Adjust Image Sizes
Since images often need to be sized according to the medium, there are several ways to specify an image size.
In most output formats, the specified width is obeyed unless the image would exceed the content width or height, in which case it scaled to fit while maintaining the original aspect ratio (i.e., responsive scaling).
width and height attributes
The primary way to specify the size of an image is to define the width
and height
attributes on the image macro.
Since these two attributes are so common, they are the second and third (unnamed) positional attributes on the image macros.
image::flower.jpg[Flower,640,480]
That’s equivalent to the long-hand version:
image::flower.jpg[alt=Flower,width=640,height=480]
While the values of width
and height
can be used to scale the image, these attributes are primarily intended to specify the intrinsic size of the image in CSS pixels.
The width
and height
attributes are mapped to attributes of the same name on the <img>
element in the HTML output.
These attributes are important because they provide a hint to the browser to tell it how much space to reserve for the image during layout to minimize page reflows.
pdfwidth attribute
Asciidoctor recognizes the following attributes to size images when converting to PDF using Asciidoctor PDF:
-
pdfwidth
- The preferred width of the image in the PDF when converting using Asciidoctor PDF.
The pdfwidth
attribute accepts the following units:
px |
Output device pixels (assumed to be 96 dpi) |
pt (or none) |
Points (1/72 of an inch) |
pc |
Picas (1/6 of an inch) |
cm |
Centimeters |
mm |
Millimeters |
in |
Inches |
% |
Percentage of the content width (area between margins) |
vw |
Percentage of the page width (edge to edge) |
If pdfwidth
is not provided, Asciidoctor PDF also accepts scaledwidth
, or width
(no units, assumed to be pixels), in that order.
See image scaling in Asciidoctor PDF for more details.
scaledwidth attribute
Asciidoctor recognizes the following attributes to size images when converting to PDF using the DocBook toolchain:
-
scaledwidth
- The preferred width of the image when converting to PDF using the DocBook toolchain. (mutually exclusive with scale) -
scale
- Scales the original image size by this amount when converting to PDF using the DocBook toolchain (mutually exclusive with scaledwidth).
scaledwidth
sizes images much like pdfwidth
, except it does not accept the vw unit.
The value of scaledwidth
when used with DocBook can have the following units:
px |
Output device pixels (assumed to be 72 dpi) |
pt |
Points (1/72 of an inch) |
pc |
Picas (1/6 of an inch) |
cm |
Centimeters |
mm |
Millimeters |
in |
Inches |
em |
Ems (current font size) |
% (or none) |
Percentage of intrinsic image size |
DocBook also accepts the width
attribute if scaledwidth
is not provided.
Image sizing recap
Backend | Absolute size | Relative to original size | Relative to content width | Relative to page width |
---|---|---|---|---|
html |
width=120 |
Not possible |
width=50% |
Not possible |
pdfwidth=100mm |
Not possible |
pdfwidth=80% |
pdfwidth=50vw |
|
docbook |
scaledwidth=100mm |
scale=80 |
scaledwidth=50% |
Not possible |
Here’s an example of how you might bring these attributes together to control the size of an image in various output formats:
image::flower.jpg[Flower,640,480,pdfwidth=50%,scaledwidth=50%]
A practice you might consider is using attributes to set the values for each output:
ifdef::backend-html5[]
:twoinches: width='144'
:full-width: width='100%'
:half-width: width='50%'
:half-size:
:thumbnail: width='60'
endif::[]
ifdef::backend-pdf[]
:twoinches: pdfwidth='2in'
:full-width: pdfwidth='100vw'
:half-width: pdfwidth='50vw'
:half-size: pdfwidth='50%'
:thumbnail: pdfwidth='20mm'
endif::[]
ifdef::backend-docbook5[]
:twoinches: width='50mm'
:full-width: scaledwidth='100%'
:half-width: scaledwidth='50%'
:half-size: width='50%'
:thumbnail: width='20mm'
endif::[]
Then you can specify a half-size image using:
image::image.jpg[{half-size}]
In addition to providing consistency across your document, this technique will help insulate you from future changes. For a more detailed example, see this thread on the discussion list.