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RLECOMP(1) RLECOMP(1)
NNAAMMEE
rlecomp - Digital image compositor
SSYYNNOOPPSSIISS
rrlleeccoommpp [ --oo _o_u_t_f_i_l_e ] _A_f_i_l_e _o_p_e_r_a_t_o_r _B_f_i_l_e
DDEESSCCRRIIPPTTIIOONN
_r_l_e_c_o_m_p implements an image compositor based on presence
of an alpha, or matte channel the image. This extra chan
nel usually defines a mask which represents a sort of a
cookie-cutter for the image. This is the case when alpha
is 255 (full coverage) for pixels inside the shape, zero
outside, and between zero and 255 on the boundary. If
_A_f_i_l_e or _B_f_i_l_e is just a single -, then _r_l_e_c_o_m_p reads that
file from the standard input.
The operations behave as follows (assuming the operation
is "_A _o_p_e_r_a_t_o_r _B"):
oovveerr The result will be the union of the two image
shapes, with _A obscuring _B in the region of over
lap.
iinn The result is simply the image _A cut by the shape
of _B. None of the image data of _B will be in the
result.
aattoopp The result is the same shape as image _B, with _A
obscuring _B where the image shapes overlap. Note
this differs from oovveerr because the portion of _A
outside _B's shape does not appear in the result.
oouutt The result image is image _A with the shape of _B cut
out.
xxoorr The result is the image data from both images that
is outside the overlap region. The overlap region
will be blank.
pplluuss The result is just the sum of the image data. Out
put values are clipped to 255 (no overflow). This
operation is actually independent of the alpha
channels.
mmiinnuuss The result of _A - _B, with underflow clipped to
zero. The alpha channel is ignored (set to 255,
full coverage).
ddiiffff The result of abs(_A - _B). This is useful for com
paring two very similar images.
aadddd The result of _A + _B, with overflow wrapping around
(_m_o_d 256).
1 December 20, 1986 1
RLECOMP(1) RLECOMP(1)
ssuubbttrraacctt
The result of _A - _B, with underflow wrapping around
(_m_o_d 256). The aadddd and ssuubbttrraacctt operators can be
used to perform reversible transformations.
SSEEEE AALLSSOO
_u_r_t(1), _R_L_E(5),
"Compositing Digital Images", Porter and Duff, _P_r_o_c_e_e_d_i_n_g_s
_o_f _S_I_G_G_R_A_P_H _'_8_4 p.255
AAUUTTHHOORRSS
Rod Bogart and John W. Peterson
BBUUGGSS
The other operations could be optimized as much as oovveerr
is.
_R_l_e_c_o_m_p assumes both input files have the same number of
channels.
1 December 20, 1986 2