LibreOffice Draw Image Filters and Raster Tools

LibreOffice Draw is primarily designed as a vector graphics editor, but it includes a versatile set of built-in image filtering and raster manipulation tools for editing bitmap graphics. Users can apply dynamic visual filters, adjust color balances, crop, manage transparency, and optimize pixel-based images directly on the canvas without relying on dedicated external photo-editing software.

Graphic Filters

Selecting an imported bitmap in LibreOffice Draw activates the Image toolbar, which includes a dedicated Filter menu offering eleven distinct real-time effects:

• Invert: Reverses the color values of a color image or the brightness values of a grayscale image.
• Smooth: Softens the image by reducing contrast between neighboring pixels, acting as a low-pass filter to blur sharp details.
• Sharpen: Increases contrast between adjacent pixels to accentuate edges and fine details.
• Remove Noise: Filters out single-pixel artifacts and random visual noise by averaging surrounding pixel values.
• Solarization: Mimics the effect of exposing photographic paper to light during development, with an adjustable threshold parameter.
• Aging: Simulates photo degradation by shifting colors toward sepia tones, configurable by an aging percentage level.
• Posterize: Reduces the total number of colors to create a stylized, flat-surfaced poster effect.
• Pop Art: Converts image colors into a high-contrast, stylized pop-art palette.
• Charcoal Sketch: Transforms the raster graphic into a monochrome drawing that simulates a charcoal sketch.
• Relief: Generates a 3D embossed look by creating artificial highlights and shadows based on a configurable virtual light source.
• Mosaic: Groups pixels into larger rectangular blocks to create a pixelated or mosaic appearance with custom block dimensions.

Color and Tonal Adjustments

Draw provides a set of basic color-grading controls within the Image properties panel:

• Color Modes: Quickly converts images between Default (standard color), Grayscale, Black/White (high-contrast bi-level), and Watermark (high brightness, low contrast for backgrounds).
• RGB Channel Sliders: Individual percentage controls to increase or decrease the Red, Green, and Blue color channels independently.
• Brightness and Contrast: Linear adjustments to control overall image luminance and tonal separation.
• Gamma Correction: Non-linear brightness adjustment to lighten or darken midtones without clipping absolute blacks and whites.

Raster Manipulation and Utility Tools

Beyond filters and color balancing, LibreOffice Draw offers functional raster-processing capabilities:

• Cropping: Allows manual trimming of bitmap edges. The cropping tool supports maintaining aspect ratios or scaling the image content within a defined frame.
• Transparency Controls: Applies uniform transparency percentages or directional gradient masks (linear, axial, radial, or ellipsoid) directly to raster objects.
• Color Replacer Tool: Located in the Tools menu, this pipette-based feature allows users to select up to four specific colors in a bitmap graphic and replace them with alternative colors or make them completely transparent within a specified tolerance.
• Resolution and Compression: Right-clicking an image allows users to compress graphics, adjust the output resolution (DPI), and choose between lossless (PNG) or lossy (JPEG) algorithms to manage document file size.
• Vectorization (Convert to Polygon): Automatically traces bitmap shapes and converts raster graphics into editable vector polygons, with controls over color count, hole reduction, and pixel grouping.