Computer graphics processing and selective visual display system – Computer graphics processing – Attributes
Reexamination Certificate
2001-02-12
2004-05-25
Luu, Matthew (Department: 2672)
Computer graphics processing and selective visual display system
Computer graphics processing
Attributes
Reexamination Certificate
active
06741261
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to the rendering of graphical elements into raster pixel images and, in particular, to the efficient rendering of such elements to pixel image data using alpha-channel compositing. The invention also relates to a computer program product including a computer readable medium having recorded thereon a computer program for the efficient rendering of graphical elements to pixel image data using alpha-channel compositing.
BACKGROUND ART
Computer generated images are typically made up of many differing components or graphical elements which are rendered and composited together to create a final image. In recent times, “alpha values” have been used in order to assist in the rendering of images. The colour associated with each element in an image is given an alpha value representing the contribution of the element to a final pixel value. An element with an alpha value of zero is typically understood to be fully transparent and an element with an alpha value of one is typically understood to be fully opaque. The alpha value is typically stored alongside each instance of a colour. Therefore, along with the RGB (Red, Green, Blue) values of a pixel, there is an alpha value representing the coverage of the pixel. The collection of all of the alpha values for an image is often called an “alpha channel” (also known as a “matte”, an “opacity channel”, or simply “opacity”).
The combining of source colour and opacity with destination colour and opacity will now be described with reference to FIG.
1
. As seen in
FIG. 1
, when a source pixel
100
and destination pixel
101
are composited together, the pixels
100
,
101
are conceptually overlaid, so that they anti-correlate to give four regions (i.e. three non-transparent regions
103
,
105
,
107
and one transparent region
111
) in the combined resultant pixel
109
. The three non-transparent regions
103
,
105
and
107
, can be defined as SOUTD (ie. source outside destination), SROPD (i.e. source intersect destination) and DOUTS (i.e. destination outside source), respectively. The colour value of each of these three regions is calculated conceptually independently. The source outside destination region
103
takes it's colour directly from the source colour. The destination outside source region takes it's colour directly from the destination colour. The source intersect destination region
105
takes it's colour from a combination of the source and destination colour. Each of the three regions
103
,
105
and
107
contributes a proportion of it's colour to the final colour of a resultant pixel according to the opacity value of that particular region. The opacity contribution from each of the three regions
103
,
105
,
107
can be defined as below, where &agr; represents the opacity value between 0 and 1 for the ‘src’ (i.e. source) and ‘dest’ (i.e. destination) pixel:
SOUTD=&agr;src(1−&agr;
dest
); (1)
DOUTS=&agr;dest(1
−&agr;src
); and (2)
SROPD=&agr;src&agr;dest. (3)
The process of combining source and destination colour is termed a raster operation and is one of a set of functions specified by raster operation code used in conjunction with many alpha-compositing systems employed by computer graphics systems. The alpha value, &agr;, for a particular pixel is typically represented as an 8 bit word in the range 0 to 255, representing the range between zero and one (i.e. normalised). As a result, the product of the multiplication of the source and destination pixels has to be ‘range corrected’ (i.e. divided by 255) to return to an 8-bit word representing the range between 0 and 1 for the resultant pixel.
FIG. 2
is a schematic diagram of a prior art alpha-compositing module
200
employed by a computer graphics system in order to combine source colour and opacity with destination colour and opacity. The module
200
comprises an opacity combination system
202
and a colour combination system
205
. The opacity combination system
202
and the colour combination system
205
calculate the resultant pixel opacity and colour values, respectively. The C
src
, C
dest
and C
result
operands represent the colour of the source, destination and resultant (i.e. ‘result’) pixels, respectively. The &agr;
src
, &agr;
dest
and &agr;
result
operands represent the opacity value of the source, destination and resultant (i.e. ‘result’) pixels, respectively. The contributions from the three regions, SOUTD, SROPD and DOUTS due to the compositing of the source and destination pixels, are controlled by three opacity flags, USE_SOUTD, USE_SROPD, and USE_DOUTS, respectively. If the flag for a region is set, there is some contribution from that region, otherwise, the contribution from that particular region is zero.
The module
200
of
FIG. 2
uses an 8-bit word to represent the alpha value for a particular pixel. Therefore, the opacity contribution from each region has to be range corrected by being divided by the number 255, using dividers
210
,
220
and
230
. The resultant opacity is calculated by summing the three range corrected opacity values for the three regions SOUTD, SROPD and DOUTS, using a summer
201
. The resultant colour C
result
is divided by the resultant opacity, using a divider
240
. As conceptually seen from
FIG. 2
, there is a long processing path from the initial colour and opacity operands to the resultant colour and opacity and typically the destination operand is taken from a compositing stack which contains the result of a previous operation. Therefore, it is not possible to pipeline such an operation and simplifying the data path while maintaining the precision is very important.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to substantially overcome, or at least ameliorate, one or more disadvantages of existing arrangements.
According to one aspect of the present invention there is provided an apparatus for compositing first and second pixel values, said apparatus comprising:
logic means for calculating combined range uncorrected colour and opacity values using opacity and colour values for each of said first and second pixel values;
selection means for selecting a reciprocal range uncorrected opacity value from a plurality of predetermined reciprocal range uncorrected opacity values using said combined range uncorrected opacity value;
multiplier means for multiplying said reciprocal range uncorrected opacity value with said combined range uncorrected colour value to determine a final range corrected resultant colour value for a third pixel; and
divider means for dividing said combined range uncorrected opacity value by a predetermined number wherein said divider means is configured to calculate a final range corrected resultant opacity value for said third pixel.
According to another aspect of the present invention there is provided apparatus for compositing first and second pixel values, said apparatus comprising:
a plurality of inputs for receiving opacity and colour values for each of said first and second pixels;
logic means for calculating combined range uncorrected colour and opacity values using said opacity and colour values;
selection means for selecting a reciprocal range uncorrected opacity value from a plurality of predetermined reciprocal range uncorrected opacity values using said combined opacity value;
multiplier means for multiplying said reciprocal range uncorrected opacity value with said combined range uncorrected colour value to produce a final range corrected resultant colour value for a third pixel; and
divider means for dividing said range uncorrected combined opacity value by a predetermined number, wherein said divider means is configured to calculate a final range corrected resultant opacity value for said third pixel.
According to still another aspect of the present invention there is provided a method of compositing first and second pixel values, said comprising the steps of:
inputting opacity and colour valu
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