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I had a look at how complex would it be to add correct downscaling to Cairo now that Pixman supports convolution filters. It turns out it this is rather easy. Here is an initial, minimal attempt. It uses convolution filters only if the image is being downscaled by more than half a pixel in at least one dimension. Some discussion: 1. The sampling and reconstruction kernels are picked in a way that gives comparable quality when upscaling and downscaling. I paired box sampling with bilinear reconstruction and impulse (point) sampling with box reconstruction. This gives the expected result for NEAREST filter. BEST filter uses Lanczos3 for both kernels. > Do we need to use a reconstruction filter for NEAREST at all? Or maybe > differentiate between NEAREST and FAST in that case? If impulse (point) sampling is used, there must be some reconstruction filter, otherwise no image is produced. That's because the sampling grid does not match the data grid, and since there is no reconstruction filter, values between data points are undefined. The alternative is to use box sampling + no reconstruction. 2. Subsampling bits are always set to 1, since this doesn't seem to affect quality at all. 3. I am not sure whether this code works correctly for matrices with a skew component. It should be OK for any combination of scale, rotation and translation. 4. This patch causes new failures in the test suite: - recording-surface*: possibly an effect of improved quality. - surface-pattern-scale-down*, surface-pattern-big-scale-down: the reference images should be updated. - pthread-same-source: I have no idea why this is failing, since this test shouldn't even trigger the new code. - large-source-roi: this test attempts to downscale an image which is 30000 pixels wide down to 7 pixels. The filter parameters seem to be created correctly, but they might trigger an overflow somewhere in the convolution code; the output rectangle is white instead of red, as if nothing was drawn. - device-offset-scale: there are subtle differences which look like convolution-related smoothing; I'm not sure whether this is OK or not. |
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| build | ||
| doc | ||
| perf | ||
| src | ||
| test | ||
| util | ||
| .gitignore | ||
| acinclude.m4 | ||
| AUTHORS | ||
| autogen.sh | ||
| BIBLIOGRAPHY | ||
| BUGS | ||
| cairo-version.h | ||
| CODING_STYLE | ||
| configure.ac | ||
| COPYING | ||
| COPYING-LGPL-2.1 | ||
| COPYING-MPL-1.1 | ||
| HACKING | ||
| INSTALL | ||
| KNOWN_ISSUES | ||
| Makefile.am | ||
| Makefile.win32 | ||
| NEWS | ||
| PORTING_GUIDE | ||
| README | ||
| README.win32 | ||
| RELEASING | ||
Cairo - Multi-platform 2D graphics library
http://cairographics.org
What is cairo
=============
Cairo is a 2D graphics library with support for multiple output
devices. Currently supported output targets include the X Window
System (via both Xlib and XCB), quartz, win32, and image buffers,
as well as PDF, PostScript, and SVG file output. Experimental backends
include OpenGL, BeOS, OS/2, and DirectFB.
Cairo is designed to produce consistent output on all output media
while taking advantage of display hardware acceleration when available
(for example, through the X Render Extension).
The cairo API provides operations similar to the drawing operators of
PostScript and PDF. Operations in cairo include stroking and filling
cubic Bézier splines, transforming and compositing translucent images,
and antialiased text rendering. All drawing operations can be
transformed by any affine transformation (scale, rotation, shear,
etc.).
Cairo has been designed to let you draw anything you want in a modern
2D graphical user interface. At the same time, the cairo API has been
designed to be as fun and easy to learn as possible. If you're not
having fun while programming with cairo, then we have failed
somewhere---let us know and we'll try to fix it next time around.
Cairo is free software and is available to be redistributed and/or
modified under the terms of either the GNU Lesser General Public
License (LGPL) version 2.1 or the Mozilla Public License (MPL) version
1.1.
