4. Transform Tools

Figuur 14.110. An overview of the transform tools

An overview of the transform tools

4.1. Algemene Eigenschappen

Inside the Transformation tool dialog, you will find eight tools to modify the presentation of the image or the presentation of an element of the image, selection, layer or path. Each transform tool has an Option dialog and an Information dialog to set parameters.

4.1.1. Tool Options

Figuur 14.111. Common options of transform tools

Common options of transform tools

Some options are shared by several transform tools. We will describe them here. More specific options will be described with their tool.

Transformeren

GIMP offers you three buttons which let you select which image element the transform tool will work on.

[Opmerking] Opmerking

Remember that the Transform option persists when you quit the tool.

  • When you activate the first button the tool works on the active layer. If no selection exists in this layer, the whole layer will be transformed.

  • When you activate the second button the tool works on the selection contour only (the whole layer contour if no selection).

  • When you activate the third button, the tool works on the path only.

Richting

This option sets which way or direction a layer is transformed:

The Normal (Forward) mode will transform the image or layer as one might expect. You just use the handles to perform the transformation you want. If you use a grid (see below), the image or layer is transformed according to the shape and position you put the grid into.

Corrective (Backward) inverts the direction. Primarily used with the Rotation tool to repair digital images that have some geometric errors (a horizon not horizontal, a wall not vertical...). See Paragraaf 4.5, “Draaien”.

Interpolatie

This drop-down list lets you choose the method and thus the quality of the transformation:

Geen

The color of each pixel is copied from its closest neighboring pixel in the original image. This often results in aliasing (the stair-step effect) and a coarse image, but it is the fastest method. Sometimes this method is called Nearest Neighbor.

Lineair

The color of each pixel is computed as the average color of the four closest pixels in the original image. This gives a satisfactory result for most images and is a good compromise between speed and quality. Sometimes this method is called Bilinear.

Kubisch

The color of each pixel is computed as the average color of the eight closest pixels in the original image. This usually gives a good result, but it naturally takes more time. Sometimes this method is called Bicubic.

LoHalo, NoHalo

Halo is an artifact that can be created by interpolation. It remembers the halo you can get when using Paragraaf 4.9, “Unsharp Mask”. Here is a note from Nicolas Robidoux, the creator of the new quality samplers for GEGL and GIMP:


                  "If haloing is not an issue with your content and use case, 
                  which of the two should you try first? 
                  (Clearly, if you want to minimize haloing, NoHalo is the 
                  ticket.)

                  If you are reducing an image, LoHalo is generally better.

                  If your transformation is not an all around reduction, for 
                  example if you enlarge, rotate or apply a perspective 
                  transformation that keeps portions of the image at the same or 
                  higher resolution, I generally prefer NoHalo. This preference, 
                  however, changes depending on the image content. If, for 
                  example, the image contains text or text-like objects or has 
                  significant areas with only a handful of different colours, 
                  like old school pixel art, I’d switch to LoHalo. Likewise if  
                  the image is quite noisy or marred by compression artifacts 
                  (as are most JPEGs found on the web). Conversely, if the 
                  image is noise free, very slightly blurry (meaning that when 
                  pixel peeking, the lines and interfaces are smeared over two 
                  or more pixels), and there are delicate skin tones to be    
                  preserved, I’d try NoHalo first. Actually, if I find that 
                  colours have not been preserved nicely after transforming an 
                  image with LoHalo, I’d immediately switch to NoHalo, even if  
                  reducing.

                  In any case, these recommendations should not be taken as 
                  gospel. I still have much to learn and figure out. For 
                  example, how best to deal with transparency and different 
                  colour spaces is something I’m likely to be thinking about 
                  for a while."
                

You can set the default interpolation method in the Tools Options Preferences dialog.

Clipping

After transformation, the image can be bigger. This option will clip the transformed image to the original image size.

You can choose between several ways to clip:

Aanpassen

Figuur 14.112. Original image for examples

Original image for examples

Original image

Original image for examples

Rotation applied with Adjust

Original image for examples

Rotation applied with Adjust and canvas enlarged to layer size


With Adjust: the layer is enlarged to contain all the rotated layer. The new layer border is visible; the whole layer becomes visible by using the ImageFit Canvas to Layers command.

Knippen

Figuur 14.113. Example for Clip

Example for Clip

Knippen


With Clip: all what exceeds image limits is deleted.

Resultaat bijsnijden

Figuur 14.114. Example for Crop to result

Example for Crop to result

Rotation 45° with Crop to result

Example for Crop to result

The crop limit is marked with red. No transparent area is included.


If this option is selected, the image is cropped so that the transparent area, created by the transform operation in corners, will not be included in the resulting image.

Bijsnijden volgens verhouding

Figuur 14.115. Example for Crop with aspect

Example for Crop with aspect

Original image

Example for Crop with aspect

Rotation -22°

Example for Crop with aspect

The rotated image


This option works like the one described before, but makes sure, that the aspect ratio is maintained.

Voorbeeld

If this is marked, which is the default setting, the transformed image will be visible on top of the original image or layer. There will also be a slider with which you may select the preview opacity.

Hulplijnen

This is a drop down list where you select the type of guide lines which suits your transforming. All the guides uses a frame to mark the image's outline in addition to the lines used by the different selections.

No guides

As the name tells you, there are no guides used.

Center lines

Uses one vertical line and one horizontal line crossing each other in the center of the image or layer.

Rule of thirds

Divides the transforming area in nine equal parts by adding two horizontal lines and two vertical lines equally spaced. According to this rule the most interesting parts of the image should be placed at the intersection points.

Rule of fifths

Just as the Rules of thirds but divides the area in five by five parts.

Golden sections

Also called The Golden Ratio. This divides the transforming area in nine parts using a mathematical formula proportioning the parts to each others and to the area to be transformed.

Diagonal lines

Divide the transforming area using diagonally lines.

Number of lines

Puts a rectangular grid with equal numbers of vertically and horizontally lines. The number of lines is set in the slider popping up when this guide is selected.

Line spacing

Puts a rectangular grid on the transforming area using the spacing between the lines set in the slider.

4.1.2. Transforming Paths

If you for some reason want to transform paths, it is possible to do this using the transform tools.

Figuur 14.116. Rotating paths

Rotating paths

Paths dialog

Rotating paths

Option modus: Path


When the path is drawn go to the path dialog and click on the first field before the path outline in the dialog window to get the eye icon visible. Then choose the transformation tool and in the upper part of the option dialog click on the path icon to tell the tool to act on the path.

Do the transformation the usual way and confirm it when finished. It could be a good idea to set the Guides to No guides to get the path more recognizable.

When the transformation is finished, choose the path tool and click on the changed path to activate it again for further working on it.