Altera Corporation
1
RGB2YCrCb &
YCrCb2RGB
Color Space Converters
February 1997, ver. 1
Data Sheet
A-DS-RGB-01
Features
s
RGB2YCrCb
and
YCrCb2RGB
MegaCore functions converting digital
video colors to television broadcast signal colors and vice versa
s
Optimized for the FLEX
10K and FLEX 8000 device architectures
s
24-bit RGB2YCrCb and YCrCb2RGB color space conversion
s
High-speed operation (100 MHz)
s
Supported by MAX+PLUS
II software
s
Full precision outputs: rounding and saturation can be performed
with the
round
and
saturate
reference designs
s
Useful for a variety of applications, including image filtering,
machine vision, and digital video
General
Description
The
RGB2YCrCb
and
YCrCb2RGB
color space converter functions are useful
for a number of image processing and filtering operations. The
RGB2YCrCb
function converts red-green-blue (RGB) color space to the
YCrCb (intensity-color red-color blue) color space; the
YCrCb2RGB
function performs the inverse operation.
Figure 1
shows the symbols for
the
RBG2YCrCb
and
YCrCb2RGB
functions.
Figure 1. RGB2YCrCb & YCrCb2RGB Symbols
RGB2YCRCB
R[7..0]
G[7..0]
B[7..0]
CLK
ACLR
Y[15..0]
CB[14..0]
CR[14..0]
YCRCB2RGB
Y[7..0]
CB[7..0]
CR[7..0]
CLK
ACLR
R[16..0]
G[16..0]
B[16..0]
2
Altera Corporation
RGB2YCrCb & YCrCb2RGB Color Space Converters Data Sheet
Function Prototypes
The Altera Hardware Description Language (AHDL) Function
Prototype of the
RGB2YCrCb
function is shown below:
FUNCTION RGB2YCrCb (r[7..0], g[7..0], b[7..0], clk, aclk)
RETURNS (y[15..0], cb[14..0], cr[14..0]);
The AHDL Function Prototype of the
YCrCb2RGB
function is shown
below:
FUNCTION YCrCb2RGB (y[7..0], cb[7..0], cr[7..0], clk, aclr)
RETURNS (r[16..0], g[16..0], b[16..0])
;
Ports
Table 1
describes the input and output ports for the
RGB2YCrCb
function.
Note:
(1)
The notation
x
.
y
represents
x
bits to the left of the radix point, and
y
bits to
the right.
Table 1. RGB2YCrCb Ports
Name
Type
Numbering
Representation
Note (1)
Description
r[7..0]
Input
Unsigned 8.0
Red data
g[7..0]
Input
Unsigned 8.0
Green data
b[7..0]
Input
Unsigned 8.0
Blue data
clk
Input
Clock
aclr
Input
Asynchronous clear
y[15..0]
Output
Unsigned 8.8
Intensity data
cb[14..0]
Output
Unsigned 8.7
Color blue data
cr[14..0]
Output
Unsigned 8.7
Color red data
Altera Corporation
3
RGB2YCrCb & YCrCb2RGB Color Space Converters Data Sheet
Table 2
describes the input and output ports for the
YCrCb2RGB
function.
Note:
(1)
Signed refers to two's compliment signed. The notation
x
.
y
represents
x
bits
to the left of the radix point, and
y
bits to the right. The RGB values may be
negative if the YCrCb values deviate outside the nominal range. For more
control over the values, rounding and saturation functions are available.
Functional
Description
The
RGB2YCrCb
function uses the following equations when
converting gamma-corrected RGB data to YCrCb data:
Y' = 0.257R' + 0.504G' + 0.098B' + 16
Cr = 0.439R' 0.368G' 0.071B' + 128
Cb = 0.148R' 0.291G' + 0.439B' + 128
The
YCrCb2RGB
function uses the following equations when
converting YCrCb data to RGB data:
R' = 1.164(Y' 16) + 1.596(Cr 128)
G' = 1.164(Y' 16) 0.813(Cr 128) 0.392(Cb 128)
B' = 1.164(Y' 16) + 2.017(Cb 128)
1
Refer to
Video Demystified
for more information on these
equations (see
"References"
for details).
Because the inputs are multiplied by constant values, the look-up
table (LUT) architecture of the FLEX 10K and FLEX 8000 devices is
ideal for efficiently performing the conversion equations.
Table 2. YCrCb2RGB Ports
Name
Type
Numbering
Representation
Note (1)
Description
y[7..0]
Input
Unsigned 8.0
Intensity data
cb[7..0]
Input
Unsigned 8.0
Color blue data
cr[7..0]
Input
Unsigned 8.0
Color red data
clk
Input
Clock
aclr
Input
Asynchronous
clear
r[16..0]
Output
Signed 10.7
Red data
g[16..0]
Output
Signed 10.7
Green data
b[16..0]
Output
Signed 10.7
Blue data
4 Altera Corporation
RGB2YCrCb & YCrCb2RGB Color Space Converters Data Sheet
Some of the coefficients are smaller in magnitude, and therefore the
multiplier only has to be as large as the equivalent number of binary
digits in the coefficient. For example in the Y' equation, when
multiplying the 8-bit input (
b[7..0]
) by 0.098
10
(or 0.00011001
2
),
only a 5
8-bit multiplier is required instead of an 8
8 bit-
multiplier.
The
y
output of the
RGB2YCrCb
function is an unsigned 16-bit binary
number with 8 bits to the left of the radix point and 8 bits to the
right. To convert the
y
output to an 8-bit binary number, the value
must be rounded.
f
For more information on data word rounding, refer to
Functional
Specification 5 (round Data Word Rounder)
.
Figure 2
shows the block diagram of the Y' component of the
RGB2YCrCb
function. The Cr and Cb components of the conversion
are calculated in a similar manner.
Figure 2. RGB2YCrCb Converter Block Diagram
b[7..0]
8
8
g[7..0]
r[7..0]
8
0.504 0.098
7
5
0.257
8
13
15
16
y[15..0]
16
16
16
Altera Corporation 5
RGB2YCrCb & YCrCb2RGB Color Space Converters Data Sheet
Figure 3
shows the block diagram for the
YCrCb2RGB
function. The
r,
g
, and
b
outputs of the
YCrCb2RGB
function are 17-bit two's
compliment signed numbers with 10 bits to the left of the radix point
and 7 bits to the right. To convert the
r
,
g
, and
b
outputs to 8-bit
signed numbers, the data words should be saturated and rounded.
f
For more information on rounding, refer to
Functional Specification 5
(round Data Word Rounder)
; for more information on saturation, refer
to
Functional Specification 6 (saturate Data Word Saturator)
.
Figure 3. YCrCb2RGB Converter Block Diagram
y[7..0]
cb[7..0]
cr[7..0]
1.596
8
r[16..0]
g[16..0]
8
8
0.392
0.813
16
8
8
128
128
(y 16)
cb 128
1.164
1.164(y 16)
cr 128
8
b[16..0]
2.017
8
+
+
+
+
+
+
+
17
17
17
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