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RadialGradientPaintContext.javaAPI DocJava SE 6 API17313Tue Jun 10 00:25:18 BST 2008java.awt

RadialGradientPaintContext

public final class RadialGradientPaintContext extends MultipleGradientPaintContext
Provides the actual implementation for the RadialGradientPaint. This is where the pixel processing is done. A RadialGradienPaint only supports circular gradients, but it should be possible to scale the circle to look approximately elliptical, by means of a gradient transform passed into the RadialGradientPaint constructor.
author
Nicholas Talian, Vincent Hardy, Jim Graham, Jerry Evans

Fields Summary
private boolean
isSimpleFocus
True when (focus == center).
private boolean
isNonCyclic
True when (cycleMethod == NO_CYCLE).
private float
radius
Radius of the outermost circle defining the 100% gradient stop.
private float
centerX
Variables representing center and focus points.
private float
centerY
private float
focusX
private float
focusY
private float
radiusSq
Radius of the gradient circle squared.
private float
constA
Constant part of X, Y user space coordinates.
private float
constB
private float
gDeltaDelta
Constant second order delta for simple loop.
private float
trivial
This value represents the solution when focusX == X. It is called trivial because it is easier to calculate than the general case.
private static final float
SCALEBACK
Amount for offset when clamping focus.
private static final int
SQRT_LUT_SIZE
private static float[]
sqrtLut
Constructors Summary
RadialGradientPaintContext(RadialGradientPaint paint, ColorModel cm, Rectangle deviceBounds, Rectangle2D userBounds, AffineTransform t, RenderingHints hints, float cx, float cy, float r, float fx, float fy, float[] fractions, Color[] colors, java.awt.MultipleGradientPaint.CycleMethod cycleMethod, java.awt.MultipleGradientPaint.ColorSpaceType colorSpace)
Constructor for RadialGradientPaintContext.

param
paint the {@code RadialGradientPaint} from which this context is created
param
cm the {@code ColorModel} that receives the {@code Paint} data (this is used only as a hint)
param
deviceBounds the device space bounding box of the graphics primitive being rendered
param
userBounds the user space bounding box of the graphics primitive being rendered
param
t the {@code AffineTransform} from user space into device space (gradientTransform should be concatenated with this)
param
hints the hints that the context object uses to choose between rendering alternatives
param
cx the center X coordinate in user space of the circle defining the gradient. The last color of the gradient is mapped to the perimeter of this circle.
param
cy the center Y coordinate in user space of the circle defining the gradient. The last color of the gradient is mapped to the perimeter of this circle.
param
r the radius of the circle defining the extents of the color gradient
param
fx the X coordinate in user space to which the first color is mapped
param
fy the Y coordinate in user space to which the first color is mapped
param
fractions the fractions specifying the gradient distribution
param
colors the gradient colors
param
cycleMethod either NO_CYCLE, REFLECT, or REPEAT
param
colorSpace which colorspace to use for interpolation, either SRGB or LINEAR_RGB

    
                                                                                                                                                                                                                                                                                                                                                                                                                                         
     
                                
                                
                                
                                
                                
                                  
                                
                                  
                                
                                
                                
                                
                   
        super(paint, cm, deviceBounds, userBounds, t, hints,
              fractions, colors, cycleMethod, colorSpace);

        // copy some parameters
        centerX = cx;
        centerY = cy;        
        focusX = fx;
        focusY = fy;
        radius = r;
        
        this.isSimpleFocus = (focusX == centerX) && (focusY == centerY);
        this.isNonCyclic = (cycleMethod == CycleMethod.NO_CYCLE);
        
        // for use in the quadractic equation
        radiusSq = radius * radius;

        float dX = focusX - centerX;
        float dY = focusY - centerY;

        double distSq = (dX * dX) + (dY * dY);

        // test if distance from focus to center is greater than the radius
        if (distSq > radiusSq * SCALEBACK) {
            // clamp focus to radius
            float scalefactor = (float)Math.sqrt(radiusSq * SCALEBACK / distSq);
            dX = dX * scalefactor;
            dY = dY * scalefactor;
            focusX = centerX + dX;
            focusY = centerY + dY;
        }

        // calculate the solution to be used in the case where X == focusX
        // in cyclicCircularGradientFillRaster()
        trivial = (float)Math.sqrt(radiusSq - (dX * dX));

        // constant parts of X, Y user space coordinates 
        constA = a02 - centerX;
        constB = a12 - centerY;

        // constant second order delta for simple loop
        gDeltaDelta = 2 * ( a00 *  a00 +  a10 *  a10) / radiusSq;
    
