环境：Win7 64位 + Matlab R2010a
本次分析的函数为im2uint8，这个函数在图像处理中要用到，主要把图像数据类转换到uint8
uint8函数有效的输入的图像数据类为：logical，uint8，uint16，double，single和int16
- 如果输入类型为logical，则输入数据为1时转换为255，数据为0还是0。
- 如果输入类型为uint8，则输入数据保持不变。
- 如果输入类型为uint16，则输入数据除以255，然后四舍五入。
- 如果输入数据为double，则输入数据乘以255，小于0的数设置为0，大于255的数设置为255，其他的数四舍五入。
- 如果输入数据为single，处理方式和double一样。
- 如果输入数据为int16，处理暂且没弄清楚。
上面输出的结果根据Matlab提供的M文件进行分析，M文件如下
function u = im2uint8(varargin)
%IM2UINT8 Convert image to 8-bit unsigned integers.
% IM2UINT8 takes an image as input, and returns an image of class uint8. If
% the input image is of class uint8, the output image is identical to it. If
% the input image is not uint8, IM2UINT8 returns the equivalent image of class
% uint8, rescaling or offsetting the data as necessary.
%
% I2 = IM2UINT8(I1) converts the intensity image I1 to uint8, rescaling the
% data if necessary.
%
% RGB2 = IM2UINT8(RGB1) converts the truecolor image RGB1 to uint8, rescaling
% the data if necessary.
%
% I = IM2UINT8(BW) converts the binary image BW to a uint8 intensity image,
% changing one-valued elements to 255.
%
% X2 = IM2UINT8(X1,'indexed') converts the indexed image X1 to uint8,
% offsetting the data if necessary. Note that it is not always possible to
% convert an indexed image to uint8. If X1 is double, then the maximum value
% of X1 must be 256 or less. If X1 is uint16, the maximum value of X1 must be
% 255 or less.
%
% Class Support
% -------------
% Intensity and truecolor images can be uint8, uint16, double, logical,
% single, or int16. Indexed images can be uint8, uint16, double or
% logical. Binary input images must be logical. The output image is uint8.
%
% Example
% -------
% I1 = reshape(uint16(linspace(0,65535,25)),[5 5])
% I2 = im2uint8(I1)
%
% See also IM2DOUBLE, IM2INT16, IM2SINGLE, IM2UINT16, UINT8. % Copyright 1993-2004 The MathWorks, Inc.
% $Revision: 1.20.4.5 $ $Date: 2005/11/15 00:58:23 $ iptchecknargin(1,2,nargin,mfilename); img = varargin{1};
iptcheckinput(img,{'double','logical','uint8','uint16','single','int16'}, ...
{'nonsparse'},mfilename,'Image',1); if nargin == 2
typestr = varargin{2};
iptcheckstrs(typestr,{'indexed'},mfilename,'type',2);
end % 如果图像类型为uint8,则不改变
if isa(img, 'uint8')
u = img; % 如果图像类型为logical(二值),则将1转为255和0仍然为0
elseif isa(img, 'logical')
u=uint8(img);
u(img)=255; else %double, single, uint16, or int16
if nargin == 1 %输入参数个数为1
if isa(img, 'int16') %如果图像类型为int16,则将其转为int16--这是因为grayto8函数不支持int16
img = int16touint16(img);
end % intensity image; call MEX-file
u = grayto8(img); else %输入参数个数为2
if isa(img, 'int16')
eid = sprintf('Images:%s:invalidIndexedImage',mfilename);
msg1 = 'An indexed image can be uint8, uint16, double, single, or ';
msg2 = 'logical.';
error(eid,'%s %s',msg1, msg2); elseif isa(img, 'uint16')
if (max(img(:)) > 255)
msg = 'Too many colors for 8-bit integer storage.';
eid = sprintf('Images:%s:tooManyColorsFor8bitStorage',mfilename);
error(eid,msg);
else
u = uint8(img);
end else %double or single 值范围1-256
if max(img(:)) >= 257
msg = 'Too many colors for 8-bit integer storage.';
eid = sprintf('Images:%s:tooManyColorsFor8bitStorage',mfilename);
error(eid,msg);
elseif min(img(:)) < 1
msg = 'Invalid indexed image: an index was less than 1.';
eid = sprintf('Images:%s:invalidIndexedImage',mfilename);
error(eid,msg);
else
u = uint8(img-1);
end
end
end
end
这其中有个grayto8函数，这个函数在MEX文件中，具体源代码google找了一份，注释了一下
/*
* GRAYTO8 MEX-file
*
* B = GRAYTO8(A) converts the double array A to uint8 by scaling A by 255
* and then rounding. NaN's in A are converted to 0. Values in A greater
* than 1.0 are converted to 255; values less than 0.0 are converted to 0.
