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thehustler

wireframe line width

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Hi, I'm currently trying to render wireframes using OpenGL. My wireframes come out fine apart from one problem. I'm setting glEnable(GL_LINE_SMOOTH) and glLineWidth(0.1f) but my wireframe lines always have a width of 1.0f. Is there anything I may be missing to get good thin antialiased lines here? Is it possible that my OpenGL implementation really doesn't support thinner lines?

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Yeah it's quite possible that your card doesn't support that line width, the easiest way to check would be to head on over to Delphi3D, download GLInfo and check what the range and granularity are for your setup. If you want a programatical way of doing this you can use glGet with GL_LINE_WIDTH_RANGE and GL_LINE_WIDTH_GRANULARITY. This is the description of glLineWidth from the Blue Book:

glLineWidth
NAME

glLineWidth - specify the width of rasterized lines
C SPECIFICATION

void glLineWidth( GLfloat width )
PARAMETERS
width

Specifies the width of rasterized lines. The default is 1.0.
DESCRIPTION

glLineWidth specifies the rasterized width of both aliased and antialiased lines. Using a line width other than 1.0 has different effects, depending on whether line antialiasing is enabled. Line antialiasing is controlled by calling glEnable and glDisable with argument GL_LINE_SMOOTH.

If line antialiasing is disabled, the actual width is determined by rounding the supplied width to the nearest integer. (If the rounding results in the value 0, it is as if the line width were 1.) If | Δx | ≥ | Δy |, i pixels are filled in each column that is rasterized, where i is the rounded value of width. Otherwise, i pixels are filled in each row that is rasterized.

If antialiasing is enabled, line rasterization produces a fragment for each pixel square that intersects the region lying within the rectangle having width equal to the current line width, length equal to the actual length of the line, and centered on the mathematical line segment. The coverage value for each fragment is the window coordinate area of the intersection of the rectangular region with the corresponding pixel square. This value is saved and used in the final rasterization step.

Not all widths can be supported when line antialiasing is enabled. If an unsupported width is requested, the nearest supported width is used. Only width 1.0 is guaranteed to be supported; others depend on the implementation. The range of supported widths and the size difference between supported widths within the range can be queried by calling glGet with arguments GL_LINE_WIDTH_RANGE and GL_LINE_WIDTH_GRANULARITY.
NOTES

The line width specified by glLineWidth is always returned when GL_LINE_WIDTH is queried. Clamping and rounding for aliased and antialiased lines have no effect on the specified value.

Non-antialiased line width may be clamped to an implementation-dependent maximum. Although this maximum cannot be queried, it must be no less than the maximum value for antialiased lines, rounded to the nearest integer value.
ERRORS

GL_INVALID_VALUE is generated if width is less than or equal to zero.

GL_INVALID_OPERATION is generated if glLineWidth is called between a call to glBegin and the corresponding call to glEnd.
ASSOCIATED GETS

glGet with argument GL_LINE_WIDTH
glGet with argument GL_LINE_WIDTH_RANGE
glGet with argument GL_LINE_WIDTH_GRANULARITY
glIsEnabled with argument GL_LINE_SMOOTH
SEE ALSO

"glEnable"


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