LÖVE

Charlie Gallie wrote: ↑Thu Feb 21, 2019 2:14 am Now, I want to create a circle just like

Code: Select all

love.graphics.circle("line", circle.x, circle.y, circle.r)

(the size and position work just fine) but I have an image as a background and I want the color of the circle to be the opposite color of which it's above.

local background = love.graphics.newCanvas()
local invert = love.graphics.newShader([[
	extern Image background;

	vec4 effect(vec4 color, Image texture, vec2 uv, vec2 fc) {
		return vec4(1.0 - Texel(background, uv).rgb, 1.0);
	}
]])
invert:send('background', background)

function love.draw()
	background:renderTo(function()
		-- draw the background contents here
		love.graphics.clear(1, 0, 0)
	end)
	love.graphics.draw(background) -- draw background on screen

	-- draw circle with invert shader
	love.graphics.setShader(invert)
	love.graphics.circle('fill', 100, 100, 100)
	love.graphics.setShader()
end

pgimeno wrote: ↑Thu Feb 21, 2019 11:31 am Edit: Oops, ninja'd by grump

pgimeno wrote: ↑Thu Feb 21, 2019 11:31 am Edit: Oops, ninja'd by grump

You need a shader. If your background is not an image, you also need a canvas where you draw the background.

The shader would be like this:

Code: Select all

local shader = love.graphics.newShader[[
  extern Image background;

  vec4 effect(vec4 colour, Image texture, vec2 texpos, vec2 scrpos)
  {
    vec4 fgpixel = Texel(texture, texpos) * colour;
    vec4 bgpixel = Texel(background, scrpos/love_ScreenSize.xy);
    return vec4(fgpixel.rgb - bgpixel.rgb, fgpixel.a);
  }
]]

You need to send the background image or canvas to the shader with:

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  shader:send('background', backgroundImageOrCanvas)

Attached is a complete example.

An 'invert' blendmode would be useful to have.

ivan wrote: ↑Thu Feb 21, 2019 2:06 pm

An 'invert' blendmode would be useful to have.

This is by far the easiest way to do it and it doesn't require shaders.
It should be possible using the "subtract" blend mode but I've never been able to get it working in Love2D.

ivan wrote: ↑Thu Feb 21, 2019 2:06 pm

An 'invert' blendmode would be useful to have.

This is by far the easiest way to do it and it doesn't require shaders.

zorg wrote: ↑Thu Feb 21, 2019 3:10 pm Yep, would be nice if these would be options; not sure whether GL and GL ES version intercompatibility is the barrier to this or not though.

https://www.khronos.org/registry/OpenGL ... parate.xml
https://www.khronos.org/registry/OpenGL ... parate.xml

Also, subtract doesn't work because to invert a color you'd want to do source minus dest, like 1-R,1-G,1-B (and some type of alpha); but löve's exposed subtract blendmode can only do dest minus source, so R-1, G-1, B-1; that doesn't invert color values (the ones are the source colors, and the letters are the destination colors)