# Bresenham's line algorithm

Bresenham's line algorithm

The Bresenham line algorithm is an algorithm that determines which points in an n-dimensional raster should be plotted in order to form a close approximation to a straight line between two given points. It is commonly used to draw lines on a computer screen, as it uses only integer addition, subtraction and bit shifting all of which are very cheap operations in standard computer architectures. It is one of the earliest algorithms developed in the field of computer graphics.

Through a minor expansion, the original algorithm for lines can also be used to draw circles. Also this can be done with simple arithmetic operations; quadratic or trigonometric expressions can be avoided or recursively dissolved into simpler steps.

The mentioned properties make it still an important algorithm, and it is used among others in plotters, in graphics chips of modern graphics cards, and in many graphics libraries. Because the algorithm is very simple, it is often implemented in both the firmware and the hardware of graphics cards.

The label "Bresenham" is today often used for a whole family of algorithms, which have been developed by others, later, yet in succession of Bresenham and with a similar basic approach. See deeper references below.

The algorithm

The common conventions that pixel coordinates increase in the down and right directions and that pixel centers have integer coordinates will be used.The endpoints of the line are the pixels at ("x"0, "y"0) and ("x"1, "y"1), where the first coordinate of the pair is the column and the second is the row.

The algorithm will be initially presented only for the octant in which the segment goes down and to the right ("x"0≤"x"1 and "y"0≤"y"1 ) , and its horizontal projection $x_1-x_0$ is longer than the vertical projection $y_1-y_0$ (in other words, the line has a slope less than 1 and greater than 0.) In this octant, for each column "x" between $x_0$ and $x_1$, there is exactly one row "y" (computed by the algorithm) containing a pixel of the line, while each row between $y_0$ and $y_1$ may contain multiple rasterized pixels.

Bresenham's algorithm chooses the integer "y" corresponding to the pixel center that is closest to the ideal (fractional) "y" for the same "x"; on successive columns y can remain the same or increase by 1.The general equation of the line through the endpoints is given by::$y - y_0 = frac\left\{y_1-y_0\right\}\left\{x_1-x_0\right\} \left(x-x_0\right).$

Since we know the column, "x", the pixel's row, "y", is given by rounding this quantity to the nearest integer:

:$frac\left\{y_1-y_0\right\}\left\{x_1-x_0\right\} \left(x-x_0\right) + y_0.$

The slope $\left(y_1-y_0\right)/\left(x_1-x_0\right)$ depends on the endpoint coordinates only and can be precomputed, and the ideal "y" for successive integer values of "x" can be computed starting from $y_0$ and repeatedly adding the slope.

In practice, the algorithm can track, instead of possibly large y values, a small "error value" between −0.5 and 0.5: the vertical distance between the rounded and the exact "y" values for the current "x". Each time "x" is increased, the error is increased by the slope; if it exceeds 0.5, the rasterization "y" is increased by 1 (the line continues on the next lower row of the raster) and the error is decremented by 1.0.

In the following pseudocode sample `plot(x,y)` plots a point and `abs` returns absolute value:

function line(x0, x1, y0, y1) "int" deltax := x1 - x0 "int" deltay := y1 - y0 "real" error := 0 "real" deltaerr := deltay / deltax // Assume deltax != 0 (line is not vertical), // note that this division needs to be done in a way that preserves the fractional part "int" y := y0 for x from x0 to x1 plot(x,y) error := error + deltaerr if abs(error) ≥ 0.5 then y := y + 1 error := error - 1.0

Generalization

Actually this first formation was done by Bresenham.This first version only handles lines that descend to the right. We would of course like to be able to draw all lines. The first case is allowing us to draw lines that still slope downwards but head in the opposite direction. This is a simple matter of swapping the initial points if `x0 > x1`. Trickier is determining how to draw lines that go up. To do this, we check if "y"0 ≥ "y"1; if so, we step "y" by -1 instead of 1. Lastly, we still need to generalize the algorithm to drawing lines in "all" directions. Up until now we have only been able to draw lines with a slope less than one. To be able to draw lines with a steeper slope, we take advantage of the fact that a steep line can be reflected across the line "y=x" to obtain a line with a small slope. The effect is to switch the "x" and "y" variables throughout, including switching the parameters to "plot". The code looks like this:

function line(x0, x1, y0, y1) "boolean" steep := abs(y1 - y0) > abs(x1 - x0) if steep then swap(x0, y0) swap(x1, y1) if x0 > x1 then swap(x0, x1) swap(y0, y1) "int" deltax := x1 - x0 "int" deltay := abs(y1 - y0) "real" error := 0 "real" deltaerr := deltay / deltax "int" ystep "int" y := y0 if y0 < y1 then ystep := 1 else ystep := -1 for x from x0 to x1 if steep then plot(y,x) else plot(x,y) error := error + deltaerr if error ≥ 0.5 then y := y + ystep error := error - 1.0

The function now handles all lines and implements the complete Bresenham's algorithm.