Where to get more information about cairo
=========================================
The primary source of information about cairo is:
http://cairographics.org/
The latest versions of cairo can always be found at:
http://cairographics.org/download
Documentation on using cairo and frequently-asked questions:
http://cairographics.org/documentation
http://cairographics.org/FAQ
Mailing lists for contacting cairo users and developers:
http://cairographics.org/lists
Roadmap and unscheduled things to do, (please feel free to help out):
http://cairographics.org/roadmap
http://cairographics.org/todo
Dependencies
============
The set of libraries needed to compile cairo depends on which backends
are enabled when cairo is configured. So look at the list below to
determine which dependencies are needed for the backends of interest.
For the surface backends, we have both "supported" and "experimental"
backends. Further, the supported backends can be divided into the
"standard" backends which can be easily built on any platform, and the
"platform" backends which depend on some underlying platform-specific
system, (such as the X Window System or some other window system).
As an example, for a standard Linux build, (with image, png, pdf,
PostScript, svg, and xlib surface backends, and the freetype font
backend), the following sample commands will install necessary
dependencies:
Debian (and similar):
apt-get install libpng12-dev libz-dev libxrender-dev libfontconfig1-dev
Fedora (and similar):
yum install libpng-devel zlib-devel libXrender-devel fontconfig-devel
(Those commands intentionally don't install pixman from a distribution
package since if you're manually compiling cairo, then you likely want
to grab pixman from the same place at the same time and compile it as
well.)
Supported, "standard" surface backends
------------------------------------
image backend (required)
------------------------
pixman >= 0.20.2 http://cairographics.org/releases
png support (can be left out if desired, but many
----------- applications expect it to be present)
libpng http://www.libpng.org/pub/png/libpng.html
pdf backend
-----------
zlib http://www.gzip.org/zlib
postscript backend
------------------
zlib http://www.gzip.org/zlib
svg backend
-----------
[none]
Supported, "platform" surface backends
-----------------------------------
xlib backend
------------
X11 http://freedesktop.org/Software/xlibs
xlib-xrender backend
--------------------
Xrender >= 0.6 http://freedesktop.org/Software/xlibs
quartz backend
--------------
MacOS X >= 10.4 with Xcode >= 2.4
win32 backend
-------------
Microsoft Windows 2000 or newer[*].
xcb backend
-----------
XCB http://xcb.freedesktop.org
Font backends (required to have at least one)
---------------------------------------------
freetype font backend
---------------------
freetype >= 2.1.9 http://freetype.org
fontconfig http://fontconfig.org
quartz-font backend
-------------------
MacOS X >= 10.4 with Xcode >= 2.4
win32 font backend
------------------
Microsoft Windows 2000 or newer[*].
[*] The Win32 backend should work on Windows 2000 and newer
(excluding Windows Me.) Most testing has been done on
Windows XP. While some portions of the code have been
adapted to work on older versions of Windows, considerable
work still needs to be done to get cairo running in those
environments.
Cairo can be compiled on Windows with either the gcc
toolchain (see http://www.mingw.org) or with Microsoft
Visual C++. If the gcc toolchain is used, the standard
build instructions using configure apply, (see INSTALL).
If Visual C++ is desired, GNU make is required and
Makefile.win32 can be used via 'make -f Makefile.win32'.
The compiler, include paths, and library paths must be set
up correctly in the environment.
MSVC versions earlier than 7.1 are known to miscompile
parts of cairo and pixman, and so should be avoided. MSVC
7.1 or later, including the free Microsoft Visual Studio
Express editions, produce correct code.
Experimental surface backends
-----------------------------
beos backend
------------
No dependencies in itself other than an installed BeOS system, but cairo
requires a font backend. See the freetype dependency list.
os2 backend
-----------
Cairo should run on any recent version of OS/2 or eComStation, but it
requires a font backend. See the freetype dependency list. Ready to use
packages and developer dependencies are available at Netlabs:
ftp://ftp.netlabs.org/pub/cairo
Compiling
=========
See the INSTALL document for build instructions.
History
=======
Cairo was originally developed by Carl Worth <cworth@cworth.org> and
Keith Packard <keithp@keithp.com>. Many thanks are due to Lyle Ramshaw
without whose patient help our ignorance would be much more apparent.
Since the original development, many more people have contributed to
cairo. See the AUTHORS files for as complete a list as we've been able
to compile so far.