Too long, didn’t watch: Black and white
images limit your print resolution to the
resolution of a pixel. Grey scale images allows
you to get sub pixel resolution.
Steve here from Autodesk. On the Ember team
we’re exploring how to use software to get
higher quality prints without changing our
hardware. Let’s see how we can achieve some
features that are smaller than the nominal
resolution of the Ember printer. So, theoretically,
it all starts with the voxel.
The voxel is the building block of DLP 3D
printing. The size of the voxel, and in turn
the resolution of the printer, is defined
by the pixel size of the projected image and
the layer thickness in the vertical direction.
So a print of a 3D model is an approximation
of many many tiny cubic voxels.
In a single layer, the location of voxels
is set by the projected image. If a pixel
is white, the projected light will cure the
resin and solidify a voxel. If a pixel is
black, there’s no light and the resin stays
liquid.
This means that if the voxels are 50 microns
on a side, we can arrange them on a grid with
50 micron spacing. We could create a group
of 4 voxels over here, and also another group
of voxels spaced 50 µm away, or 100, or 200,
or some other multiple of 50 µm. But we cannot
arrange them, say, 125 µm apart because voxels
are the indivisible atoms of DLP printing
(in theory).
So far we’ve been assuming that our image
is composed of either purely black or purely
white pixels. But what would happen with a
grey pixel? Would it print half  a voxel?
If so, which half? Top half, bottom half,
left half, right half, center half?
Richard Greene on the Ember team created an
experiment where he printed a solid row voxels
followed by a row of pixels ranging in brightness
from very dark grey (almost black) up to a
fully white pixel. He found that pixels darker
than a certain shade don’t print at all.
Then at a certain point a small, hemispherical
bump forms attached to the previously printed
layer. A brighter pixel produces a taller
bump, and as the pixel gets brighter the voxel
grows wider and slightly taller. This means the size
of the voxel is can be controlled by varying
the luminosity of a single pixel. In practice
we found that a grey-pixel or “half-voxel”
will tend to merge with adjacent voxel. So
2 white pixels on the left and a grey pixel
in the middle, the half voxel will form on
the left. If the two white pixels are on the
right, the same  grey pixel will merge toward
the right.
In practice this allows us to do some really
cool tricks. Here is an image slice of printing
a small cube with 10-pixel by 10-pixel cross
section and another cube of the same size
that’s offset one pixel. By using just black
and white images that’s the best we can
do with arranging their relative locations.
Now if we use grey values in the image, we
can locate the cube with more precision that
the projector should theoretically allow.
This also allows us to create very precise
vertical slopes. Here, in the X-Y plane, is
a cross section through a stack of slice images.
Note the voxels are rectangular because the
pixels are 50 µm in the X and Y dimensions
but here we’ve chosen to have 25 µm steps
in the Z direction. There’s a vertical pillar
of solid white voxels which forms a 90 degree
angle with the base. But by adding a one pixel
wide gradient of 32 grey values along the
side, what we get is a very precise slope
where each layer is roughly 1 and a half microns
thinner than the layer below it. So instead
being perpendicular, we’ve added a precise
draft angle of 3.6 degrees. All within a single
50 micron wide pixel.
Here’s another great example. Printing a
slope of 75 degrees with just black
and white pixels results in a stairstep pattern.
Because for every 7 or 8 25 µm z-layers, there
is 1 50 µm step over in the x-direction.
However, if we use grey scale we get a much
flatter slope.
This also works with shallow horizontal slopes.
By using a controlled gradient of 32 grey
values we can create a very shallow slope
-- all within a single printed layer -- that
has an average Z step of less than 2 microns.
The slope is less than 1 degree above horizontal.
These greyscale tricks don’t magically allow
you to print sub-pixel features, but they
do allow you to reduce the layer lines and
some other artifacts you see on 3D prints.
Check out some other related videos like this
one about the difference between video and
pattern mode in DLP projectors. There’s
also an instructable in the description that
goes into more depth about printing single
voxels with greyscale. Thanks for watching!
And have a beautiful tomorrow!
