[Back] [Up] [Next]

## Controlling image quality

When the Volume plot is drawn with graphics hardware, the database is resampled onto a rectilinear grid that is used to place the polygons that are drawn to produce the image. You can control the coarseness of the resampled grid with the Number of samples text field and slider. To increase the number of sample points, enter a larger number into the Number of samples text field or move the slider to the right. Note that the slider is on an exponential scale and moving it to the right increases the number of sample points exponentially.

In addition to setting the number of samples, when the Volume plot is using the 3D texturing method, you can set the number of planes to be drawn from back to front. Increasing the number of planes can help to reduce the amount of aliasing in the resulting image. However, as the Volume plot uses a higher number of planes, more work must be done to draw the plot and it takes a little longer to draw. To set the number of planes, enter a new number of planes into the Number of slices text field.

When the Volume plot is drawn in ray casting mode, the number of samples along each ray that is cast through the data becomes important. Having too few sample points along a ray gives rise to sampling artifacts such as rings or voids. You should adjust this number until you are satisfied with the image. More samples generally produce a better image, though the image will take longer to produce. To change the number of samples per ray, enter a new number of samples per ray into the Samples per ray text field.

When using lighting, the gradient calculation method that the Volume plot uses influences the quality of the images that are produced. By default, VisIt uses the Sobel operator, which uses more information from adjacent cells to calculate a gradient. When the Sobel operator is used to calculate the gradient, lighting usually looks better. The alternative gradient calculation method is centered-differences and while it is much less compute intensive than the Sobel operator, it also produces lesser quality gradient vectors, which results in images that are not lit as well. To change the gradient calculation method, click on either the Centered diff or Sobel radio buttons.