From Free Pascal wiki
Jump to: navigation, search

Deutsch (de) English (en) español (es) français (fr) 中文(中国大陆)‎ (zh_CN)

See also: Developing with Graphics


BGRABitmap is a set of units designed to modify and create images with transparency (alpha channel). Direct pixel access allows fast image processing. The library has been tested on Windows, Ubuntu and Mac OS X (The last version doesn't work on Mac), with widgetsets win32, gtk1, gtk2 and carbon.

The main class is TBGRABitmap, which is derived from TFPCustomImage. There is also TBGRAPtrBitmap which allows to edit BGRA data that are already allocated. This format consists of 4 bytes for each pixel (blue, green, red and alpha in that order).

Reference of BGRABitmapTypes unit

Using BGRABitmap



Functions have long names in order to be understandable. Almost everything is accessible as a function or using a property of the TBGRABitmap object. For example, you can use CanvasBGRA to have some canvas similar to TCanvas (with opacity and antialiasing) and Canvas2D to have the same features as the HTML canvas.

Some special features require the use of units, but you may not need them :

  • TBGRAMultishapeFiller to have an antialiased junctions of polygons is in BGRAPolygon
  • TBGRATextEffect is in BGRATextFX
  • 2D transformations are in BGRATransform
  • TBGRAScene3D is in BGRAScene3D
  • If you need to have layers, BGRALayers provides TBGRALayeredBitmap

Double-buffering is not really part of BGRABitmap, because it is more about how to handle forms. To do double-buffering, you can use TBGRAVirtualScreen which is in the BGRAControls package. Apart from that, double-buffering in BGRABitmap works like any double-buffering. You need to have a bitmap where you store your drawing and that you display with a single Draw instruction.


After unpacking a checkout, BGRA often does not compile in Linux. Try using the IDE Macro


in such cases. Still some other part may not compile.

Simple example

Create a project and open bgrabitmappackage.lpk with Lazarus. In the package window, click on "Use > Add to Project". Then in the source code of the main file (main form or main program), add to uses clause BGRABitmap units. You may need to add BGRAGraphics unit as well if you use certain types that are inherited from the LCL.

Uses Classes, SysUtils, BGRABitmap, BGRABitmapTypes;

The unit BGRABitmapTypes contains common definitions, but one can declare only BGRABitmap in order to load and show a bitmap. Then, the first step is to create a TBGRABitmap object:

var bmp: TBGRABitmap;
  bmp := TBGRABitmap.Create(100,100,BGRABlack); //creates a 100x100 pixels image with black background
  bmp.FillRect(20,20,60,60,BGRAWhite, dmSet); //draws a white square without transparency
  bmp.FillRect(40,40,80,80,BGRA(0,0,255,128), dmDrawWithTransparency); //draws a transparent blue square

Finally to show the bitmap:

procedure TFMain.FormPaint(Sender: TObject);
  bmp.Draw(Canvas, 0, 0, True); // draw the bitmap in opaque mode (faster)

See a full source code in tutorial 1.


Pixels in a bitmap with transparency are stored with 4 values, here 4 bytes in the order Blue, Green, Red, Alpha. The last channel defines the level of opacity (0 signifies transparent, 255 signifies opaque), other channels define color and luminosity.

There are basically two drawing modes. The first consists in replacing the content of the pixel information, the second consists in blending the pixel already here with the new one, which is called alpha blending.

BGRABitmap functions propose 4 modes:

  • dmSet : replaces the four bytes of the drawn pixel, transparency not handled
  • dmDrawWithTransparency : draws with alphablending and with gamma correction (see below)
  • dmFastBlend or dmLinearBlend : draws with alphablending but without gamma correction (faster but entails color distortions with low intensities).
  • dmXor : apply Xor to each component including alpha (if you want to invert color but keep alpha, use BGRA(255,255,255,0) )

Integrated drawing functions

  • draw/erase pixels
  • draw a line with or without antialiasing
  • floating point coordinates
  • floating point pen width
  • rectangle (frame or fill)
  • ellipse and polygons with antialiasing
  • spline computation (rounded curve)
  • simple fill (Floodfill) or progressive fill
  • color gradient rendering (linear, radial...)
  • round rectangles
  • texts with transparency

Drawing with the Canvas

It is possible to draw with a Canvas object, with usual functions but without antialiasing. Opacity of drawing is defined by the CanvasOpacity property. This way is slower because it needs image transformations. If you can, use CanvasBGRA instead, which allows transparency and antialiasing while having the same function names as with TCanvas.

