8.54. Umbral

8.54. Umbral
	8. El menú “Colores”

The Threshold filter transforms the current layer or the selection into a black and white image, where white pixels represent the pixels of the image where the value for the chosen Channel is in the threshold range, and black pixels represent pixels with a value outside the threshold range.

Se puede usar para realzar una imagen blanca y negra (por ejemplo, un texto escaneado) o para crear máscaras de selección.

	Nota
	As this filter creates a black and white image, the anti-aliasing of the original image disappears. If this poses a problem, use the Levels tool instead.

8.54.1. Activating the Filter

There are different possibilities to activate this:

This filter is found in the main menu under Colors → Threshold…,
or by clicking the icon in the Toolbox if Threshold has been enabled as a tool (which is disabled by default). For this, please refer to Sección 6.13, “Caja de herramientas”.

8.54.2. Opciones

Figura 16.214. Threshold filter dialog

Ajustes predefinidos: “Presets” are a common feature for several Colors commands. You can find its description in Sección 8.1.1, “Características comunes de los colores”.
Canal: This allows you to select which channel(s) will be used to apply the Threshold. The default is Value, which will use all color channels, but you can also choose to use a single channel: Red, Green, Blue and Alpha (only available when the layer has an alpha channel), Luminance, or RGB.
Linear Histogram, Logarithmic Histogram: These two buttons to the right of the Channel selection let you choose the type of histogram shown.
Rango del umbral: The Threshold tool provides a visual graph, a histogram, of the intensity value of the active layer or selection. You can set the threshold range either using the minimum and maximum input boxes, or by clicking and dragging on the graph. It allows you to select a part of the image with some intensity from a background with another intensity. Pixels inside the range are white, and the others are black. Adjust the range to get the selection you want in white on black background. The Auto button can be used to automatically adjust the threshold.
Blending Options, Preview, Merge filter and Split view: These are common features described in Sección 8.1.1, “Características comunes de los colores”.

8.54.3. Usar el umbral y la máscara rápida para crear un máscara de selección

Sometimes an element you want to extract from an image can stand out well against the background. In this case, you can use the Threshold tool to select this element as a whole, and then use a Quick mask.

First start decomposing your image into its RGB and HSV components by using the Decompose filter. A new grayscale image is created and the components are displayed as layers in the Layers Dialog. These layers come with a thumbnail but it is too small for an easy study. You can, of course, increase the size of this preview with the dialog menu (the small triangular button), but playing with the “eyes” is more simple to display the wanted layer in the decompose image. Select the layer that isolates the element the best.

Figura 16.215. The original image, the decompose image and its Layers dialog

Active la herramienta umbral de la imagen descompuesta. Moviendo el cursor negro, ajuste el umbral para aislar mejor el elemento a extraer. No será perfecto: se realzará el resultado con la máscara de selección que se está creando.

	Aviso
	Asegúrese de seleccionar la capa correcta antes de activar la herramienta umbral: cuando se abra, no podrá cambiar a otra capa.

Figura 16.216. La capa seleccionada después de ajustar el umbral

Se obtiene el mejor contorno para la flor. Hay muchos objetos rojos que se deben quitar.

Asegúrese de que la imagen mostrada de la capa seleccionada es la activa y cópiela en el portapapeles con Ctrl+C.
Now, make the original image active. Click the Quick Mask button at the bottom-left corner of the image window: the image gets covered with a red (default) translucent mask. This red color does not suit well to our image with much red: go to the Channels dialog, activate the “Quick mask” channel and change this color with the Edit Channel Attributes. Come back to the original image. Press Ctrl+V to paste the previously copied layer.

Figura 16.217. La máscara
Voilà. Your selection mask is ready: you can improve the selection as usual. When the selection is ready, disable the Quick mask by clicking its button again: you will see the marching ants around the selection.

Figura 16.218. El resultado

We used the Zoom to work at a pixel level, Free Select to remove large unwanted areas, the pencil (to get hard limits), black paint to remove selected areas, white paint to add selected areas, especially for stem.