Is Photoshop "Motion Blur" possible with GLSL?

Question

I'm using sfml to make a 2D game with sprites and trying to get this effect:

I use Photoshop's "Motion Blur" in the example. As you can see, the effect is directional.

My game uses paperdolled sprites so it would be much easier to have this as a post effect instead of blurring every single equipment combination on every sprite.

Would it be possible to get this effect with a shader? An example would be appreciated.

Answer 1

Here I post the best result I could do in one night. I did it with shaders as you ask. I didn't implement the angles but that something not too hard.

Here is part of the cpp file:

...
if (!shader.loadFromFile("dMotionBlur_v05.frag", sf::Shader::Fragment)){
...

window.clear(sf::Color(120,120,120));

// Passing parameters to shader.
shader.setUniform("dir", dir); //direction of blur
shader.setUniform("nSamplesF", (float)std::atoi(argv[3])); // number of samples
shader.setUniform("radius", (float)std::atof(argv[4]) ); //radius of blur

window.draw(sprite, &shader);
window.display();
...

This is the fragment shader:

uniform sampler2D u_texture;
uniform float nSamplesF;
uniform float radius;
uniform vec2 dir;

void main(){
    vec2 tc = gl_TexCoord[0].xy;
    float blur = radius;
    float hstep = dir.x;
    float vstep = dir.y;
    float total = 0.0;
    int nSamplesI = int(nSamplesF);

    vec4 sum = vec4(0.0);

    for (int i=1; i<=nSamplesI; i++){
        float floatI = float(i);
        float counter = nSamplesF-floatI+1.0;

        float p = floatI/nSamplesF;
        float tO = (p * 0.1783783784) + 0.0162162162;
        total += tO;

        sum += texture2D(u_texture, vec2(tc.x - counter*blur*hstep, tc.y - counter*blur*vstep)) * tO;
    }

    sum += texture2D(u_texture, vec2(tc.x, tc.y)) * 0.2270270270;

    for (int i=nSamplesI; i>=1; i--){
        float floatI = float(i);
        float counter = nSamplesF-floatI+1.0;

        float p = floatI/nSamplesF;
        float tO = (p * 0.1783783784) + 0.0162162162;
        total += tO;

        sum += texture2D(u_texture, vec2(tc.x + counter*blur*hstep, tc.y + counter*blur*vstep)) * tO;
    }

    gl_FragColor =  gl_Color * (sum/total);
}

I upload the entire code to my repository so you can downloaded and try. You can set x/Y direction, sample, and blur radius. X/Y directions are from 0-1. You can play with the number of samples. The blur radius is very sensitive you can start try with 0.01

with my example would be:

$ ./sfml-app [x(0-1)] [Y(0-1] [number of sample] [ radius blur]

Some pics:

Answer 2

Generating points over a line

A little bit on the manual side but, I can suggest an approach.
Lets imagine a circle, crossed by a line on a determined angle. Over that line, we place some random points.

Something like this:

^{Circle with radius 100, angle 45º (PI / 4 radians) and 100 random points}

Then, wherever each one of that points are, we will draw a sprite of our texture, with some alpha (transparency). The result will look like so:

Changing the radius of the circle, and the number of points, the result may vary

---

Further explanation

In my example, I use a class, which represents the BlurredSprite. It will hold those points, and the sf::Texture to draw.

class BlurredSprite : public sf::Drawable, sf::Transformable {
    public:
        BlurredSprite(const sf::Texture &t, float radius, float angle, unsigned int points = 30) :
            m_texture(t)
        {
            auto getPointOverLine = [=]{
                auto angleOverX = angle - std::_Pi*0.5;  // Angle 0 is a vertical line, so I rotate it 45 degrees clockwise (substract)
                if (rand() % 2){
                    angleOverX += std::_Pi;   // Half of the points will be on the "first quadrant", the other half over the "third", if you consider (0,0) the center of the circle
                }
                auto l = radius * ((rand() * 1.f) / RAND_MAX);
                auto x = cos(angleOverX) * (l);
                auto y = sin(angleOverX) * (l);

                return sf::Vector2f(x, y);
            };

            while (m_points.size() < points){
                m_points.push_back(getPointOverLine());
            }

        }
    private:
        std::vector<sf::Vector2f> m_points;
        sf::Texture m_texture;
};

With getPointOverLine lambda, I create a random point over the line, but can be other options to do this. In fact, the way you spread those points will affect on the final result.

I need to override the draw method, in order to make this class Drawable on the window. I also override the setPosition method (from sf::Transformable) because, if you move it, you should move all the points with it.

public:
    virtual void draw(sf::RenderTarget& target, sf::RenderStates states) const{
        auto it = m_points.begin();
        while (it != m_points.end()){    
            sf::Sprite sp(m_texture);
            auto col = sp.getColor();
            auto rect = sp.getTextureRect();
            auto orig = sp.getOrigin() + sf::Vector2f(rect.width, rect.height)*0.5f;
            col.a = 25;

            sp.setColor(col);
            sp.setOrigin(orig);
            sp.setPosition(*it);
            target.draw(sp);

            it++;
        }
    }

    virtual void setPosition(sf::Vector2f pos){
        sf::Transformable::setPosition(pos);
        for (int i = 0; i < m_points.size(); ++i){
            m_points[i] = pos + m_points[i];
        }
    }