4. Transform Tools
| Hoofdstuk 14. Tools |

## 4. Transform Tools

Afbeelding 14.111. An overview of the transform tools

### 4.1. Common Features

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

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

Transform

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

  | 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.

Direction

  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, “Rotate”.

  With GIMP-2.10.10, you can link these two options in Rotate, Scale, Perspective, Unified transform and Handle transform tools. This allows moving handles without affecting the transformation, letting you manually readjust their position.

Interpolation

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

  None

    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”.

  Linear

    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”.

  Cubic

    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 4.8, “Sharpen (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:

  Adjust

    Afbeelding 14.112. Original image for examples

    Original image

    Rotation applied with “Adjust”

    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 Image → Fit Canvas to Layers command.

  Clip

    Afbeelding 14.113. Example for Clip

    Clip

    With Clip: all what exceeds image limits is deleted.

  Crop to result

    Afbeelding 14.114. Example for Crop to result

    Rotation 45° with 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.

  Crop with aspect

    Afbeelding 14.115. Example for Crop with aspect

    Original image

    Rotation -22°

    The rotated image

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

Show image preview

  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.

Guides

  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.

Afbeelding 14.116. Rotating paths

Paths dialog

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.

3.17. Dodge/Burn |  | 4.2. Align

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