How to Convert Images Using Linux

This guide will show you how to manipulate images using the Linux command line.

You will find out how to resize an image both in terms of file size and in scale. You will also learn how to convert between multiple file types such as from JPG to PNG or GIF to TIF.

The Convert Command

The convert command is used to convert an image. The format is as follows:

convert [input options] input file [output options] output file.

How to Resize an Image

If you are going to include an image on a webpage and you want it to be a particular size then you could use some CSS to resize the image.

It is actually better though to upload the image as the correct size in the first place and insert it into the page.

This is of course just one example why you might want to resize an image.

To resize an image use the following command

convert imagename.jpg -resize dimensions newimagename.jpg

For example, to convert an image to be 800x600 you would use the following command:

convert imagename.jpg -resize 800x600 newimagename.jpg

If by converting to the specified dimensions the aspect ratio will be messed up the image will be resized to the closest ratio.

To force the conversion to be the exact size, use the following command:

convert imagename.jpg -resize 800x600! newimagename.jpg

You don't have to specify the height and the width as part of the resize command.

For example, if you want the width to be 800 and you don't care about the height you can use the following command:

convert imagename.jpg -resize 800 newimagename.jpg

To resize an image to be a specified height use the following command:

convert imagename,jpg -resize x600 newimagename.jpg

How to Convert From One Image Format to Another

If you have a JPG file and you wish to convert it to a PNG then you would use the following command:

convert image.jpg image.png

You can combine many different file formats. For example

convert image.png image.gif

convert image.jpg image.bmp

convert image.gif image.tif

How to Adjust the File Size for an Image

There are a number of ways to change the physical file size of an image.

Change the aspect ratio (make it smaller)
Change the file format
Change the compression quality

Reducing the size of the image will make the file size smaller. In addition, using a file format that includes compression such as JPG will enable you to reduce the physical file size.

Finally adjusting the quality will make the physical file size smaller.

The previous 2 sections showed you how to adjust the size and file type. To compress the image try the following command:

convert imagename.jpg -quality 90 newimage.jpg

The quality is specified as a percentage. The lower the percentage the smaller the output file but obviously the final output quality is not as good.

How to Rotate Images

If you have taken a photo in portrait but you want it to be a landscape image you can rotate the image using the following command:

convert imagename.jpg -rotate 90 newimage.jpg

You can specify any angle for rotation.

For example, try this out:

convert imagename.jpg -rotate 45 newimage.jpg

Convert Command Line Options

There are dozens of command line options that can be used with the convert command as shown here:

Options are processed in command line order. Any option you specify on the command line remains in effect for the set of images that follows, until the set is terminated by the appearance of any option or -noop. Some options only affect the decoding of images and others only the encoding. The latter can appear after the final group of input images.

For a more detailed description of each option, see ImageMagick.

