I am surprised that AutoCAD can't show a perfect circle or arc. I have done all the tricks I searched online - increase resolution, VIEWRES, etc. But no use. Those were for "polygon" circle. What I want is a perfect circle like what we did in mechanical drawing by using a compass. No matter how high resolution I use, the circle is jagged, like an earthworm in a close look. Why is that? Why can't this software just draw a perfect circle/arc?? How could this be used??
I can't even have a username that I can set up by myself here??
Found it in security... How do I delete this post then?
Seems I can only edit this post. No option to delete this reply ??? ..... and my new username will take a while to update???
New to here, have to say this is not very user-friendly. Very interesting......
@Anonymous wrote:
.... No matter how high resolution I use, the circle is jagged, like an earthworm in a close look. Why is that? Why can't this software just draw a perfect circle/arc?? How could this be used??
"Perfect" is a matter of degree. Your screen is made up of pixels in rows, so some level of pixelation is unavoidable. Do you have the LINESMOOTHING System Variable set to ON to decrease the effect? And regardless of screen display, when you PLOT, are they still jagged? [Even there, "perfect" is limited by the DPI setting.]
The graphic display is just that. Curves are made with lines to save computational demands on the computer. It's much easier graphically to draw a bunch of line segments than it is to draw a curve. It has no effect on printing.
As to your name, not sure what you are talking about. We all have our own chosen SN based on availability.
@Anonymous wrote:
... How do I delete this post then? .....
You could use the Report button at lower right in your post, and ask the Moderator to remove it.
Computers don't generate perfect circles. You can approximate something close to a curve on a computer screen, such as with a high enough resolution, dpi, anti-aliasing, etc. But a software like Autocad doesn't have much need to show perfect circles on screen. Autocad is vector based. Meaning essentially it uses lines for all its graphics, including curves and circles. So even though you may zoom in real close and see jagged lines, those circles and curves are still defined as such. So as others have stated, when you print or publish to pdf, the vector geometry will be converted to look like actual curves on the print. Ultimately the way you draw circles by hand with pen and paper is different than how computers "draw" circles.
It would be cool to figure out what he did wrong. Did he extrude an arc in 3d and look at it in an iso view? Was he rendering his screen? working on a vga monitor?
Agreed with everything except for these:
@apatterson2APPDE wrote:Computers don't generate perfect circles.
They don't display perfect circles. The circle is defined in the file. The pixels on the screen prohibit the drawing of arcs.
@apatterson2APPDE wrote:Autocad is vector based. Meaning essentially it uses lines for all its graphics, including curves and circles.
There is no correlation between vector and graphics. The graphics replace the vector data with a grid representation. See next comment.
@apatterson2APPDE wrote:the vector geometry will be converted to look like actual curves on the print.
The curves output to the print are vector geometry. Now, what the printer driver does with that information I would expect to vary depending on the printer. I'm just not familiar enough with what happens with the driver to know if they can actually print the vector data or there is a conversion to segmented arcs.
It was worse than not accurate. Some of the fundamentals are wrong and needed to be corrected before you confused the OP.
Just to be clear, my comments only mentioned your definition of vector geometry. Nothing more. Anything else you read into my reply is all on you.
@RobDraw wrote:Agreed with everything except for these:
...
@apatterson2APPDE wrote:the vector geometry will be converted to look like actual curves on the print.
The curves output to the print are vector geometry. Now, what the printer driver does with that information I would expect to vary depending on the printer. I'm just not familiar enough with what happens with the driver to know if they can actually print the vector data or there is a conversion to segmented arcs.
The process of drawing a circle on paper with a compass is an analog, as opposed to digital, process. The curve produced is smooth and continuous.
Computer printers of graphics rely on one of two methods to make a print. They are either raster printers that use a series of dots to make an image or they are what is often referred to as XY plotters. Raster printers may make lines other than horizontal and vertical appear like a step of stairs. This is especially true with lines of small angles. To reduce the "jaggies" cause by this stair stepping antialiasing is used. A simple form of anti-aliasing makes the dot (or pixel) at the beginning of a step half the intensity of the line. This softens the appearance of the step. I feel the effect is more akin to making the image slightly out of focus. XY plotters in contrast can move a pen from one location on directly to anther location on the paper in a perfectly straight line. XY plotters use stepping motors to precisely position a pen on paper. A stepping motor can be considered a digital device in that there is a finite size to the step. To draw a circle, arc or spline, the XY plotter is given a series of positions it should move to in order to create the curve. Therefore the curve is draw by moving the pen to specific coordinates. A circle is thus interpreted as a series of points along its circumference although the data for the circle may be just its center location and radius. Mathematically, the location of the pen is simply:
x = x_center + radius * cos(theta) and y = y_center + radius * sin(theta)
To draw the circle theta must go from 0° to 360°. If you incremented theta by 45° (0, 45, 90,...) then the result would look like an octagon IF THE SIZE OF THE PLOTTED CIRCLE WERE LARGE ENOUGH TO SEE THE EIGHT SIDES. Typically it would be large enough so a step size much smaller than 45° is used. The increment value for theta that is used when plotting is a function of the radius of the printed circle and the minimum step size for the stepping motors. Bigger circles will use smaller angle steps. Similarly splines will be drawn by determining the step size for the independent variable of the parametric spline definition whether they are Bezier, B-splines or NURBS. These splines store the curve not as a series of points that are along the curve but as a series of points that define Control Vertices. Just like the circle, points along the curve are calculated on-the-fly as it is printed (or displayed). One nice feature of these splines is that for a given value of delta_t (the step size of the independent variable) the length of the line segments used to draw the spline are shorter where the radius of curvature is small and longer where the spline flattens out. This yields the same degree of perceived smoothness for the entire spline.
Bottom line? Whether printed with a raster printer (e.g. laser printer) or XY plotter, at some level a curve will not be smooth. The question is, will it be smooth enough. This leads me to the age old question, "what's the difference between a scientist and an engineer?" 😀
Is an old school pen plotter considered an X,Y plotter or a true vector plotter?
@RobDraw wrote:Is an old school pen plotter considered an X,Y plotter or a true vector plotter?
Sorry @RobDraw , I read your question on my cell and did not see " or a true vector". Pen plotters, XY plotters and vector plotters all refer to essentially the same concept. The ability to move a marking device, e.g., a pen, from one XY location directly to another. The marking device could be a pen but also a light beam for exposing photographic film or a knife to cut Rubylith for making the mask for printed circuit boards. When I first got into CAD (> 50 years ago!) all we had were "vector" printing and display devices . Raster displays had not been invented. They didn't come onto the CAD market until the early 1980's. My early days in CAD (1970's) were spent viewing DVST displays, a vector device that drew a green straight line from one location to another. It could take several minutes to display an entire PCB. Here's a video from the company I worked for in the 70s. Circles were drawn as they are plotted today via a series of short straight lines.
Raster printers are typically either laser or ink jet.
The reason why is (beside that it is vector based) - for performance.
Don't worry, the circles and arcs work perfect (with highest possible precision) if you extend a line to them or lay a line tangent to it. The simplification is only in the viewport.
To get them look right temporarily, you can use the command REGENALL.
Another reason for real loss of precision could be, if you work far off from the 0,0 coordinates.
If you stay within 100 000 (just a rough guess) of your base units (doesn't matter if it's inch or mm), your drafts should be fine.
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