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table of contents

pamhomography(1) | General Commands Manual | pamhomography(1) |

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# NAME¶

pamhomography - stretch/shrink a quadrilateral region of an image
to

another quadrilateral shape

# SYNOPSIS¶

**pamhomography**

[**-from**=*coords*]

[**-to**=*coords*]

[**-mapfile**=*map_file*]

[**-view**=*coords*]

[**-fill**=*color*]

[**-verbose**

[*pam_file*]

You can abbreviate any option to its shortest unique prefix. You can use two hyphens instead of one to delimit an option. You can separate an option from its value with whitespace instead of =.

# DESCRIPTION¶

This program is part of Netpbm .

**pamhomography** stretches and shrinks an arbitrary
quadrilateral portion of an input image you specify (not necessarily
rectangular), into a new quadrilateral shape you specify, producing a new
image.

You can do any
affine
image transformation : translation, reflection, scaling, rotation, and
shearing/skewing. However, **pamhomography** additionally can do
*bilinear* transforms, which means it can warp any quadrilateral to any
other quadrilateral, even when this mapping cannot be described using a
single set of linear equations. This can be useful, for example, for
creating perspective views of rectangular images or for reverse-mapping a
perspective view back to a rectangular projection.

# OPTIONS¶

In addition to the options common to all programs based on
libnetpbm (most notably **-quiet**, see
Common Options ),
**pamhomography** recognizes the following command line options:

**-from**=*coords*-
This defines the source quadrilateral.

*coords*is a list of four

integer-valued (*x*,*y*) coordinates. If you do not specify the

source with either**-from**or**-mapfile**, the source quadrilateral

is the entire input image. **-to**=*coords*-
This defines the target quadrilateral.

*coords*is a list of four integer-valued (*x*,*y*) coordinates. If you do not specify the target with either**-to**or**-mapfile**, the target quadrilateral is the same as the entire input image. **-mapfile**=*map_file*-
This names a text file that describes the mapping from the source to the target quadrilateral. The file

*map_file*must contain either eight integer-valued (*x*,*y*) coordinates, being the four source coordinates followed by the corresponding four target coordinates, or only four (*x*,*y*) coordinates, being only the four target coordinates. In the latter case, the source quadrilateral is taken to be the four corners of the input image in clockwise order, starting from the upper left.Anything you specify with

**-to**or**-from**overrides what is in

this file. **-view**=*coords*-
This defines the target view.

*coords*is a list of two integer-valued (*x*,*y*) coordinates: the upper left and lower right boundaries, respectively, of the pixels that will be visible in the output image. If**-view**is not specified, the target view will fit precisely the target quadrilateral. **-fill**=*color*-
This is the color with which the program fills all pixels that lie outside of the target quadrilateral. Specify the color as described for the

argument of the pnm_parsecolor() library routine .The default is black, and for images with a transparency plane, transparent.

**-verbose**- This makes the program issue some informational messages about what it is doing.

Cooordinates should normally be specified in clockwise order. The syntax is fairly flexible: all characters other than the plus sign, minus sign, and digits are treated as separators. Although coordinates need to be integers, they may lie outside the image's boundary.

# PARAMETERS¶

**pamhomography**'s only parameter, *pam_file*, is the
name of the

file containing the input image. If you don't specify *pam_file*, the

image comes from Standard Input.

# NOTES¶

The output image uses the same Netpbm format as the input image.

Simple transformations are best handled by other Netpbm programs,
such as those listed in the 'SEE ALSO'
section below. Use **pamhomography** for more sophisticated
transformations such as perspective adjustments, rotations around an
arbitrary point in the image, extraction of non-rectangular quadrilaterals,
shearings by coordinates rather than by angle, and, in general, all
transformations that are most easily expressed as mapping four points in one
image to four points in another image.

# EXAMPLES¶

The following examples use the
park_row.ppm test image, which is a

photograph of New York City's Park Row Building , scaled to
441×640, converted to a PPM file, and redistributed under the
terms of the

GFDL .

