resources/images: Allow to set background fill colour

Closes #6298

1 files changed, 36 insertions, 9 deletions

diff --git a/docs/content/en/content-management/image-processing/index.md b/docs/content/en/content-management/image-processing/index.md
index b83a6c103..f03c5bee6 100644
--- a/docs/content/en/content-management/image-processing/index.md
+++ b/docs/content/en/content-management/image-processing/index.md
@@ -2,7 +2,6 @@
 title: "Image Processing"
 description: "Image Page resources can be resized and cropped."
 date: 2018-01-24T13:10:00-05:00
-lastmod: 2018-01-26T15:59:07-05:00
 linktitle: "Image Processing"
 categories: ["content management"]
 keywords: [bundle,content,resources,images]
@@ -72,31 +71,42 @@ Image operations in Hugo currently **do not preserve EXIF data** as this is not
 
 In addition to the dimensions (e.g. `600x400`), Hugo supports a set of additional image options.
 
+### Background Color
 
-JPEG Quality
-: Only relevant for JPEG images, values 1 to 100 inclusive, higher is better. Default is 75.
+The background color to fill into the transparency layer. This is mostly useful when converting to a format that does not support transparency, e.g. `JPEG`.
+
+You can set the background color to use with a 3 or 6 digit hex code starting with `#`.
+
+```go
+{{ $image.Resize "600x jpg #b31280" }}
+```
+
+For color codes, see https://www.google.com/search?q=color+picker
+
+### JPEG Quality
+Only relevant for JPEG images, values 1 to 100 inclusive, higher is better. Default is 75.
 
 ```go
 {{ $image.Resize "600x q50" }}
 ```
 
-Rotate
-: Rotates an image by the given angle counter-clockwise. The rotation will be performed first to get the dimensions correct. The main use of this is to be able to manually correct for [EXIF orientation](https://github.com/golang/go/issues/4341) of JPEG images.
+### Rotate
+Rotates an image by the given angle counter-clockwise. The rotation will be performed first to get the dimensions correct. The main use of this is to be able to manually correct for [EXIF orientation](https://github.com/golang/go/issues/4341) of JPEG images.
 
 ```go
 {{ $image.Resize "600x r90" }}
 ```
 
-Anchor
-: Only relevant for the `Fill` method. This is useful for thumbnail generation where the main motive is located in, say, the left corner. 
+###  Anchor
+Only relevant for the `Fill` method. This is useful for thumbnail generation where the main motive is located in, say, the left corner. 
 Valid are `Center`, `TopLeft`, `Top`, `TopRight`, `Left`, `Right`, `BottomLeft`, `Bottom`, `BottomRight`.
 
 ```go
 {{ $image.Fill "300x200 BottomLeft" }}
 ```
 
-Resample Filter
-: Filter used in resizing. Default is `Box`, a simple and fast resampling filter appropriate for downscaling. 
+### Resample Filter
+Filter used in resizing. Default is `Box`, a simple and fast resampling filter appropriate for downscaling. 
 
 Examples are: `Box`, `NearestNeighbor`, `Linear`, `Gaussian`.
 
@@ -106,6 +116,16 @@ See https://github.com/disintegration/imaging for more. If you want to trade qua
 {{ $image.Resize "600x400 Gaussian" }}
 ```
 
+### Target Format
+
+By default the images is encoded in the source format, but you can set the target format as an option.
+
+Valid values are `jpg`, `png`, `tif`, `bmp`, and `gif`.
+
+```go
+{{ $image.Resize "600x jpg" }}
+```
+
 ## Image Processing Examples
 
 _The photo of the sunset used in the examples below is Copyright [Bjørn Erik Pedersen](https://commons.wikimedia.org/wiki/User:Bep) (Creative Commons Attribution-Share Alike 4.0 International license)_
@@ -160,6 +180,13 @@ quality = 75
 # Valid values are Smart, Center, TopLeft, Top, TopRight, Left, Right, BottomLeft, Bottom, BottomRight
 anchor = "smart"
 
+# Default background color. 
+# Hugo will preserve transparency for target formats that supports it,
+# but will fall back to this color for JPEG.
+# Expects a standard HEX color string with 3 or 6 digits.
+# See https://www.google.com/search?q=color+picker
+bgColor = "#ffffff"
+
 ```
 
 All of the above settings can also be set per image procecssing.