carol.gimp.org

The first thing to note is TheGIMP will open the image at a viewable size. It does this without changing the file size, instead it changes the "resolution" of the view. TheGIMP simply zooms the view so that it will fit on your workspace. You can see the actual size in the title bar on the window. The image in this screen shot is actually 1008 pixels wide and 720 pixels long.

The titlebar also tells the mode of the image file. When working on an image with TheGIMP this should always read RGB. If your image says something else (Indexed or Grayscale), you should read the mode changing quickie and change it. You will see the difference in the quality of your changes.

Use the mouse and right click on the image. Follow the menu <Image> Image ->Scale Image.... When ever you click an option from the menu that has "..." behind it, expect a dialog. In this case, you should get the "Scale Image" dialog.

If you have a desired width, put it in the dialog at the top where it says "Width". 300 pixels is a nice width for web pages or email. After entering the number 300 into the Width box, the "Height" displayed will automatically change to the correct number which is 214 pixels for the sample image. It is best to only use one dimension when altering the image size like this, otherwise the image will become distorted (larger in one direction only). You can see the image that this scale dialog produced here.

Perhaps you want your image to look more like a 4 x 6 inch photo on most image rendering web browsers. Simply switch the units to "inches" and put "4" inches in the height box (opting for smaller than 4x6 inches rather than bigger). The transformed image should now look like a 4 x 6 inch photograph on most computer screens.

If you need for the image to be exactly a certain pixel size that is different than the size the dialog will automatically adjust itself to be, for example 200 x 200 pixels, use the dialog to scale the image so that one side is 200 pixels and the other side is larger than 200 pixels. This will leave some area to "crop" away (cropping is demonstrated in another "quickie"). The sample image scales to 200 X 280 pixels.

For demonstation purposes only, see the distortion that happens when you force the image to scale to an unconstrained size (without the cropping step). This frustrating fact about images on computers is shared with real printed photographs as well. It was just as difficult to make printed photographs or the film negatives fit into a pre-specified format. You can do this with computers, but you will distort the image.

The goal here is to reduce the image for a web page, so the first step would be to scale the image to a width of 600 pixels, which is a reasonable pixel size for web viewing. The Info Window for this image reports a weight of 18.7 MB. Now for the task of reducing this weight. Using File ->Save As...

Right click with the mouse on the Image and follow the menu path to <Image> File ->Save As... and type a different name into the text box when the new dialog appears making certain that the file extension (the stuff behind the dot) says jpg or jpeg.

If everything is in order, the "Save as JPEG" dialog should appear. If you see instead the "Export File" you need to change the Mode of the image. Any other sort of error or dialog would mean that you mis-typed the file extension (the ".jpg" at the end of the file name).

The "Save as JPEG" dialog has a slider and some "spinning" numbers and should appear set at a default value of 85 (this is a percentage -- 85 percent of the original quality). Right under the slider is an estimate of the weight of the image with the shown percentage of quality (File size). Using the default value reduces the weight of the image by almost one kB. Underneath the weight information is an option to see a preview of the image in the image window. By default, this is toggled to Off; toggle it On. This will cause your quality changes to show on the image you are saving. Once it is toggled, you can see what the default setting of 85% will have on your image. In this case, it is difficult to see any changes in the image.

Using either the slider or the number spinners or by typing numbers into the little box, you can experiment with the different image quality levels and see what happens to the image and the weight. Very small quality results in a fairly lightweight image however, it almost destroys the original, as shown here when the image quality was set to 3%.

Use your eyes and perhaps other peoples opinion to find a nice balance between what you see and how big the file size is. I opted for a quality of 42%; at this quality you can see the image change some but the savings in weight is still worth it (8.6 kB, less than half of the original weight).

Compare four different quality settings on this jpeg race page. People who are still blessed with a slow internet connection might be able to see the loading speed differences as well.

Once the crop tool has been engaged, a left click with the mouse on the image will do two things. First it will shade the areas of the image which have been marked for cropping and draw lines between at the edges of the marked area. One click will shade the whole image and draw only two lines -- you will need to tell the tool the other point. This action is difficult to describe, best to try/see it for yourself instead of trying to make sense of this description. The second thing that happens is the launching of the crop tool dialog.

