iOS 彩色图片置灰,灰度图片


理论依据:
所谓颜色或灰度级指黑白显示器中显示像素点的亮暗差别,在彩色显示器中表现为颜色的不同,灰度级越多,图像层次越清楚逼真。灰度级取决于每个像素对应的刷新
存储单元的位数和显示器本身的性能。如每个象素的颜色用16位
二进制数表示,我们就叫它16位图,它可以表达2的16次方即65536种颜色。如每一个象素采用24位二进制数表示,我们就叫它24位图,它可以表达2的24次方即16777216种颜色。
灰度就是没有色彩,RGB色彩分量全部相等。如果是一个二值灰度图象,它的象素值只能为0或1,我们说它的灰度级为2。用个例子来说明吧:一个256级灰度的图象,如果RGB三个量相同时,如:RGB(100,100,100)就代表灰度为100,RGB(50,50,50)代表灰度为50。
彩色图象的灰度其实在转化为黑白图像后的像素值(是一种广义的提法),转化的方法看应用的领域而定,一般按加权的方法转换,R, G,B 的比一般为3:6:1。

任何颜色都由红、绿、蓝
三原色组成,假如原来某点的颜色为RGB(R,G,B),那么,我们可以通过下面几种方法,将其转换为灰度:

1.
浮点
算法:Gray=R*0.3+G*0.59+B*0.11
2.整数方法:Gray=(R*30+G*59+B*11)/100
3.移位方法:Gray =(R*77+G*151+B*28)>>8;
4.
平均值法:Gray=(R+G+B)/3;
5.仅取绿色:Gray=G;
通过上述任一种方法求得Gray后,将原来的RGB(R,G,B)中的R,G,B统一用Gray替换,形成新的颜色RGB(Gray,Gray,Gray),用它替换原来的RGB(R,G,B)就是灰度图了。
综上所述,就有了面两种方法,其实也就算一种吧,仅供大家参考。


方法1
//UIImage:
去色功能的实现(图片灰色显示)
– (
UIImage
*)grayImage:(
UIImage
*)sourceImage {
   

int
width = sourceImage.
size
.
width
;
   

int
height = sourceImage.
size
.
height
;
   

CGColorSpaceRef
colorSpace =
CGColorSpaceCreateDeviceGray
();
   

CGContextRef
context =
CGBitmapContextCreate
(
nil
, width, height,
8
,
0
, colorSpace,
kCGImageAlphaNone
);
   

CGColorSpaceRelease
(colorSpace);
   

if
(context ==
NULL
) {
       

return
nil
;
    }
   

CGContextDrawImage
(context,
CGRectMake
(
0
,
0
, width, height), sourceImage.
CGImage
);
   

UIImage
*grayImage = [
UIImage
imageWithCGImage
:
CGBitmapContextCreateImage
(context)];
   

CGContextRelease
(context);
   

return
grayImage;

}

方法2
– (
UIImage
*)grayscaleImageForImage:(
UIImage
*)image {
   

// Adapted from this thread: http://stackoverflow.com/questions/1298867/convert-image-to-grayscale

   

const
int
RED =
1
;
   

const
int
GREEN =
2
;
   

const
int
BLUE =
3
;
   
   

// Create image rectangle with current image width/height

   

CGRect
imageRect =
CGRectMake
(
0
,
0
, image.
size
.
width
* image.
scale
, image.
size
.
height
* image.
scale
);
   
   

int
width = imageRect.
size
.
width
;
   

int
height = imageRect.
size
.
height
;
   
   

// the pixels will be painted to this array

   

uint32_t
*pixels = (
uint32_t
*)
malloc
(width * height *
sizeof
(
uint32_t
));
   
   

// clear the pixels so any transparency is preserved

   

memset
(pixels,
0
, width * height *
sizeof
(
uint32_t
));
   
   

CGColorSpaceRef
colorSpace =
CGColorSpaceCreateDeviceRGB
();
   
   

// create a context with RGBA pixels

   

CGContextRef
context =
CGBitmapContextCreate
(pixels, width, height,
8
, width *
sizeof
(
uint32_t
), colorSpace,
kCGBitmapByteOrder32Little
|
kCGImageAlphaPremultipliedLast
);
   
   

// paint the bitmap to our context which will fill in the pixels array

   

CGContextDrawImage
(context,
CGRectMake
(
0
,
0
, width, height), [image
CGImage
]);
   
   

for
(
int
y =
0
; y < height; y++) {
       

for
(
int
x =
0
; x < width; x++) {
           

uint8_t
*rgbaPixel = (
uint8_t
*) &pixels[y * width + x];
           
           

// convert to grayscale using recommended method: http://en.wikipedia.org/wiki/Grayscale#Converting_color_to_grayscale

           

uint32_t
gray =
0.3
* rgbaPixel[RED] +
0.59
* rgbaPixel[GREEN] +
0.11
* rgbaPixel[BLUE];
           
           

// set the pixels to gray

            rgbaPixel[RED] = gray;
            rgbaPixel[GREEN] = gray;
            rgbaPixel[BLUE] = gray;
        }
    }
   
   

// create a new CGImageRef from our context with the modified pixels

   

CGImageRef
imageRef =
CGBitmapContextCreateImage
(context);
   
   

// we’re done with the context, color space, and pixels

   

CGContextRelease
(context);
   

CGColorSpaceRelease
(colorSpace);
   

free
(pixels);
   
   

// make a new UIImage to return

   

UIImage
*resultUIImage = [
UIImage
imageWithCGImage
:imageRef
scale
:image.
scale
orientation
:
UIImageOrientationUp
];
   
   

// we’re done with image now too

   

CGImageRelease
(imageRef);
   
   

return
resultUIImage;
}