*
* B = GRAYTO8(A) converts the uint16 array A by scaling the elements of A
* 255/65535, rounding, and then casting to uint8.
*
* Copyright 1993-2007 The MathWorks, Inc.
*
*/ #include "mex.h"
#include "math.h"
#include "mwsize.h" // double类型转换
void ConvertFromDouble(double *pr, uint8_T *qr, mwSize numElements) {
mwSize k;
double val; for (k = ; k < numElements; k++) {
val = *pr++;
if (mxIsNaN(val)) {
*qr++ = ;
}
else {
val = val * 255.0 + 0.5; // 数据加0.5是为了四舍五入需要 例如 uint8(245.1+0.5)=245 和 uint8(245.6+0.5)=246
if (val > 255.0) val = 255.0;
if (val < 0.0) val = 0.0;
*qr++ = (uint8_T) val;
}
}
} // single类型转换(处理方式和double类型一样)
void ConvertFromSingle(float *pr, uint8_T *qr, mwSize numElements) {
mwSize k;
float val; for (k = ; k < numElements; k++) {
val = *pr++;
if (mxIsNaN(val))
*qr++ = ;
else {
val = val * 255.0f + 0.5f;
if (val > 255.0) val = 255.0;
if (val < 0.0) val = 0.0;
*qr++ = (uint8_T) val;
}
}
} // uint16类型转换
void ConvertFromUint16(uint16_T *pr, uint8_T *qr, mwSize numElements) {
mwSize k;
/*
除以257的原因来自于uint16数字映射到uint8
例如数字x转换到uint8
y = x*255/65535
由于257 = 65535/255，则y=x/257
*/
double factor = 1.0 / 257.0; for (k = ; k < numElements; k++)
*qr++ = (uint8_T) ( (double) (*pr++) * factor + 0.5 );
} void ValidateInputs(int nrhs, const mxArray *prhs[]) {
if (nrhs < ) {
mexErrMsgIdAndTxt("Images:grayto8:tooFewInputs",
"%s","Too few input arguments.");
}
if (nrhs > ) {
mexErrMsgIdAndTxt("Images:grayto8:tooManyInputs",
"%s","Too many input arguments.");
}
if (!mxIsDouble(prhs[]) && !mxIsUint16(prhs[]) && !mxIsSingle(prhs[])) {
mexErrMsgIdAndTxt("Images:grayto8:invalidType",
"%s","Input must be double, single, or uint16.");
}
if (mxIsComplex(prhs[])) {
mexWarnMsgIdAndTxt("Images:grayto8:ignoringImaginaryPartOfInput",
"%s","Ignoring imaginary part of input.");
}
} void mexFunction(int nlhs, mxArray *plhs[], int nrhs, const mxArray *prhs[]) {
uint8_T *qr; (void) nlhs; /* unused parameter */ ValidateInputs(nrhs, prhs); plhs[] = mxCreateNumericArray(mxGetNumberOfDimensions(prhs[]),
mxGetDimensions(prhs[]),
mxUINT8_CLASS, mxREAL);
qr = (uint8_T *) mxGetData(plhs[]); if (mxIsDouble(prhs[]))
ConvertFromDouble((double *) mxGetData(prhs[]), qr, mxGetNumberOfElements(prhs[])); else if (mxIsUint16(prhs[]))
ConvertFromUint16((uint16_T *) mxGetData(prhs[]), qr, mxGetNumberOfElements(prhs[])); else
ConvertFromSingle((float *) mxGetData(prhs[]), qr, mxGetNumberOfElements(prhs[])); }