javascript - How to consider alpha channel(opacity) for canvas blending(context.globalCompositeOperation) -

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we have following sample code blend 2 images global composition.

image1

image2

canvas blending works perfect. need detect formulas/algorithms make operation imagedatas , same result.

    image.src = "landscape.png";     image.onload = function() {         img = new image;          img.src = "gradient.png";         img.onload = function(){             canvas.width = img.width;             canvas.height = img.height;             context.save();             context.drawimage(image, 0, 0, canvas.width, canvas.height);             drawcustomlogo(context);             var id1 = getcontextfromimg(image);             var id2 = getcontextfromimg(img);             var b = new blending();             var sid = b.overlay(id1, id2);             var bc = document.getelementbyid('custom');             bc.width = image.width;             bc.height = image.height;             var bctx = bc.getcontext('2d');             bctx.putimagedata(sid, 0, 0);         }

but problem in processing images alpha channel. tried following algorithms:

    (target > 0.5)  (1 - (1-2*(target-0.5)) * (1-blend)) +     (target <= 0.5) ((2*target) * blend)

and

dst[px  ] = (dra<=0.5) ? (2*src[px  ]*dra/da) : 255 - (2 - 2*dra/da) * (255-src[px  ]);                 dst[px+1] = (dga<=0.5) ? (2*src[px+1]*dga/da) : 255 - (2 - 2*dga/da) * (255-src[px+1]);                 dst[px+2] = (dba<=0.5) ? (2*src[px+2]*dba/da) : 255 - (2 - 2*dba/da) * (255-src[px+2]);

but result different. need find strict overlay blending algorithm

looks found right decision. best explanation stores on w3c

general formula blending filter is

color(red) = (1 - alphablending)*sourcecolor(red) + alphablending*func(colorblending(red),colorsource(red)); .... // continue other color

so blending functions should used general formula compositing , blending


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