Providing the image resource

There are several things you need to know about providing an image resource in a Genero program.

Supported image formats

Genero supports various image data formats, typically PNG, JPEG and SVG.

True Type Font (TTF) files are also supported: The TTF format is used when image-to-font-glyph mapping is enabled by specifying a mapping file in the FGLIMAGEPATH environment variable.

Table 1. List of supported image file formats
Extension Format name
.bmp Bitmap image file
.gif Graphics Interchange Format
.ico ICO file format (Microsoft™ Windows® Icons)
.jpg JPEG Joint Photographic Experts Group
.png Portable Network Graphics
.svg Scalable Vector Graphics
.tiff Tag Image File Format

Image resolution

Consider using the appropriate image resolution for the target front-end platform. For example, mobile devices have a much higher pixel density (a higher resolution) than desktop monitors. An image which looks nice on a desktop can appear small or as an unscaled image on a mobile device.

Static versus dynamic images

The image resource specification is different for static and dynamic images:
  • For static images (such as button icons), set the image resource in the image attribute (IMAGE, IMAGELEAF, and so on). See Static images.
  • For dynamic images (such as image fields displaying photos from a database), the image resource is specified with the field/variable value, to be rendered in a form field. The form field is typically defined as an IMAGE item, or an IMAGECOLUMN in a table view. For more details, see Runtime images.

Image resource lookup

The image data can be provided in different ways, depending on the image resource specification:

  1. As a Uniform Resource Locator (URL), such as "https://4js.com/files/images/fourjs_logo.jpg".
  2. As a simple image name (for TTF icons), such as "smiley".
  3. As a simple file name, relative or absolute file path, with image extension (such as .png or .jpg).

Using a URL image resource

If the image specification starts with a URL prefix, the front-end will try to download the image from the location specified by the URL.

The network access to the Web server must exist and network bandwidth must be sufficient to rapidly download the images.

Table 3. Supported image resource locations (URLs)
Image resource location (URL) Description
http://location-specification HTTP server
https://location-specification HTTPS server (HTTP over SSL/TLS)

Using a simple image name (centralized TTF icons)

If the image specification is a simple name (without a file extension), and the FGLIMAGEPATH environment variable defines an icon mapping file for the runtime system, the image name is converted to a TrueType (.ttf) font file and font glyph, based on the mapping file entries, and the image form item displays the glyph/icon found in the font definition file. The mapping file and the .ttf font definition file are centralized on the application server.

A line in the image-to-font-glyph mapping file must have the following format:
image-name=font-file:hexa-ordinal[:color-spec]
Where:
  • image-name is the name of the image resource used in form elements.
  • font-file is the .ttf file where the images/glyphs are defined.
  • hexa-ordinal is the number (in hexadecimal) defined by the creator of the .ttf file, to identify the image/glyph in the file.
  • color-spec is an RGB color specification in hexidecimal format #RRGGBB.
For example, if the image mapping file defines the following lines:
smiley=FontAwesome.ttf:f118
red_smiley=FontAwesome.ttf:f118:#8B0000

An image resource (IMAGE attribute, IMAGECOLUMN value, and so on) with the name "smiley" will be mapped to the glyph identified by the code 0xf118 in the FontAwesome.ttf font file, and the image resources using "red_smiley" will use the same glyph, but will get a red color.

Important:

The directory to the font file must be specified in FGLIMAGEPATH, except if the font file is located in the same directory as the mapping file.

A default color can be defined for all TTF icons, by using the defaultTTFColor style attribute, for a container element such as Window, or specifically for a given type for element:
<StyleList>
  <Style name="Window.important">
     <StyleAttribute name="defaultTTFColor" value="red" />
  </Style>
  <Style name="Button.validate">
     <StyleAttribute name="defaultTTFColor" value="#AAFFAA" />
  </Style>
  ...

A default mapping file named "image2font.txt" and the "FontAwesome.ttf" font file are provided in $FGLDIR/lib. If FGLIMAGEPATH is not defined, the runtime system will use these files to make the image to font glyph mapping.

Important:

When providing your own customized font file, it must be a valid TTF file. For example, changing the filename is not sufficient to turn it into a different font: In order to produce a valid TTF file, use font management tools such as FontForge (http://fontforge.github.io/en-US/) or Fontello (http://fontello.com). Furthermore, to target Microsoft Internet Explorer (version 11), you will need to patch the generated TTF file to remove embedding limitations from TrueType fonts, by setting the fsType field in the OS/2 table to zero. This modification can be done with freeware tools like ttembed.

