crop--"Image Cropper"

Current Version

1.1 (16 January 1997)




crop <input file> {<identifier> {<number>}}


input fileone PPM image
identifierany string legal for filenames (default: swatch)
numbera positive integer (default: 1)

Starting the Program

The program starts by opening the input image in a new window. The file name and image size are displayed in the bottom left corner, while the current image coordinates are displayed in the lower right. The upper left holds three menus.

Extracting a Sub-Image

The process is started by clicking the left mouse button in the upper left corner of the portion of the image you wish to crop. A small circle will appear at the coordinates you have chosen. If you have chosen fixed-size rectangles from the Size Menu, a rectangle will appear. If not, you can specify the other corner by clicking the left mouse button again, which will cause a rectangle to appear. The circle(s) can always be moved after placement by dragging them with the middle mouse button.

The Coordinate System

Most image applications number the columns of an MxN image as 0 through M-1 and the rows as 0 through N-1. This utility differs from that scheme in two major ways. First, the columns are numbered 1 through M and the rows 1 through N. However, if this was the only place you could put your mouse, it would be impossible to capture any pixels along the border of the image. For this reason a 1-pixel wide padding has been placed around the image, meaning that valid mouse coordinates are 0 through M+1 and 0 through N+1.

The File Menu

Once a rectangle has been drawn, it can be saved by choosing Save from the menu. The program will save the file using the filename of the format identifier + number + ".ppm". For example, if no options were specified on the command line, the sub-image would be saved as swatch1.ppm. After a save, the number is incremented so that multiple saves can be performed easily. The identifier could be an entire path, allowing the save to be performed to a different directory. The identifier or number can be changed at any point by choosing Options from the menu. New images can be loaded with the Open command, but currently only images in the present working directory ending in ".ppm" can be opened. The fourth option, Quit, allows an exit from the program.

It is important to note that only the pixels that are visible within the rectangle are saved. The pixels underneath the rectangle itself are not. In order to be able to save pixels at the borders of the image, it is possible to place the circles at points whose coordinates are (0,0) or (x+1,y+1). The upper-left corner of the image has coordinates (1,1). The padding mentioned above will not show up in any saved images, because it can never be seen inside the rectangle drawn.

The Edit Menu

The Edit Menu has only one command, Erase, which removes any circles and/or rectangles that have been placed on the screen.

The Size Menu

Rectangles can be of fixed or of variable sizes. The default is fixed rectangles of size 128 by 128. The Fixed Size and Variable Size radio buttons allow you to choose between them. In addition, the Change Fixed Size option allows you to change the width and height of fixed-size rectangles to any value you wish. Changing the type of rectangle automatically erases any circles or rectangles on the screen.


If you try hard enough, you can move a circle placed on the screen so that the ensuing rectangle incorporates part of the border. This will result in a Tcl error if you try to save the sub-image. The problem is that the rectangle is drawn from the center of the circle, but moving a circle involves only the mouse, which does not have to be in the center of the circle in order to begin moving.