Noisy cairo-created custom cursor in GTK3

Question

I'm working on an image annotation tool. It involves drawing circles of different size on top of an image. For convenience, I'd like to modify the cursor to be a circle of some radius. The snippet below (custom_cursor() function in particular) allows me to use a custom cursor (drawn with cairo) and change its radius with bracket keys ("[" and "]"). The issue I have is obvious in this video: https://i.stack.imgur.com/N2v4v.jpg. Basically, at some values of radii there's a lot of artifacts covering the whole cursor image. What's the reason for this and is it possible to remove that noise?

import gi
import numpy as np
import cairo
gi.require_version('Gtk', '3.0')
from gi.repository import Gtk, Gdk, GdkPixbuf

class MainWindow(Gtk.Window):

  def __init__(self):
    Gtk.Window.__init__(self, title = "Test")
    self.maximize()
    self.radius = 137
    viewport = Gtk.Viewport()
    self.darea = Gtk.DrawingArea()
    self.darea.connect("draw", self.draw)
    self.pixbuf = GdkPixbuf.Pixbuf.new_from_file("any_image.jpg")
    self.darea.set_size_request(self.pixbuf.get_width(), self.pixbuf.get_height());
    grid = Gtk.Grid()
    self.add(grid)
    scrolled = Gtk.ScrolledWindow()
    scrolled.set_hexpand(True)
    scrolled.set_vexpand(True)
    scrolled.connect("key-press-event",self.press)
    viewport.add(self.darea)
    scrolled.add(viewport)
    grid.add(scrolled)
    self.custom_cursor(self.radius)

  def draw(self, widget, cr):
    Gdk.cairo_set_source_pixbuf(cr, self.pixbuf, 0, 0)
    cr.paint()
    self.darea.queue_draw()

  def custom_cursor(self, radius):
    display = self.get_screen().get_display()
    pb = GdkPixbuf.Pixbuf.new(GdkPixbuf.Colorspace.RGB, True, 8, 2 * radius, 2 * radius)
    surface = Gdk.cairo_surface_create_from_pixbuf(pb, 0, None)
    context = cairo.Context(surface)
    context.arc(radius, radius, radius, 0, 2 * np.pi)
    context.set_source_rgba(0, 0, 0, 1)
    context.stroke()
    pbdrawn = Gdk.pixbuf_get_from_surface(surface, 0, 0, surface.get_width(), surface.get_height())
    cursor = Gdk.Cursor.new_from_pixbuf(display, pbdrawn, radius, radius)
    self.darea.get_screen().get_root_window().set_cursor(cursor)

  def press(self, widget, event):
    if (event.keyval == 93):
      self.radius = self.radius + 2
      self.custom_cursor(self.radius)
    elif (event.keyval == 91):
      self.radius = self.radius - 2
      self.custom_cursor(self.radius)

win = MainWindow()
win.connect("destroy", Gtk.main_quit)
win.show_all()
Gtk.main()

Edit: It could be a graphics issue. I'd appreciate it if anybody could confirm that there's (no) noise.

What is your `custom_cursor` function doing exactly? It creates a pixbuf, converts it to a cairo surface, and then converts it to a pixbuf again? Why do you even start with a pixbuf? — Uli Schlachter, Mar 21 '19 at 17:22
That's a fair question. You're right, there's no need to start with pixbuf. Problem solved. Thank you! — Juozas Miškinis, Mar 21 '19 at 18:30

score 0 · Accepted Answer · answered Mar 21 '19 at 17:23

From the documentation for gdk_pixbuf_new:

Note that the buffer is not cleared; you will have to fill it completely yourself.

https://developer.gnome.org/gdk-pixbuf/2.36/gdk-pixbuf-Image-Data-in-Memory.html#gdk-pixbuf-new

Basically, you are seeing uninitialised memory here.

I would suggest to just create a cairo image surface directly, instead of creating a pixbuf just to convert it to a cairo surface. Cairo initialises new image surface for you.

Noisy cairo-created custom cursor in GTK3

1 Answers1