It is possible to mix several plain image file directories with several image-to-font glyph mapping files in FGLIMAGEPATH. The list of mapping files and directories defines the order of precedence, for example:

$ export FGLIMAGEPATH="/var/myapp/myimages:\
/var/myapp/myicons.txt:/var/myapp/fontfiles:\
$FGLDIR/lib/image2font.txt:$FGLDIR/lib"
In the above FGLIMAGEPATH configuration:
  • /var/myapp/myimages: Directory where plain image files can be found
  • /var/myapp/myicons.txt: Custom image-to-font-glyph mapping file (icons)
  • /var/myapp/fontfiles: Font files used by the myicons.txt mapping file
  • $FGLDIR/lib/image2font.txt: Default icon mapping files (using FontAwesome.ttf)
  • $FGLDIR/lib: Location of the default FontAwesome.ttf file

Consider defining your own image mapping file and make FGLIMAGEPATH point to your own files.

When executing the application on a mobile device, you must define the FGLIMAGEPATH environment variable with the mobile.environment.FGLIMAGEPATH entry in FGLPROFILE. Use $FGLAPPDIR and $FGLDIR placeholders to include the current appdir (program file directory) and the FGL runtime system directory, respectively.

See FGLIMAGEPATH for more details about this environment variable.

Important:

On Microsoft Windows platforms, there is a system setting that can block the use of untrusted TTF font files to avoid EOP attacks. For more details, see Microsoft knowledge base for more details.

Using file names or paths

If the image specification is a simple file name, relative or absolute path (without a URL prefix, and with a image file extension), the front-end gets the image file from the runtime system.
DISPLAY "bird.jpg" TO img1

The image file is searched on the platform where the program executes. The runtime system uses the FGLIMAGEPATH environment variable when searching for the images. If FGLIMAGEPATH is not set, the current working directory is searched for the image files.

If FGLIMAGEPATH is defined, the current working directory is not searched. To find image files in the current working directory and in other directories, add "." to the FGLIMAGEPATH path list.

Important:

Always use the image file extension (.png, .jpg): If the image file is specified without a file extension, Genero will try to find the image file with a predefined list of extensions (.png, .jpg, etc). The search depends on the name of the image file, the list of directories defined in FGLIMAGEPATH, and the predefined list of file extensions. This search procedure has been implemented to allow different types of front-ends to pass the preferred image compression formats, and allow you to specify the image name without any file extension in programs and forms. However, it is much more efficient to use images with common formats (such as .png or .jpg), and specify this extension in the filename. The file extension will also be used by the front-end to easily identify the compression format (for example, to define the Content-Type in an HTML entity).

The resource file cache of the front-end

The resource files transmitted by the runtime system to the front-end are automatically cached on the front-end workstation, to be reused by several programs and program instances.

If the content of the resource file on the program server side changes, and the image filename is redisplayed to the form field (by a DISPLAY TO / DISPLAY BY NAME or implicitly with the UNBUFFERED mode) as a new field value, the resource file content will be retransferred to the front-end.

For example, when a form holds an IMAGE field, the following code sequence will produce the results described in the inline comments (starting with an empty front-end file cache): 
DISPLAY "/tmp/bird.png" TO image1  -- bird.png is transferred and cached.
DISPLAY "/tmp/cat.png"  TO image1  -- cat.png is transferred and cached.
DISPLAY "/tmp/bird.png" TO image1  -- bird.png is cached, no transfer needed.
DISPLAY "/tmp/cat.png"  TO image1  -- cat.png is cached, no transfer needed.
RUN "touch /tmp/bird.png"          -- updates modification timestamp of bird.png.
DISPLAY "/tmp/bird.png" TO image1  -- bird.png is retransferred and recached.
The standard.clearFileCache front call can be used in development or for debug purpose, to force the front-end to clear its resource file cache. However, this front call is not required in a production context:
DEFINE res BOOLEAN
CALL ui.Interface.frontCall("standard","clearFileCache",[],[res])
IF NOT res THEN
   ERROR "A problem occurred while clearing the front-end cache..."
END IF
Important:

This feature is provided for development and debug purpose, and must not be used in a production environment.

Clearing the front-end file cache will retransfer resources that have already been transferred (and previously cached):
DISPLAY "/tmp/bird.png" TO image1  -- bird.png is transferred and cached.
DISPLAY "/tmp/cat.png"  TO image1  -- cat.png is transferred and cached.
CALL ui.Interface.frontCall("standard","clearFileCache",[],[res])
DISPLAY "/tmp/bird.png" TO image1  -- bird.png is transferred and cached.
DISPLAY "/tmp/cat.png"  TO image1  -- cat.png is transferred and cached.

Application images in Web Components

Web Components can display static images (part of the Web Component assets), and application images provided at runtime (for example, a photo gallery web component). In order to provide application images to a Web Component, the program must use the ui.Interface.filenameToURI() method to convert the local file name to a URI that can be accessed by the front-end.

For more details, see Using image resources with the gICAPI web component.