Hello Forum,

I wrote a little painting software and now i try to mimic the behavior of photoshops "normal mode" blending style.

I did a lot of tests, but i cant find the right blend mode settings/formulas.

Essentially i have a circle object with a shader on it (for smooth borders) which draws onto a RenderTexture.

I think the problem lies in the blend mode and not in the shader.

Since its hard to explain in words i made a little video to show what iam talking about.

Any help is much appreciated!

Thanks! --> Video

Ha!!

I was just making another video with the blendmode that @Kylotan recommended.

It kinda works at a first glance, but not completly.

I also noticed that it must be a 2 Step process somehow with a layer that is created on Mouse Down and then blended down onto an existing Layer.

@Prototype Yes, you are right!! If the brush is set to soft, then the outer alpha "Ring" deletes the destination pixels.

I made another (short!!) video to demonstrate what i mean -> Photoshop vs My Program

Blend Settings i used:

sf::BlendMode BlendPre = sf::BlendMode(sf::BlendMode::SrcAlpha, sf::BlendMode::OneMinusSrcAlpha, sf::BlendMode::Equation::Add);

@lawnjelly Thank you for your tips!

When i looked closer to what actually happens in PS i started to think that it must be a 2 step process too.

Thanks for confirming that.

Also i work with full colors and extra Alpha.

I think i know already how to implement an extra temporary layer that gets blended down after Mouse Released.

The only problem i see at the moment is with the soft brush that deletes itself.

Because this will also happen on the temp. layer.

@lawnjelly How did you solve the problem with a softbrush that is mostly deleting itself ?

@lawnjelly Hello and thanks for your answer. I dont fully understand.

In SFML i have a circle object that is my brush. Its color and alpha is all zero.

Then i have a shader ON this brush object.

The shader has a color glFragColor.rgb and the alpha is glFragColor.a = GradientAlpha

Then on every mouse change i stamp down this circle object with the shader onto a RenderTexture with a previously set BlendMode.

So where in this process do i have to introduce a "first" and "second" pass?

As far as i know i can only draw the whole thing at once to the RenderTexture (i might be wrong).

This is my shader code. (expand creates the gradient / alpha is an additional transparency(opacity))

const char VertexShader[] =
		"void main()"
		"{"
		"gl_Position = gl_ModelViewProjectionMatrix * gl_Vertex;"
		"gl_TexCoord[0] = gl_TextureMatrix[0] * gl_MultiTexCoord0;"
		"gl_FrontColor = gl_Color;"
		"}";
 
	const char RadialGradient[] =
		"uniform vec3 color;"
		"uniform float alpha;"
		"uniform vec2 center;"
		"uniform float radius;"
		"uniform float expand;"
		"uniform float LayerHeight;"
 
		"void main(void)"
		"{"
		"vec2 SfmlCenter = vec2(center.x, LayerHeight - center.y);"
		"vec2 p = (gl_FragCoord.xy - SfmlCenter) / radius;"
		"float d = length(p);"
		"float GradientAlpha = smoothstep(0.0, 1.0 - expand, 1.0 - d);"
		"gl_FragColor.rgb = color;"
		"gl_FragColor.a = GradientAlpha - alpha;"
		"}";