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function newSourceMat=AlphaSourceTerm(sourceFactorStr, sourceMat, destinationMat)
% newSourceMat=AlphaSourceTerm(sourceFactorStr, sourceMat, destinationMat)
%
% AlphaSourceTerm simuluates the step of multplying an alpha factor with the
% source image in OpenGL alpha blending.
%
% Apply an OpenGL source factor rule such as 'GL_ZERO' to the image matrix
% 'sourceMat', returing the dot product of sourceMat and the alpha factor
% selected by string 'sourceFactorStr'.
%
% The destination image matrix 'destinationMat' is required because source
% factor strings may select alpha values from the destination.
%
% AlphaSourceTerm calculates with double-precision (64-bit) floating point
% arithmatitic whereas the precision of an OpenGL renderer which it
% simulates is unspecified, except that OpenGL guarantees perfect precision
% for alpha values 0 and 1 (255 via Screen). Comparison of AlphaSourceTerm
% with the OpenGL renderer shows that alpha multiplicaion discards up to
% one bit of precision.
%
% see also: AlphaDestinationProduct PsychAlphaBlending
% HISTORY
%
% mm/dd/yy
%
% 2/11/05 awi wrote it.
sourceMatDims=size(sourceMat);
destinationMatDims=size(destinationMat);
if length(sourceMatDims) ~= 3 | sourceMatDims(3) ~= 4
error('Argument ''sourceMat'' must be a matrix with dimensions [x,y,4]');
end
if length(destinationMatDims) ~= 3 | destinationMatDims(3) ~= 4
error('Argument ''destinationMat'' must be a matrix with dimensions [x,y,4]');
end
sourceMatAlphaPlane=sourceMat(:,:,4);
destinationMatAlphaPlane=destinationMat(:,:,4);
if strcmp(sourceFactorStr, 'GL_ZERO')
newSourceMat=AlphaTimes(sourceMat, 0);
elseif strcmp(sourceFactorStr, 'GL_ONE')
newSourceMat=AlphaTimes(sourceMat, 255);
elseif strcmp(sourceFactorStr, 'GL_DST_COLOR')
newSourceMat=AlphaTimes(sourceMat, destinationMat);
elseif strcmp(sourceFactorStr, 'GL_ONE_MINUS_DST_COLOR')
newSourceMat=AlphaTimes(sourceMat, 255 - destinationMat);
elseif strcmp(sourceFactorStr, 'GL_SRC_ALPHA');
newSourceMat=AlphaTimes(sourceMat, sourceMat(:,:,4));
elseif strcmp(sourceFactorStr, 'GL_ONE_MINUS_SRC_ALPHA')
newSourceMat=AlphaTimes(sourceMat, 255 - sourceMat(:,:,4));
elseif strcmp(sourceFactorStr, 'GL_DST_ALPHA');
newSourceMat=AlphaTimes(sourceMat, destinationMat(:,:,4));
elseif strcmp(sourceFactorStr, 'GL_ONE_MINUS_DST_ALPHA');
newSourceMat=AlphaTimes(sourceMat, 255-destinationMat(:,:,4));
elseif strcmp(sourceFactorStr, 'GL_SRC_ALPHA_SATURATE');
minAlphaPlane=min(sourceMat(:,:,4), 255-destinationMat(:,:,4));
newSourceMat=AlphaTimes(sourceMat, minAlphaPlane);
newSourceMat(:,:,4)=ones(sourceMatDims(1), sourceMatDims(2)) * 255;
else
error('Argument "sourceFactorStr" is unrecognized or invalid');
end
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