 文档中的注释，不幸的是，我在 PyGTK 文档中不到任何地方。 在上面的代码中，您应该在按键处理程序中生成随机线的坐标和颜，并将它们保存在一个数组中。然后在曝光处理程序中，按顺序绘制数组中的每条线。 代码实现如下：

import pygtk
pygtk.require('2.0')
import gtk, gobject, cairo, math, random
# Create a GTK+ widget on which we will draw using Cairo
class Screen(gtk.DrawingArea):
# Draw in respe to an expose-event
  __gsignals__ = { "expose-event": "override" }
​
  def __init__(self):
      super(Screen, self).__init__()
      # Create an array to store the coordinates and color of the random lines
      self.lines = []
​
  # Handle the expose-event by drawing
  def do_expose_event(self, event):
    # Create the cairo context
     = self._create()
    # Restrict Cairo to the exposed area; avoid extra work
    .rectangle(event.area.x, event.area.y, event.area.width, event.area.height)
    .clip()
​
    # Draw the background with gray
    .set_source_rgb(.5,.5,.5)
    .rectangle(0, 0, width, height)
    .fill()
​
    .set_source_rgb(1,0,0)
    .arc(width/2.0, height/2.0, min(width,height)/2.0 - 20.0, 0.0, 2.0*math.pi)
    .stroke()
​
    # Draw the random lines
    for line in self.lines:
      .set_source_rgb(*line['color'])
      .move_to(*line['start'])
      .line_to(*line['end'])
      .stroke()
​
  def key_press_event(self, *args):
    # Generate the coordinates and color of the random line
    start = [z/2.0 for z in self.window.get_size()]
    end = [z*random.random() for z in self.window.get_size()]
    color = [random.random(), random.random(), random.random()]
​
    # Add the line to the array
    self.lines.append({'start': start, 'end': end, 'color': color})
​
    # Queue a redraw of the widget
    self.queue_draw()
​
# Create a GTK+ window, attach to the exit button, and whatever is passed as arg becomes the body of the window.
def run(Widget):
  window = gtk.Window()
  widget = Widget()
  ("delete-event", _quit)
  ('key-press-event',widget.key_press_event)
  widget.show()
  window.add(widget)
  window.present()
  ()
​
if __name__ == "__main__":
  run(Screen)

解决方案二：

禁用双缓冲

虽然每次运行都必须创建上下文，但您可以通过禁用小部件的双缓冲来实现您正在寻的持久性。

在代码中，您可以调用 set_double_buffered(False) 方法来禁用双缓冲。 以下是使用 Hamster Graphics 库的一个示例，它可以满足您的要求：

import hamster
from hamster import graphics
​
# Create a window
window = graphics.window(640, 480, "Many Lines")
​
# Disable double buffering
window.set_double_buffered(False)
​
# Create an array to store the lines
lines = []
​
# Draw a line when a key is pressed
def on_key_press(event):
  # Generate the coordinates and color of the random line
  start = [random.randint(0, window.width), random.randint(0, window.height)]
  end = [random.randint(0, window.width), random.randint(0, window.height)]
  color = [random.random(), random.random(), random.random()]
​
  # Add the line to the array
  lines.append({'start': start, 'end': end, 'color': color})
​
  # Draw the line
  graphics.set_color(*color)
  _to(*start)
  graphics.line_to(*end)
  graphics.stroke()
​
# Handle key press events
window.bind("<KeyPress>", on_key_press)
​
# Start the event loop
()

在这两个例子中，Cairo context 的持久性是通过将绘图结果保存到文件中实现的。这使得 Cairo 非常适合于需要高度图形处理和跨平台图形输出的应用程序。