Circos > Documentation > Tutorials > Configuration > Png Output
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Use the latest version of Circos and read Circos best practices—these list recent important changes and identify sources of common problems.
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0 — Configuration and Installation

10. PNG Output

Circos is capable of producing both PNG (24-bit) and SVG images. This section discusses PNG files, as well as the <image> block which controls the location, size and other characteristics of the output file.

<image> block

I suggest that you always import the default image settings.

# import defaults from Circos distribution
<<include etc/image.conf>>

The settings define the output file to be 3,000 x 3,000 pixels, with white background, named circos.png, which will be placed in the current directory.

Take a look at etc/image.conf in the Circos distribution to see the parameter definitions. You'll find that this file is actually composed of two additional includes, a practise common in Circos to try to modularize configuration as much as possible.

# etc/image.conf
<<include image.generic.conf>>
<<include background.white.conf>>

# image.generic.conf
dir                = .
file               = circos.png
png                = yes
svg                = yes
radius             = 1500p
angle_offset       = -90
#angle_orientation = counterclockwise
auto_alpha_colors  = yes
auto_alpha_steps   = 5

# background.white.conf
background         = white

If you would like to overwrite any of these parameters, use the * suffix syntax.

# circos.conf
<<include etc/image.conf>>
file*   = myfile.png
radius* = 1000p

24-bit images

PNG files are created in 24-bit mode. The 24bit flag previously found in the <image> block has been deprecated.

image size

Output image directory and filename are defined in the dir and file parameters of the <image> block. The produced image is always square, and its size set by the radius parameter (this is the size of the inscribed circle). If radius=1500p, then the image will be 3,000 x 3,000 pixels in size.

image background

Image background is controlled using the background parameter, which may be set to transparent or an image location. For more details, see the Image Transparency Tutorial

angle offset

You can adjust the angle offset of the circular layout using the angle_offset parameter. The value of this parameter determines where the start of the first ideogram appears. By default, the value is set to angle_offset=-90 to make the first ideogram start at the top of the image.

This is the prefered way to rotate the image, rather than in post-production, because it will maintain legibility of all all text labels, which will be oriented right-side-up. If you rotate the image yourself, some labels may be upside-down.

scale orientation

You can orient all the ideogram directions to point counterclockwise by setting angle_orientation=counterclockwise. By default, everything is oriented clockwise.

PNG dpi image resolution

There's often confusion about what "dpi" means—I hope to clear this up here.

The default settings in Circos generate PNG images at 3,000 x 3,000 pixels, which provides you with enough pixels for most journal size/resolution requirements.

The PNG output doesn't have any inherent dpi resolution. When loaded into an application like Photoshop, this lack of value will be replaced by the default 72 dpi, which itself is both historic and useless. Let's just call it a default. You might see this 72 dpi value and think that it's too low for your needs, because the journal requirements stipulate "at least 300 dpi".

When journals say 300 dpi, what they really mean is "give us enough pixels so that when we go to print, we have at least 300 pixels for every inch of figure". To know whether you have enough "dots" per "inch", you need to not only know how many "dots" you have (pixels), but also how many "inches" the pixels will map to. Without having this physical size in mind, dpi resolution is meaningless.

Let's look at an example.

Nature's two-column figure is 7.2" wide (183 mm) as described in their Guidelines to Authors. To achieve 300 dpi resolution (at least 300 pixels per inch of figure), you'll need a figure that is

300 * 7.2 = 2,160 pixels

in each dimension. The default Circos 3,000 x 3,000 pixel output is sufficient (it is 417 dpi if the output is 7.2").

Using the radius parameter in the <image> block, you control the output pixel size of the image. Match this to the size of the physical image in the journal. It usually doesn't hurt to have an image that exceeds the minimum resolution.

Limitations in PNG output

If you susspect there may be a problem with drawing images, please run


and look at the output gddiag.png. It should look like the second image in this tutorial.

Circos uses perl's GD module to draw its graphics, which in turn depends on the gd library for its core implementation.

anti-aliasing bug in libgd

If you find artefacts in your image (e.g. squares or elements in unexpected places), it is likely that your libgd is one of the versions which has an anti-aliasing bug. Please turn anti-aliasing off by toggling the anti_aliasing parameter in etc/housekeeping.conf.

You can overwrite this parameter in circos.conf, right after importing etc/housekeeping,

# in circos.conf
<<include etc/housekeeping.conf>>
anti_aliasing* = no

or change it permanently in etc/housekeeping.conf,

# etc/housekeeping.conf
anti_aliasing = no

other anti-aliasing limitations

As of libgd 2.0.35, antialiasing for lines is not supported when the line is drawn at a thickness >1 or with a color that has an alpha channel. The consequence is that you cannot have thick antialiased lines, or partially transparent antialiased lines.

Until these features are implemented in gd, the solution is to create an image magnified by a factor of x (e.g. 2x) and use a command-line image manipulation tool like ImageMagick's convert to shrink the image back to its desired size. The process of resizing the image will subsample the pixels and give the effect of antialiasing.