Shaders and RenderTextures

[BlueCobold]

Hi.

I don't know if I'm doing something wrong or if there's a problem with how SFML shaders and render-textures work, but here's what I got.

I'm using latest SFML master from github on Win7 x64 in a 32-bit-build.

======= Sample 1 =======
I'm rending a rect to a RenderTexture and then rendering this texture via shader to the window.

(click to show/hide)

#include <SFML/Graphics/Shader.hpp>
#include <SFML/Graphics/RenderTexture.hpp>
#include <SFML/Graphics/RectangleShape.hpp>
#include <SFML/Window/Event.hpp>

int main(int argc, char* argv[])
{
    sf::RenderWindow window(sf::VideoMode(550, 350), "SFML window");

    sf::RenderTexture target;
    target.create(window.getSize().x, window.getSize().y);
    sf::Shader shader;
    if(!shader.loadFromMemory(
        "uniform sampler2D inputTex;\
        void main()\
        {\
           vec4 renderTargetColor = texture2D(inputTex, gl_TexCoord[0].xy);\
           gl_FragColor = vec4(0, renderTargetColor.g, renderTargetColor.r, 1);\
        }", sf::Shader::Fragment))
         return 1;

    while (window.isOpen())
    {
        sf::Event event;
        while (window.pollEvent(event))
        {
            if (event.type == sf::Event::Closed) {
                window.close();
            }
        }
        sf::RectangleShape a(sf::Vector2f(250, 250));
        a.setFillColor(sf::Color(0, 255, 0, 255));
        target.clear(sf::Color(255, 0, 0, 255));
        target.draw(a);
        target.display();
        glFinish();
       
        window.clear();
        shader.setParameter("inputTex", target.getTexture());
        sf::Shader::bind(&shader);
        sf::Sprite b(target.getTexture());
        window.draw(b);
        window.display();
        sf::Shader::bind(0);
    }
    return 0;
}

The first rendered frame looks like this (green rect on red ground) and it is wrong - its basically the result without use of the shader at all:

All consecutive rendered frames look like this (green rect on blue ground), which is correct use of the shader:

[eXpl0it3r]

With the provided code, I'm experiencing the same issue, however by using the "SFML way" it seems to work fine. I've no idea about OpenGL, so I'll leave the interpretation to others.

(click to show/hide)

#include <SFML/Graphics.hpp>
#include <SFML/OpenGL.hpp>

int main(int argc, char* argv[])
{
    sf::RenderWindow window(sf::VideoMode(550, 350), "SFML window");

    sf::RenderTexture target;
    target.create(window.getSize().x, window.getSize().y);
    sf::Shader shader;
    if(!shader.loadFromMemory(
        "uniform sampler2D inputTex;\
       void main()\
       {\
          vec4 renderTargetColor = texture2D(inputTex, gl_TexCoord[0].xy);\
          gl_FragColor = vec4(0, renderTargetColor.g, renderTargetColor.r, 1);\
       }", sf::Shader::Fragment))
         return 1;

    while (window.isOpen())
    {
        sf::Event event;
        while (window.pollEvent(event))
        {
            if (event.type == sf::Event::Closed) {
                window.close();
            }
        }
        sf::RectangleShape a(sf::Vector2f(250, 250));
        a.setFillColor(sf::Color(0, 255, 0, 255));
        target.clear(sf::Color(255, 0, 0, 255));
        target.draw(a);
        target.display();

        shader.setParameter("inputTex", target.getTexture());
        sf::Sprite b(target.getTexture());

        window.clear();
        window.draw(b, &shader);
        window.display();

        sf::sleep(sf::seconds(5.f));
    }
    return 0;
}
 

[BlueCobold]

Well, the "draw(drawable, &shader)" is not a solution I could use, because I'd have to pass down the shader to all draw-calls which is inacceptable. Independently binding the shader once should be enough.
If that's indeed the only correct way to use a shader, I'll have to find a way around it. The Shader::bind method shouldn't be publicly accessible then though.
The performance of this is horrible, because she shader will get bound and enabled for each drawable, the parameters set again and again.

[eXpl0it3r]

If that's indeed the only correct way to use a shader, I'll have to find a way around it. The Shader::bind method shouldn't be publicly accessible then though.

You make it sound like it's the worst issue ever.
I mean sure we want it fixed, but one malformed first frame isn't going to harm anyone. If you don't pause at that point, you won't even notice it.

[BlueCobold]

One does notice, because it flickers.
Anyway, the draw(drawable, &shader) solution is really horrible for performance. I guess I'll have to make a hack in my app and use this for the first frame and afterwards use the sf::Shader::bind myself. That looks to be working, but it just can't be a preferred solution.

[binary1248]

Call window.resetGLStates() right after you create your window. It will initialize the internal GL state and make sure your shader doesn't get unbound the first time it is drawn to. You should be able to manually bind/unbind all the shaders you want after that, SFML doesn't cache the currently bound shader.

[BlueCobold]

Works like charm, perfect. Thanks a lot.
(plus: the solution actually makes sense by looking at the SFML source)