Resize

Resize

Screenshot of the whole Resize tool in RawTherapee 5.4-789-ga7b7f20b8.

Resize

Resizing is one of the last things to happen when saving an image - this tool runs after most other tools and transformations. As downscaling an image involves a certain loss of detail, this tool includes a "Post-Resize Sharpening" component which you can use to make the downscaled image crisp.

The effects of the Resize tool will not be shown in the preview. The saved image will of course be resized.

Size limits:

• The downscaling limit is 32x32px.
• The upscaling limit is 16-times the image's size as of RawTherapee version 5.5. In versions prior to and including 5.4, it was 4-times the image's size.

Applies To

You will typically want the width and height to apply to the cropped area, however you can also have it apply to the full image. The crop will be scaled accordingly.

Demonstration of the effects of the resizing methods "nearest neighbor" vs "Lanczos".

Method

Resizing can be performed using various algorithms, each with its own strengths and weaknesses. We narrowed the choice down to the two most significant:

• Nearest

The "nearest neighbor" method of interpolation is meant to be used when you want to upscale an image in order to make some detail larger without introducing smooth blending between pixels. It preserves the pixels as they are. It is usually used when upscaling by squares of 2: 2x, 4x, 8x, 16x, etc.. The result will be very sharp, but also very blocky. This method is not meant to be used for everyday photography.

• Lanczos

Lanczos is the default method. It is meant to be used for everyday photography and in all cases except for the one described above. It results in a smooth yet sharp and high quality image. Use it to resize by any amount.

Specify

You can specify what dimensions to scale to or by:

• Scale

Uniformly scale the image by a factor. Ranges from 0.01 (100 times smaller) to 16 (16 times bigger).

• Width / Height

Set the desired absolute width or height in pixels, regardless of the image orientation. The other dimension is updated according to the aspect ratio.

• Bounding Box

Resize to fit the image within a box of certain width and height in pixels, regardless of the image orientation. The image aspect ratio is kept. E.g. for a bounding box of 1920×1080 px and a landscape image of 3000×2000 px the resulting image will be 1620×1080 px. For the same bounding box and a portrait image of 2000×3000 px the resulting image will be 720×1080 px.

• Long Edge / Short Edge (Since RawTherapee 5.9)

Set the desired size of either the long or short edge of the image in pixels. The other dimension is updated according to the aspect ratio. E.g. setting the long edge to 1500 px for a landscape image of 3000×2000 px gives a resulting image of 1500×1000 px. Setting the short edge to 1500 px for the same image gives a resulting image of 2250×1500 px. Setting the long edge to 1500 px for a portrait image of 2000×3000 px gives a resulting image of 1000×1500 px.

Allow Upscaling

This setting controls whether the resize tool will also upscale your images, or only downscale. If it is disabled and the combination of image dimensions, crop dimensions and resize dimensions is such that the image would need to be upscaled to meet your target, this setting will prevent it from being upscaled - it will remain at its current size.

This setting was introduced in RawTherapee 5.5. If in RawTherapee 5.5 you use a sidecar file from an older version of RawTherapee, this setting will automatically be disabled.

Post-Resize Sharpening

Resizing an image often leads to a loss of sharpness, so it is common practice to sharpen the image again after having resized it. With the Post-Resize Sharpening tool you can save crystal-clear images straight away with no further hassle. Because this tool works on the image after it is resized, you cannot use the preview to see what it will do, though this is not a problem because the procedure for finding the right values is straightforward.

The default values work great, but if you want to change them, here's how to preview what the image will look like:

1. Tweak your image as you usually would and enable the Resize tool, e.g. downscale using the Lanczos method to a 900x900px bounding box.
2. Save the image to a lossless format such as TIFF.
3. Open that saved TIFF in RawTherapee, apply the Neutral processing profile if that wasn't done automatically, and enable the Sharpening tool in the Detail tab.
4. Zoom to 100% (1:1) and tweak the Sharpening tool's parameters until you get a result that satisfies you. These are the values you should use in the Post-Resize Sharpening tool.
5. Go back to the raw image, enable the Post-Resize Sharpening tool and set it up with the values from the previous step.

The Post-Resize Sharpening tool is only available when you use the "Lanczos" resizing method.

As the Post-Resize Sharpening tool works identically to the standard Sharpening tool (except that it takes place right at the end of processing), refer to the Sharpening article to learn more about how the sharpening tools work.