Methods Summary
private voidcyclicCircularGradientFillRaster(int[] pixels, int off, int adjust, int x, int y, int w, int h)
Fill the raster, cycling the gradient colors when a point falls outside of the perimeter of the 100% stop circle. This calculation first computes the intersection point of the line from the focus through the current point in the raster, and the perimeter of the gradient circle. Then it determines the percentage distance of the current point along that line (focus is 0%, perimeter is 100%). Equation of a circle centered at (a,b) with radius r: (x-a)^2 + (y-b)^2 = r^2 Equation of a line with slope m and y-intercept b: y = mx + b Replacing y in the circle equation and solving using the quadratic formula produces the following set of equations. Constant factors have been extracted out of the inner loop.

     
        for (int i = 0; i < sqrtLut.length; i++) {
            sqrtLut[i] = (float) Math.sqrt(i / ((float) SQRT_LUT_SIZE));
        }
    
        // constant part of the C factor of the quadratic equation
        final double constC = 
            -radiusSq + (centerX * centerX) + (centerY * centerY);

        // coefficients of the quadratic equation (Ax^2 + Bx + C = 0)
        double A, B, C;

        // slope and y-intercept of the focus-perimeter line
        double slope, yintcpt;

        // intersection with circle X,Y coordinate
        double solutionX, solutionY;

        // constant parts of X, Y coordinates
        final float constX = (a00*x) + (a01*y) + a02;
        final float constY = (a10*x) + (a11*y) + a12;

        // constants in inner loop quadratic formula
        final float precalc2 =  2 * centerY;
        final float precalc3 = -2 * centerX;

        // value between 0 and 1 specifying position in the gradient
        float g;

        // determinant of quadratic formula (should always be > 0)
        float det;

        // sq distance from the current point to focus
        float currentToFocusSq;

        // sq distance from the intersect point to focus
        float intersectToFocusSq;

        // temp variables for change in X,Y squared
        float deltaXSq, deltaYSq;

        // used to index pixels array
        int indexer = off;

        // incremental index change for pixels array
        int pixInc = w+adjust;
        
        // for every row
        for (int j = 0; j < h; j++) {

            // user space point; these are constant from column to column
            float X = (a01*j) + constX;
            float Y = (a11*j) + constY;

            // for every column (inner loop begins here)
            for (int i = 0; i < w; i++) {
        
                if (X == focusX) {                   
                    // special case to avoid divide by zero
                    solutionX = focusX;
                    solutionY = centerY;
                    solutionY += (Y > focusY) ? trivial : -trivial;
                } else {    
                    // slope and y-intercept of the focus-perimeter line
                    slope = (Y - focusY) / (X - focusX);
                    yintcpt = Y - (slope * X);
                    
                    // use the quadratic formula to calculate the
                    // intersection point                  
                    A = (slope * slope) + 1; 
                    B = precalc3 + (-2 * slope * (centerY - yintcpt));
                    C = constC + (yintcpt* (yintcpt - precalc2));
                    
                    det = (float)Math.sqrt((B * B) - (4 * A * C));
                    solutionX = -B;
                    
                    // choose the positive or negative root depending
                    // on where the X coord lies with respect to the focus
                    solutionX += (X < focusX)? -det : det;
                    solutionX = solutionX / (2 * A); // divisor
                    solutionY = (slope * solutionX) + yintcpt;
                }                                    

                // Calculate the square of the distance from the current point
                // to the focus and the square of the distance from the
                // intersection point to the focus. Want the squares so we can
                // do 1 square root after division instead of 2 before.

                deltaXSq = X - focusX;
                deltaXSq = deltaXSq * deltaXSq;

                deltaYSq = Y - focusY;
                deltaYSq = deltaYSq * deltaYSq;

                currentToFocusSq = deltaXSq + deltaYSq;

                deltaXSq = (float)solutionX - focusX;
                deltaXSq = deltaXSq * deltaXSq;

                deltaYSq = (float)solutionY - focusY;
                deltaYSq = deltaYSq * deltaYSq;

                intersectToFocusSq = deltaXSq + deltaYSq;

                // get the percentage (0-1) of the current point along the 
                // focus-circumference line
                g = (float)Math.sqrt(currentToFocusSq / intersectToFocusSq);
                                              
                // store the color at this point
                pixels[indexer + i] = indexIntoGradientsArrays(g);

                // incremental change in X, Y
                X += a00;
                Y += a10;        
            } //end inner loop

            indexer += pixInc;
        } //end outer loop
    
protected voidfillRaster(int[] pixels, int off, int adjust, int x, int y, int w, int h)
Return a Raster containing the colors generated for the graphics operation.

param
x,y,w,h the area in device space for which colors are generated.

        if (isSimpleFocus && isNonCyclic && isSimpleLookup) {
            simpleNonCyclicFillRaster(pixels, off, adjust, x, y, w, h);
        } else {
            cyclicCircularGradientFillRaster(pixels, off, adjust, x, y, w, h);
        }
    
private voidsimpleNonCyclicFillRaster(int[] pixels, int off, int adjust, int x, int y, int w, int h)
This code works in the simplest of cases, where the focus == center point, the gradient is noncyclic, and the gradient lookup method is fast (single array index, no conversion necessary).

