ndg/lib/lvgl/docs/widgets/core/img.md

Image (lv_img)

Overview

Images are the basic object to display images from flash (as arrays) or from files. Images can display symbols (LV_SYMBOL_...) too.

Using the Image decoder interface custom image formats can be supported as well.

Parts and Styles

• LV_PART_MAIN A background rectangle that uses the typical background style properties and the image itself using the image style properties.

Usage

Image source

To provide maximum flexibility, the source of the image can be:

• a variable in code (a C array with the pixels).
• a file stored externally (e.g. on an SD card).
• a text with Symbols.

To set the source of an image, use lv_img_set_src(img, src).

To generate a pixel array from a PNG, JPG or BMP image, use the Online image converter tool and set the converted image with its pointer: lv_img_set_src(img1, &converted_img_var); To make the variable visible in the C file, you need to declare it with LV_IMG_DECLARE(converted_img_var).

To use external files, you also need to convert the image files using the online converter tool but now you should select the binary output format. You also need to use LVGL's file system module and register a driver with some functions for the basic file operation. Go to the File system to learn more. To set an image sourced from a file, use lv_img_set_src(img, "S:folder1/my_img.bin").

You can also set a symbol similarly to Labels. In this case, the image will be rendered as text according to the font specified in the style. It enables to use of light-weight monochrome "letters" instead of real images. You can set symbol like lv_img_set_src(img1, LV_SYMBOL_OK).

Label as an image

Images and labels are sometimes used to convey the same thing. For example, to describe what a button does. Therefore, images and labels are somewhat interchangeable, that is the images can display texts by using LV_SYMBOL_DUMMY as the prefix of the text. For example, lv_img_set_src(img, LV_SYMBOL_DUMMY "Some text").

Transparency

The internal (variable) and external images support 2 transparency handling methods:

• Chroma-keying - Pixels with LV_COLOR_CHROMA_KEY (lv_conf.h) color will be transparent.
• Alpha byte - An alpha byte is added to every pixel that contains the pixel's opacity

Palette and Alpha index

Besides the True color (RGB) color format, the following formats are supported:

• Indexed - Image has a palette.
• Alpha indexed - Only alpha values are stored.

These options can be selected in the image converter. To learn more about the color formats, read the Images section.

Recolor

A color can be mixed with every pixel of an image with a given intensity. This can be useful to show different states (checked, inactive, pressed, etc.) of an image without storing more versions of the same image. This feature can be enabled in the style by setting img_recolor_opa between LV_OPA_TRANSP (no recolor, value: 0) and LV_OPA_COVER (full recolor, value: 255). The default value is LV_OPA_TRANSP so this feature is disabled.

The color to mix is set by img_recolor.

Auto-size

If the width or height of the image object is set to LV_SIZE_CONTENT the object's size will be set according to the size of the image source in the respective direction.

Mosaic

If the object's size is greater than the image size in any directions, then the image will be repeated like a mosaic. This allows creation a large image from only a very narrow source. For example, you can have a 300 x 5 image with a special gradient and set it as a wallpaper using the mosaic feature.

Offset

With lv_img_set_offset_x(img, x_ofs) and lv_img_set_offset_y(img, y_ofs), you can add some offset to the displayed image. Useful if the object size is smaller than the image source size. Using the offset parameter a Texture atlas or a "running image" effect can be created by Animating the x or y offset.

Transformations

Using the lv_img_set_zoom(img, factor) the images will be zoomed. Set factor to 256 or LV_IMG_ZOOM_NONE to disable zooming. A larger value enlarges the images (e.g. 512 double size), a smaller value shrinks it (e.g. 128 half size). Fractional scale works as well. E.g. 281 for 10% enlargement.

To rotate the image use lv_img_set_angle(img, angle). Angle has 0.1 degree precision, so for 45.8° set 458.

The transform_zoom and transform_angle style properties are also used to determine the final zoom and angle.

By default, the pivot point of the rotation is the center of the image. It can be changed with lv_img_set_pivot(img, pivot_x, pivot_y). 0;0 is the top left corner.

The quality of the transformation can be adjusted with lv_img_set_antialias(img, true/false). With enabled anti-aliasing the transformations are higher quality but slower.

The transformations require the whole image to be available. Therefore indexed images (LV_IMG_CF_INDEXED_...), alpha only images (LV_IMG_CF_ALPHA_...) or images from files can not be transformed. In other words transformations work only on true color images stored as C array, or if a custom Image decoder returns the whole image.

Note that the real coordinates of image objects won't change during transformation. That is lv_obj_get_width/height/x/y() will return the original, non-zoomed coordinates.

IMPORTANT The transformation of the image is independent of the transformation properties coming from styles. (See here). The main differences are that pure image widget transformation

• doesn't transform the children of the image widget
• image is transformed directly without creating an intermediate layer (buffer) to snapshot the widget

Size mode

By default, when the image is zoomed or rotated the real coordinates of the image object are not changed. The larger content simply overflows the object's boundaries. It also means the layouts are not affected the by the transformations.

If you need the object size to be updated to the transformed size set lv_img_set_size_mode(img, LV_IMG_SIZE_MODE_REAL). (The previous mode is the default and called LV_IMG_SIZE_MODE_VIRTUAL). In this case if the width/height of the object is set to LV_SIZE_CONTENT the object's size will be set to the zoomed and rotated size. If an explicit size is set then the overflowing content will be cropped.

Rounded image

You can use lv_obj_set_style_radius to set radius to an image, and enable lv_obj_set_style_clip_corner to clip the content to rounded rectangle or circular shape. Please note this will have some negative performance impact to CPU based renderers.

Events

No special events are sent by image objects.

See the events of the Base object too.

Learn more about Events.

Keys

No Keys are processed by the object type.

Learn more about Keys.

Example

.. include:: ../../../examples/widgets/img/index.rst

API

.. doxygenfile:: lv_img.h
  :project: lvgl