The first example showcases the real power of bilinear
transformations. Assuming *park_row_rect.map* has the following
contents:

(0, 0) (440, 0) (440, 639) (0, 639)

then

projects the building's facade from a perspective view to a
rectilinear front-on view. Remember that **pamhomography** ignores the
parentheses and commas used in *park_row_rect.map*; they merely make
the file more human-readable. We equivalently could have written

or any of myriad other variations.

**pamhomography** can warp the image to a trapezoid to make it
look like it's leaning backwards in 3-D:

As a very simple example,

flips the image left-to-right. Note that in this case the target quadrilateral's coordinates are listed in counterclockwise order because that represents the correspondence between points (0, 0) ↔ (440, 0) and (0, 639) ↔ (639, 0).

Scaling is also straightforward. The following command scales down the image from 441×640 to 341×540:

Let's add 100 pixels of tan border to the above. We use
**-view** and **-fill** to accomplish that task:

We can add a border without having to scale the image:

The **-view** option can also be used to extract a rectangle
out of an image, discarding the rest of the image:

Specifying the same set of coordinates to **-from** and
**-to** has the same effect but also allows you to extract
non-rectangular quadrilaterals from an image:

Rotation is doable but takes some effort. The challenge is that you need to compute the rotated coordinates yourself. The matrix expression to rotate points \((x_1, y_1)\) \((x_2, y_2)\), \((x_3, y_3)\), and \((x_4, y_4)\) clockwise by \(\theta\) degrees around point \((c_x, c_y)\) is

\[ \begin{bmatrix} 1 & 0 & c_x \\ 0 & 1 & c_y \\ 0 & 0 & 1 \end{bmatrix} \begin{bmatrix} \cos \theta & -\sin \theta & 0 \\ \sin \theta & \cos \theta & 0 \\ 0 & 0 & 1 \end{bmatrix} \begin{bmatrix} 1 & 0 & -c_x \\ 0 & 1 & -c_y \\ 0 & 0 & 1 \end{bmatrix} \begin{bmatrix} x_1 & x_2 & x_3 & x_4 \\ y_1 & y_2 & y_3 & y_4 \\ 1 & 1 & 1 & 1 \end{bmatrix} \quad. \]

For example, to rotate *park_row.ppm* 30° clockwise
around (220, 320) you would compute

\[ \begin{bmatrix} 1 & 0 & 220 \\ 0 & 1 & 320 \\ 0 & 0 & 1 \end{bmatrix} \begin{bmatrix} \cos 30^{\circ} & -\sin 30^{\circ} & 0 \\ \sin 30^{\circ} & \cos 30^{\circ} & 0 \\ 0 & 0 & 1 \end{bmatrix} \begin{bmatrix} 1 & 0 & -220 \\ 0 & 1 & -320 \\ 0 & 0 & 1 \end{bmatrix} \begin{bmatrix} 0 & 440 & 440 & 0 \\ 0 & 0 & 639 & 639 \\ 1 & 1 & 1 & 1 \end{bmatrix} = \begin{bmatrix} 189.4744 & 570.5256 & 251.0256 & -130.0256 \\ -67.1281 & 152.8719 & 706.2621 & 486.2621 \\ 1.0000 & 1.0000 & 1.0000 & 1.0000 \end{bmatrix} \quad, \]

round these coordinates to integers, transpose the matrix, and
produce the following map file, *park_row_rot30.map*:

571 153

251 706

-130 486

(These are the 'to' coordinates; we use the default, full-image 'from' coordinates.) The mapping then works as in all of the preceding examples:

# SEE ALSO¶

# HISTORY¶

**pamhomography** was new in Netpbm 10.94 (March 2021).

# AUTHOR¶

Copyright © 2020 Scott Pakin,
*scott+pbm@pakin.org*

# Table of Contents¶

# DOCUMENT SOURCE¶

This manual page was generated by the Netpbm tool 'makeman' from HTML source. The master documentation is at

04 December 2022 | netpbm documentation |