The crop tool is a fairly complex tool; it can be used scientifically (entering the numbers of the desired area into the dialog) or using the corners of the border marks to visually move the selected area or a combination of both. The goal of this demonstration is to make a rectangular image into a square, and both methods should be used to get the best image possible for the goal.

Square means the same number of pixels on all sides. 900 pixels is a good number of pixels for this image; type 900 into both the Width and Height of the crop dialog.

With the amount of square "scientifically" selected, it is now time to use visual placement to select which pixels are saved and which are discarded. The area selected so far is square but the image contained within is not so good.

The corners where the crop lines meet are very special. The upper left and lower right corners allow you to change the dimensions of the crop selection. The lower left and upper right corners allow you to move the selection without changing the dimensions of the selected area. The mouse will tell you what the corner will do; dimension changing corners get a double arrow curser and corners that move the crop selection get the move star.

The task at this point in the demonstration is to move the area to a better position for the image. Using only the move corners of the crop area, position the area to display the best area of the image.

The actual cropping is initiated by left clicking on the image within the crop selection area or by touching the button marked "Crop" at the bottom of the dialog.

<Image> View -> Info Window initiates a window with the information.

Much of this information appears in other places; the Info Window simply presents it all in one tidy area. New users of TheGIMP can safely ignore "Print Size" and "Resolution"; this embedded information in each image seems to be ignored by every printer or image display; so you can also.

Perhaps you have a task similar to the task demonstrated in the Crop Quickie where you need to make an image square, only this task demands that you retain as much of the original information as possible. This is so easy to accomplish by simply determining the smaller of the two pixel dimensions and using this number in the Width and Height area of the crop dialog (in this case, 1548 pixels would be the largest square area that can be made out of this image.

The more you play with computer graphics, the more uses you will have for this information.

Change to RGB

It is so simple to change the mode of an image to RGB that it hardly makes a whole section to this tutorial worth it, however, it is so important that the importance overrides the simplicity.

Simply right click with the mouse on the image and follow the menu path <Image> Image ->Mode ->RGB.

Change to Indexed

So much more complex to make an image simpler -- you get a few more choices when reducing an image from millions of colors to only 256. Using the same image, only this time grabbed from Earth Observatory in full RGB and really really big, a demonstration of converting from indexed back to RGB.

Right click on the image and follow the menus <Image> Image ->Mode ->Indexed ... (the three dots at the end of the menu entry tell you to expect a dialog).

Trust the default settings and touch the "Ok" button with the mouse.

Convert from RGB to indexed only as a last step with TheGIMP. If you are saving your image as a GIF, you will need to take this step. If you are not so worried about perfect color information and more about file size, saving as an indexed PNG is the best option.

RGB to Grayscale

The quickest way to change your photographs from color to "black & white" is to follow the menu <Image> Image ->Mode ->Grayscale. If you would like photographs that seem more like those really wonderful old black and white prints you will need to use some tools that are beyond the scope of this new users tutorial.

The ideal use for this mode is to save scanned documents and line drawings with.

Select the Flip Tool from the Toolbox or right click on the image and follow the menus <Image> Tools ->Transform Tools ->Flip.

The default behavior of this tool is to flip images horizontally. Touch your image with this tool and everything in the image that was facing right will now face left.

The tool will flip images vertically as well; the tool needs some instructions and gets these instructions from the "Tool Options" dialog which can be found via two menu paths <File> Dialogs ->Tool Options or <Image> Dialogs ->Tool Options. A word about this dialog; the more you want to do with TheGIMP, the more you will probably need to use this tool option dialog. It will show the options for which ever tool is engaged at the moment.

When using the Flip Tool you should be able to make 3 images different from your original; play with the tool yourself or see the demonstration.

Fixing this particular image needs a clockwise (the top goes 90 degrees to the right) rotation. 180 degree rotation is the same as a vertical flip.

An easily cropped and rotated image ....