Direct access to pixels

To access pixels, there are two properties, Data and Scanline. The first gives a pointer to the first pixel of the image, and the second gives a pointer to the first pixel of a given line.

  bmp: TBGRABitmap;
  p: PBGRAPixel;
  n: integer;
  bmp := TBGRABitmap.Create('image.png');
  p := bmp.Data;
  for n := bmp.NbPixels-1 downto 0 do
    p^.red := not p^.red;  //invert red channel
  bmp.InvalidateBitmap;  //note that we have accessed pixels directly

It is necessary to call the function InvalidateBitmap to rebuild the image in a next call to Draw' for example. Notice that the line order can be reverse, depending on the LineOrder property.

See also the comparison between direct pixel access methods.

Image manipulation

Available filters (prefixed with Filter) :

  • Radial blur : non directional blur
  • Motion blur : directional blur
  • Custom blur : blur according to a mask
  • Median : computes the median of colors around each pixel, which softens corners
  • Pixelate : simplifies the image with rectangles of the same color
  • Smooth : soften whole image, complementary to Sharpen
  • Sharpen : makes contours more accute, complementary to Smooth
  • Contour : draws contours on a white background (like a pencil drawing)
  • Emboss : draws contours with shadow
  • EmbossHighlight : draws contours of a selection defined with grayscale
  • Grayscale : converts colors to grayscale with gamma correction
  • Normalize : uses whole range of color luminosity
  • Rotate : rotation of the image around a point
  • Sphere : distorts the image to make it look like projected on a sphere
  • Twirl : distorts the image with a twirl effect
  • Cylinder : distorts the image to make it look like projected on a cylinder
  • Plane : computes a high precision projection on a horizontal plane. This is quite slow.
  • SmartZoom3 : resizes the image x3 and detects borders, to have a useful zoom with ancient games sprites

Some functions are not prefixed with Filter, because they do not return a newly allocated image. They modify the image in-place :

  • VerticalFlip : flips the image vertically
  • HorizontalFlip : flips the image horizontally
  • Negative : inverse of colors
  • LinearNegative : inverse without gamma correction
  • SwapRedBlue : swap red and blue channels (to convert between BGRA and RGBA)
  • ConvertToLinearRGB : to convert from sRGB to RGB. Note the format used by BGRABitmap is sRGB when using

dmDrawWithTransparency and RGB when using dmLinearBlend.

  • ConvertFromLinearRGB : convert from RGB to sRGB.

Images combination

PutImage is the basic image drawing function and BlendImage allows to combine images, like layers of image editing softwares. Available modes are the following:

  • LinearBlend : simple superimposition without gamma correction (equivalent to dmFastBlend)
  • Transparent : superimposition with gamma correction
  • Multiply : multiplication of color values (with gamma correction)
  • LinearMultiply : multiplication of color values (without gamma correction)
  • Additive : addition of color values (with gamma correction)
  • LinearAdd : addition of color values (without gamma correction)
  • Difference : difference of color values (with gamma correction)
  • LinearDifference : difference of color values (without gamma correction)
  • Negation : makes common colors disappear (with gamma correction)
  • LinearNegation : makes common colors disappear (without gamma correction)
  • Reflect, Glow : for light effects
  • ColorBurn, ColorDodge, Overlay, Screen : misc. filters
  • Lighten : keeps the lightest color values
  • Darken : keeps the darkest color values
  • Xor : exclusive or of color values

These modes can be used in TBGRALayeredBitmap, which makes it easier because BlendImage only provides the basic blending operations.


Lazpaint contour.png Lazpaint curve redim.png Bgra wirecube.png Bgra chessboard.jpg


modified LGPL

Author: Johann ELSASS (Facebook)


Sourceforge with LazPaint and BGRABitmap :


Old link :

If you did not find the information you are looking for or have a suggestion, you can send feedback here.