如何在objective-c中绘制色轮 [英] How to draw a color wheel in objective-c

问题描述

我正在尝试为iPhone绘制色轮，但我不能让渐变围绕一个点旋转。我试图使用渐变，但objective-c提供了一个线性渐变，它绘制了一条直线的渐变，如下所示：

I am trying to draw a color wheel for an iPhone, but I can't get the gradient to rotate around a point. I am trying to use gradients but objective-c provides a linear gradient, which draws a gradient in a straight line like this:

和径向渐变，它从点开始绘制渐变并向所有方向辐射，如下所示：

and a radial gradient, which draws the gradient starting at point and radiating out in all directions like this:

我想绘制一个旋转的线性渐变围绕这样的观点：

I want to draw a linear gradient that rotates around a point like this:

推荐答案

以下内容在UIView子类中绘制HSL色轮。它通过为每个像素计算正确的颜色值来生成位图来实现这一点。这并不是你想要做的（看起来它只是在恒定亮度/饱和度的圆圈中变化），但你应该能够根据你的需要进行调整。

The following draws an HSL color wheel in a UIView subclass. It does this by generating a bitmap by computing, for each pixel, the correct color value. This is not exactly what you're trying to do (looks like it's just hue varies in the circle with a constant luminance/saturation), but you should be able to adapt it for your needs.

请注意，这可能没有最佳性能，但它应该让您入门。此外，您可以使用 getColorWheelValue（）来处理用户输入（给定坐标处的点击/触摸）。

Note that this may not have optimal performance, but it should get you started. Also, you can use getColorWheelValue() to handle user input (clicks/touches at a given coordinate).

- (void)drawRect:(CGRect)rect
{
    int dim = self.bounds.size.width; // should always be square.
    bitmapData = CFDataCreateMutable(NULL, 0);
    CFDataSetLength(bitmapData, dim * dim * 4);
    generateColorWheelBitmap(CFDataGetMutableBytePtr(bitmapData), dim, luminance);
    UIImage *image = createUIImageWithRGBAData(bitmapData, self.bounds.size.width, self.bounds.size.height);
    CFRelease(bitmapData);
    [image drawAtPoint:CGPointZero];
    [image release];
}

void generateColorWheelBitmap(UInt8 *bitmap, int widthHeight, float l)
{
    // I think maybe you can do 1/3 of the pie, then do something smart to generate the other two parts, but for now we'll brute force it.
    for (int y = 0; y < widthHeight; y++)
    {
        for (int x = 0; x < widthHeight; x++)
        {
            float h, s, r, g, b, a;
            getColorWheelValue(widthHeight, x, y, &h, &s);
            if (s < 1.0)
            {
                // Antialias the edge of the circle.
                if (s > 0.99) a = (1.0 - s) * 100;
                else a = 1.0;

                HSL2RGB(h, s, l, &r, &g, &b);
            }
            else
            {
                r = g = b = a = 0.0f;
            }

            int i = 4 * (x + y * widthHeight);
            bitmap[i] = r * 0xff;
            bitmap[i+1] = g * 0xff;
            bitmap[i+2] = b * 0xff;
            bitmap[i+3] = a * 0xff;
        }
    }
}

void getColorWheelValue(int widthHeight, int x, int y, float *outH, float *outS)
{
    int c = widthHeight / 2;
    float dx = (float)(x - c) / c;
    float dy = (float)(y - c) / c;
    float d = sqrtf((float)(dx*dx + dy*dy));
    *outS = d;
    *outH = acosf((float)dx / d) / M_PI / 2.0f;
    if (dy < 0) *outH = 1.0 - *outH;
}

UIImage *createUIImageWithRGBAData(CFDataRef data, int width, int height)
{
    CGDataProviderRef dataProvider = CGDataProviderCreateWithCFData(data);
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    CGImageRef imageRef = CGImageCreate(width, height, 8, 32, width * 4, colorSpace, kCGImageAlphaLast, dataProvider, NULL, 0, kCGRenderingIntentDefault);
    UIImage *image = [[UIImage alloc] initWithCGImage:imageRef];
    CGDataProviderRelease(dataProvider);
    CGColorSpaceRelease(colorSpace);
    CGImageRelease(imageRef);
    return image;
}

// Adapted from Apple sample code.  See http://en.wikipedia.org/wiki/HSV_color_space#Comparison_of_HSL_and_HSV
void HSL2RGB(float h, float s, float l, float* outR, float* outG, float* outB)
{
    float temp1, temp2;
    float temp[3];
    int i;

    // Check for saturation. If there isn't any just return the luminance value for each, which results in gray.
    if(s == 0.0)
    {
        *outR = l;
        *outG = l;
        *outB = l;
        return;
    }

    // Test for luminance and compute temporary values based on luminance and saturation 
    if(l < 0.5)
        temp2 = l * (1.0 + s);
    else
        temp2 = l + s - l * s;
    temp1 = 2.0 * l - temp2;

    // Compute intermediate values based on hue
    temp[0] = h + 1.0 / 3.0;
    temp[1] = h;
    temp[2] = h - 1.0 / 3.0;

    for(i = 0; i < 3; ++i)
    {
        // Adjust the range
        if(temp[i] < 0.0)
            temp[i] += 1.0;
        if(temp[i] > 1.0)
            temp[i] -= 1.0;


        if(6.0 * temp[i] < 1.0)
            temp[i] = temp1 + (temp2 - temp1) * 6.0 * temp[i];
        else {
            if(2.0 * temp[i] < 1.0)
                temp[i] = temp2;
            else {
                if(3.0 * temp[i] < 2.0)
                    temp[i] = temp1 + (temp2 - temp1) * ((2.0 / 3.0) - temp[i]) * 6.0;
                else
                    temp[i] = temp1;
            }
        }
    }

    // Assign temporary values to R, G, B
    *outR = temp[0];
    *outG = temp[1];
    *outB = temp[2];
}

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