-adjoin	join images into a single multi-image file
-affine	drawing transform matrix
-antialias	remove pixel aliasing
-append	append a set of images
-average	average a set of images
-background	the background color
-blur x	blur the image with a gaussian operator
-border x	surround the image with a border of color
-bordercolor	the border color
-box	set the color of the annotation bounding box
-cache	megabytes of memory available to the pixel cache
-channel	the type of channel
-charcoal	simulate a charcoal drawing
-chop x{+-}{+-}{%}	remove pixels from the interior of an image
-clip	apply the clipping path, if one is present
-coalesce	merge a sequence of images
-colorize	colorize the image with the pen color
-colors	preferred number of colors in the image
-colorspace	the type of colorspace
-comment	annotate an image with a comment
-compose	the type of image composition
-compress	the type of image compression
-contrast	enhance or reduce the image contrast
-crop x{+-}{+-}{%}	preferred size and location of the cropped image
-cycle	displace image colormap by amount
-debug	enable debug printout
-deconstruct	break down an image sequence into constituent parts
-delay <1/100ths of a second>	display the next image after pausing
-density x	vertical and horizontal resolution in pixels of the image
-depth	depth of the image
-despeckle	reduce the speckles within an image
-display	specifies the X server to contact
-dispose	GIF disposal method
-dither	apply Floyd/Steinberg error diffusion to the image
-draw	annotate an image with one or more graphic primitives
-edge	detect edges within an image
-emboss	emboss an image
-encoding	specify the font encoding
-endian	specify endianness (MSB or LSB) of output image
-enhance	apply a digital filter to enhance a noisy image
-equalize	perform histogram equalization to the image
-fill	color to use when filling a graphic primitive
-filter	use this type of filter when resizing an image
-flatten	flatten a sequence of images
-flip	create a "mirror image"
-flop	create a "mirror image"
-font	use this font when annotating the image with text
-frame x++	surround the image with an ornamental border
-fuzz {%}	colors within this distance are considered equal
-gamma	level of gamma correction
-gaussian x	blur the image with a gaussian operator
-geometry x{+-}{+-}{%}{@} {!}{<}{>}	preferred size and location of the Image window.
-gravity	direction primitive gravitates to when annotating the image.
-help	print usage instructions
-implode	implode image pixels about the center
-intent	use this type of rendering intent when managing the image color
-interlace	the type of interlacing scheme
-label	assign a label to an image
-level	adjust the level of image contrast
-list	the type of list
-loop	add Netscape loop extension to your GIF animation
-map	choose a particular set of colors from this image
-mask	Specify a clipping mask
-matte	store matte channel if the image has one
-median	apply a median filter to the image
-modulate	vary the brightness, saturation, and hue of an image
-monochrome	transform the image to black and white
-morph	morphs an image sequence
-mosaic	create a mosaic from an image sequence
-negate	replace every pixel with its complementary color
-noise	add or reduce noise in an image
-noop	NOOP (no option)
-normalize	transform image to span the full range of color values
-opaque	change this color to the pen color within the image
-page x{+-}{+-}{%}{!}{<}{>}	size and location of an image canvas
-paint	simulate an oil painting
-pen	specify the pen color for drawing operations
-ping	efficiently determine image characteristics
-pointsize	pointsize of the Postscript, OPTION1, or TrueType font
-preview	image preview type
-process	process a sequence of images
-profile	add ICM, IPTC, or generic profile to image
-quality	JPEG/MIFF/PNG compression level
-raise x	lighten or darken image edges
-region x{+-}{+-}	apply options to a portion of the image
-resize x{%}{@}{!}{<}{>}	resize an image
-roll {+-}{+-}	roll an image vertically or horizontally
-rotate {<}{>}	apply Paeth image rotation to the image
-sample	scale image with pixel sampling
-sampling_factor x	sampling factors used by JPEG or MPEG-2 encoder and YUV decoder/encoder.
-scale	scale the image.
-scene	set scene number
-seed	pseudo-random number generator seed value
-segment x	segment an image
-shade x	shade the image using a distant light source
-sharpen x	sharpen the image
-shave x	shave pixels from the image edges
-shear x	shear the image along the X or Y axis
-size x{+offset}	width and height of the image
-solarize	negate all pixels above the threshold level
-spread	displace image pixels by a random amount
-stroke	color to use when stroking a graphic primitive
-strokewidth	set the stroke width
-swirl	swirl image pixels about the center
-texture	name of texture to tile onto the image background
-threshold	threshold the image
-tile	tile image when filling a graphic primitive
-transform	transform the image
-transparent	make this color transparent within the image
-treedepth	tree depth for the color reduction algorithm
-trim	trim an image
-type	the image type
-units	the type of image resolution
-unsharp x	sharpen the image with an unsharp mask operator
-use_pixmap	use the pixmap
-verbose	print detailed information about the image
-view	FlashPix viewing parameters
-wave x	alter an image along a sine wave
-write	write an image sequence [convert, composite]