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Blending modes are a little difficult to explain. Basically, when one layer is above the other, the computer uses a bit of programming to decide how the combination of both layers will look.
Blending modes can not just apply to Layers, but also to individual strokes.
Let's start explaining with the Normal Blending mode: As you may have guessed this is the default Blending mode for all layers.
In this mode, the computer checks on the upper layer how transparent a pixel is, which colour it is, and then mixes the colour of the upper layer with the lower layer measured by transparency.
These blending modes are based on simple maths.
Adds the numerical values of two colours together:
Divides the numerical value from the lower color by the upper color.
This, unlike regular substract, will consider 0,0,0 the lowest number.
So black(0,0,0)-grey(0.5,0.5,0.5) doesn't become grey, but stays black.
Multiplies the two colors with each other, but does not go beyond the upper limit.
This is often used to color in a black and white lineart. One puts the black and white lineart on top, and sets the layer to 'Multiply', and then draw in color on a layer beneath. Multiply will all the colour to go through.
White(1,1,1)xWhite(1,1,1)=White(1,1,1) White(1,1,1)xGrey(0.5,0.5,0.5)=Grey(0.5,0.5,0.5) Grey(0.5,0.5,0.5)xGrey(0.5,0.5,0.5)=Dark grey(0.25,0.25,0.25)
White(1,1,1)-White(1,1,1)=Black(0,0,0) White(1,1,1)-Grey(0.5,0.5,0.5)=Grey(0.5,0.5,0.5) Grey(0.5,0.5,0.5)-Grey(0.5,0.5,0.5)=Black(0,0,0)
With the darken, the upper layer's colors are checked for their lightness. Only if they are darker than the underlying color on the lower layer, will they be visible.
HSI is a color coordinate system, using Hue, Saturation and Intensity to catagorise a color. Hue is roughly the wavelength, whether the colour is red, yellow, green, cyan, blue or purple. It is measure in 360°, with 0 being red. Saturation is the measurement of how close a color is to grey. Intensity, in this case is the tone of the color. What makes intensity special is that it recognises Yellow(rgb:1,1,0) having a higher combined rgb value than blue(rgb:0,0,1). This is a non-linear tone dimension, which means it's gamma-corrected.
This takes the Intensity of the colours on the lower layer, and combines them with the Saturation and Hue of the upper pixels.
HSL is also a color coordinate system. It describes colors in Hue, Saturation and Lightness. Lightness specifically puts both yellow(rgb:1,1,0), blue(rgb:0,0,1) and middle grey(rgb:0.5,0.5,0.5) at the same lightness(0.5).
This takes the Lightness of the colours on the lower layer, and combines them with the Saturation and Hue of the upper pixels.
HSV, occasionally called HSB, is a color coordinate system. It measures colors in Hue, Saturation, and Value(also called Brightness). Value or Brightness specifically reffers to strength at which the pixel-lights on your monitor have to shine. It sets Yellow(rgb:1,1,0), Blue(rgb:0,0,1) and White(rgb:1,1,0) at the same Value(100%)
This takes the Value of the colours on the lower layer, and combines them with the Saturation and Hue of the upper pixels.
HSY is a color coordinate system. It catagorises colors in Hue, Saturation and Luminosity. Luminosity is based on scientific studies of how much light a color reflects in real-life. Unlike the others it's linear, and while like intensity it acknowledges that Yellow(rgb:1,1,0) is light than blue(rgb:0,0,1), it also acknowledges that Yellow(rgb:1,1,0) is bright than Cyan(rgb(0,0,1), based on these studies.
This takes the Luminosity of the colours on the lower layer, and combines them with the Saturation and Hue of the upper pixels.
Blending modes that lighten the image.
Blends the upper layer as half-transparent with the lower.
As far as I can tell this seems to premultiply the alpha, as is common in some file-formats.
Does the opposite of normal, and tries to have the upper layer rendered below the lower layer.
This substracts the opaque pixels of the upper layer from the lower layer, effectively erasing.
Similar to substract, the colors of the upper layer are substracted from the colors of the lower layer, and then 50% grey is added.
Similar to addition, the colors of the upper layer are added to the colors, and then 50% grey is substracted.
The standard blending mode. This looks at how transparant a given pixel is, and mixes the upper layer with the lower according to that.
These are all blending modes which seem to make the image go negative.