Optimization

The problem with this approach is that computers operate relatively slowly on fractional numbers like `error` and `deltaerr`; moreover, errors can accumulate over many floating-point additions. Working with integers will be both faster and more accurate. The trick we use is to multiply all the fractional numbers above by `deltax`, which enables us to express them as integers. The only problem remaining is the constant 0.5&mdash;to deal with this, we change the initialization of the variable `error`, and invert it for an additional small optimization. The new program looks like this:

function line(x0, x1, y0, y1) "boolean" steep := abs(y1 - y0) > abs(x1 - x0) if steep then swap(x0, y0) swap(x1, y1) if x0 > x1 then swap(x0, x1) swap(y0, y1) "int" deltax := x1 - x0 "int" deltay := abs(y1 - y0) "int" error := deltax / 2 "int" ystep "int" y := y0 if y0 < y1 then ystep := 1 else ystep := -1 for x from x0 to x1 if steep then plot(y,x) else plot(x,y) error := error - deltay if error < 0 then y := y + ystep error := error + deltax

History

The algorithm was developed by Jack E. Bresenham in 1962 at IBM. In 2001 Bresenham wrotePaul E. Black. "Dictionary of Algorithms and Data Structures," NIST. http://www.nist.gov/dads/HTML/bresenham.html] ::"I was working in the computation lab at IBM's San Jose development lab. A Calcomp plotter had been attached to an IBM 1401 via the 1407 typewriter console. [The algorithm] was in production use by summer 1962, possibly a month or so earlier. Programs in those days were freely exchanged among corporations so Calcomp (Jim Newland and Calvin Hefte) had copies. When I returned to Stanford in Fall 1962, I put a copy in the Stanford comp center library."

:"A description of the line drawing routine was accepted for presentation at the 1963 ACM national convention in Denver, Colorado. It was a year in which no proceedings were published, only the agenda of speakers and topics in an issue of Communications of the ACM. A person from the IBM Systems Journal asked me after I made my presentation if they could publish the paper. I happily agreed, and they printed it in 1965."

Bresenham's algorithm was later modified to produce circles, the resulting algorithm being sometimes known as either "Bresenham's circle algorithm" or Midpoint circle algorithm.

Similar algorithms

The Bresenham algorithm can be interpreted as slightly modified DDA (Using 0.5 as error threshold instead of 0, which is required for non-overlapping polygon rasterizing).

The principle of using an incremental error in place of division operations has other applications in graphics. It is possible to use this technique to calculate the U,V co-ordinates during raster scan of texture mapped polygons. The voxel heightmap software-rendering engines seen in some PC games also used this principle.

Bresenham also published a Run-Slice (as opposed to the Run-Length) computational algorithm.

References

* Jack E. Bresenham, [http://www.research.ibm.com/journal/sj/041/ibmsjIVRIC.pdf "Algorithm for computer control of a digital plotter"] , "IBM Systems Journal", Vol. 4, No.1, January 1965, pp. 25-30

* [http://www.cs.helsinki.fi/group/goa/mallinnus/lines/bresenh.html "The Bresenham Line-Drawing Algorithm"] , by Colin Flanagan

See also

* [http://www.chez.com/pmaillot Patrick-Gilles Maillot's Thesis] an extension of the Bresenham line drawing algorithm to perform 3D hidden lines removal; also published in MICAD '87 proceedings on CAD/CAM and Computer Graphics, page 591 - ISBN 2-86601-084-1.
* Digital Differential Analyzer (graphics algorithm) is a simple and general method for rasterizing lines and triangles.

*Xiaolin Wu's line algorithm, a similarly fast method of drawing lines with antialiasing.

*Midpoint circle algorithm, a similar algorithm for drawing circles.

External links

* [http://tide4javascript.com/?s=Bresenham Analyze Bresenham's line algorithm in an online Javascript IDE]
* [http://programmers-lounge-basicgraphics.blogspot.com/ Basic Graphics Programs]
* [http://www.cs.helsinki.fi/group/goa/mallinnus/lines/bresenh.html "The Bresenham Line-Drawing Algorithm" by Colin Flanagan]
* [http://www.nist.gov/dads/HTML/bresenham.html National Institute of Standards and Technology page on Bresenham's algorithm]
* [http://www.pdp8.net/563/563.shtml Calcomp 563 Incremental Plotter Information]
* [http://www.codecodex.com/wiki/index.php?title=Bresenham%27s_line_algorithm Implementations in Java, C, and OCaml] at the Code Codex
* [http://plagiata.net.ru/?p=277 Implementations in Delphi]

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