        /* We calculate sqrt(X^2 + Y^2) relative to the radius
         * size to get the fraction for the color to use.
         *
         * Each step along the scanline adds (a00, a10) to (X, Y).
         * If we precalculate:
         *   gRel = X^2+Y^2
         * for the start of the row, then for each step we need to
         * calculate:
         *   gRel' = (X+a00)^2 + (Y+a10)^2
         *         = X^2 + 2*X*a00 + a00^2 + Y^2 + 2*Y*a10 + a10^2
         *         = (X^2+Y^2) + 2*(X*a00+Y*a10) + (a00^2+a10^2)
         *         = gRel + 2*(X*a00+Y*a10) + (a00^2+a10^2)
         *         = gRel + 2*DP + SD
         * (where DP = dot product between X,Y and a00,a10
         *  and   SD = dot product square of the delta vector)
         * For the step after that we get:
         *   gRel'' = (X+2*a00)^2 + (Y+2*a10)^2
         *          = X^2 + 4*X*a00 + 4*a00^2 + Y^2 + 4*Y*a10 + 4*a10^2
         *          = (X^2+Y^2) + 4*(X*a00+Y*a10) + 4*(a00^2+a10^2)
         *          = gRel  + 4*DP + 4*SD
         *          = gRel' + 2*DP + 3*SD
         * The increment changed by:
         *     (gRel'' - gRel') - (gRel' - gRel)
         *   = (2*DP + 3*SD) - (2*DP + SD)
         *   = 2*SD
         * Note that this value depends only on the (inverse of the)
         * transformation matrix and so is a constant for the loop.
         * To make this all relative to the unit circle, we need to
         * divide all values as follows:
         *   [XY] /= radius
         *   gRel /= radiusSq
         *   DP   /= radiusSq
         *   SD   /= radiusSq
         */
        // coordinates of UL corner in "user space" relative to center
        float rowX = (a00*x) + (a01*y) + constA;
        float rowY = (a10*x) + (a11*y) + constB;

        // second order delta calculated in constructor
        float gDeltaDelta = this.gDeltaDelta;

        // adjust is (scan-w) of pixels array, we need (scan)
        adjust += w;

        // rgb of the 1.0 color used when the distance exceeds gradient radius
        int rgbclip = gradient[fastGradientArraySize];

        for (int j = 0; j < h; j++) {
            // these values depend on the coordinates of the start of the row
            float gRel   =      (rowX * rowX + rowY * rowY) / radiusSq;
            float gDelta = (2 * ( a00 * rowX +  a10 * rowY) / radiusSq +
                            gDeltaDelta/2);

            /* Use optimized loops for any cases where gRel >= 1.
             * We do not need to calculate sqrt(gRel) for these
             * values since sqrt(N>=1) == (M>=1).
             * Note that gRel follows a parabola which can only be < 1
             * for a small region around the center on each scanline. In
             * particular:
             *   gDeltaDelta is always positive
             *   gDelta is <0 until it crosses the midpoint, then >0
             * To the left and right of that region, it will always be
             * >=1 out to infinity, so we can process the line in 3
             * regions:
             *   out to the left  - quick fill until gRel < 1, updating gRel
             *   in the heart     - slow fraction=sqrt fill while gRel < 1
             *   out to the right - quick fill rest of scanline, ignore gRel
             */
            int i = 0;
            // Quick fill for "out to the left"
            while (i < w && gRel >= 1.0f) {
                pixels[off + i] = rgbclip;
                gRel += gDelta;
                gDelta += gDeltaDelta;
                i++;
            }
            // Slow fill for "in the heart"
            while (i < w && gRel < 1.0f) {
                int gIndex;

                if (gRel <= 0) {
                    gIndex = 0;
                } else {
                    float fIndex = gRel * SQRT_LUT_SIZE;
                    int iIndex = (int) (fIndex);
                    float s0 = sqrtLut[iIndex];
                    float s1 = sqrtLut[iIndex+1] - s0;
                    fIndex = s0 + (fIndex - iIndex) * s1;
                    gIndex = (int) (fIndex * fastGradientArraySize);
                }

                // store the color at this point
                pixels[off + i] = gradient[gIndex];

                // incremental calculation
                gRel += gDelta;
                gDelta += gDeltaDelta;
                i++;
            }
            // Quick fill to end of line for "out to the right"
            while (i < w) {
                pixels[off + i] = rgbclip;
                i++;
            }

            off += adjust;
            rowX += a01;
            rowY += a11;
        }