Houdini 19.0 Nodes Compositing nodes Atop

Atop compositing node

Composites the first input (Foreground) over the second (background), but only where the background alpha exists.

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Atop is similar to an Over operation, but the over is restricted to the area of the background image’s alpha matte. The atop operation is defined as

C = C2 * A1 + C1 *(1-A2)

The foreground is considered to be premultiplied, so it will not be multiplied by its alpha channel. If your alpha plane has been replaced (by a ChannelCopy or SwitchAlpha COP, for instance), use a Premultiply COP to correct the color plane.

Planes containing geometric data are handled differently (like Zdepth, Normal, Velocity). Alpha blending does not occur on these planes; the operation either takes the foreground (A>0) or the background pixel data (A=0). This avoids edges artifacts when using geometric data after compositing.

Tip

The Layer COP allows multiple inputs to be composited instead of just two, each with its own compositing operation. The Composite COP is the more general version of this COP, allowing you to choose any compositing operation, and an optionally different operation for alpha.

Masking

This operation may be masked, which restricts the operation to an area of the image. The mask may be inverted, brightened or dimmed.

The mask input is on the side of the node. The label on the connector indicates the plane being used as a mask.

The mask input can also be scaled to fit the output image’s resolution, if they differ. If this node is changing constantly, and the mask is not, it is somewhat faster to put a Scale COP down to do the resize for the mask image. Otherwise, the scale will occur every time this node cooks.

Parameters

Composite

Image Units

Selects pixels or UV units for the transform.

Translate, Rotate, Scale, Pivot

Transforms the foreground in X,Y relative to the background.

Image Filter

Specifies the filter to use when transforming.

Image Wrap

Specifies the foreground image’s wrapping style.

Motion Blur

Toggles on motion blur, and specifies the time range around the current frame to blur.

Motion Frame Bias

Shifts the time range for the blur.

Motion Blur Segments

Number of samples to blur together in the specified time range.

Motion Blur Method

Velocity

Only uses the current frame and the transform to compute past and future positions.

Deformation

Cooks the images at all time samples.

Merge

Overview

These parameters specify how to build the output sequence when there are differences between the input sequences.

Plane Merge

If the planes in the inputs differ, this specifies what the output planes should be.

Raster Depth

If the raster depth for a given plane differs between inputs, this specifies what the output raster depth for the plane should be.

Frame Range

If the frame ranges of the inputs differ, this determines what the output range should be.

Frame Rate

If the frame rates of the inputs differ, this determines what the output frame rate should be.

Frame Match

If the frame rates of the inputs differ, this determines which input frame to pick when the cook time doesn’t reside on a frame boundary.

Mask

A mask can be chosen to limit the effect of the operator to areas defined by the mask. The mask can be taken from the mask input (side input) or from the first input itself.

Effect Amount

If no mask is present, this blends the output with the input by a constant amount (0 = all input, 1 = all output).

If a mask is present, this amount multiplies the mask.

Operation Mask

Selects the mask plane to use as a mask from the mask input. The mask can be selected from:

A mask can be a component of a plane or an entire plane. If a vector plane is supplied as a mask, its components are multiplied by the images' components.

Scalar Mask ('A', 'C.r')

C.r = I.r * M
C.g = I.g * M
C.b = I.b * M

Vector Mask ('C')

C.r = I.r * M.r
C.g = I.g * M.g
C.b = I.b * M.b

First Input

Useful for masking the operation to the image’s own alpha plane.

Mask Input

Selects the mask from the side mask input.

Off

Turns off masking, without requiring disconnection of the mask input (useful for temporarily disabling the mask).

Resize Mask to Fit Image

If the mask image is a different resolution than the output image, turning on this parameter will scale the mask to the output image’s resolution.

If this node is changing constantly, and the mask is not, it is somewhat faster to put a Scale COP down to do the resize for the mask image. Otherwise, the scale will occur every time this node cooks.

Invert Mask

Inverts the mask so that all fully 'masked' portions become unmasked. This saves you from inserting an Invert COP after the node with the mask.

Locals

L

Sequence length

S

Start of sequence

E

End of sequence

IL

Input sequence length

SR

Sequence frame rate

NP

Number of planes in sequence

W,H

Width and height of image

I

Image index (0 at start frame)

IT

Image time (0 at start frame)

AI

Current plane array index

PI

Current plane index

PC

Num of channels in current plane

CXRES

Composite Project X resolution

CYRES

Composite Project Y resolution

CPIXA

Composite Project pixel aspect ratio

CDEPTH

Composite Project raster depth

CBP

Composite Project black point

CWP

Composite Project white point

See also

Atop