Hi! The GLSL may be easier than you think.
I created a little proof-of-concept for 4x upscaling. So a 4x4 block of screen pixels corresponds to 1 texture pixel. My code just does some smoothing in the y direction but it should be enough to give you an idea of how to do more sophisticated upscaling.
To get your head around it, you just need to understand that the main() function is called once for each pixel on screen (as if in a pair of for loops; although in reality they run in parallel), and the colour of that pixel is determined by what ends up in
gl_FragColor.
Any questions, just ask.
Here is the shader code:
#version 130
uniform sampler2D texture;
void main () {
//get window co-ordinates
int window_x = int(gl_FragCoord.x); //varies from 0 to 799 (0,0 is bottom-left)
int window_y = int(gl_FragCoord.y); //varies from 0 to 599
//get the index into the texture corresponding to this pixel
int tex_x = window_x / 4;
int tex_y = window_y / 4;
//figure out which "subpixel" this one is
int xoff = window_x % 4;
int yoff = window_y % 4;
//get colour of corresponding texture pixel
vec4 here = texelFetch(texture, ivec2(tex_x,tex_y), 0);
//get colour of texture 1 pixel up
vec4 up1 = texelFetch(texture, ivec2(tex_x,tex_y+1), 0);
//get colour of texture 1 pixel down
vec4 down1 = texelFetch(texture, ivec2(tex_x,tex_y-1), 0);
if (yoff == 0) //mix with below colour
gl_FragColor = (here + down1) / 2.0;
else if (yoff == 3) //mix with above colour
gl_FragColor = (here + up1) / 2.0;
else //mix them all!
gl_FragColor = (2.0 * here + up1 + down1) / 4.0;
//How the heck do you debug shaders??
//You can't print stuff but you CAN colour pixels instead
//try uncommenting the following to get an idea:
//gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0); //will make all pixels red
//if (window_x >= 400 && window_x < 404)
// gl_FragColor = vec4(0.0, float(window_y)/600.0, 0.0, 1.0); //a bar ranging from black to green
}
Here a full example:
#include <SFML/Graphics.hpp>
#include <SFML/Graphics/Image.hpp>
const std::string FRAG_SHADER = "#version 130\n\
\
uniform sampler2D texture; \
\
void main () { \
int window_x = int(gl_FragCoord.x); \
int window_y = int(gl_FragCoord.y); \
int tex_x = window_x / 4; \
int tex_y = window_y / 4; \
int xoff = window_x % 4; \
int yoff = window_y % 4; \
vec4 here = texelFetch(texture, ivec2(tex_x,tex_y), 0); \
vec4 up1 = texelFetch(texture, ivec2(tex_x,tex_y+1), 0); \
vec4 down1 = texelFetch(texture, ivec2(tex_x,tex_y-1), 0); \
if (yoff == 0) \
gl_FragColor = (here + down1) / 2.0; \
else if (yoff == 3) \
gl_FragColor = (here + up1) / 2.0; \
else \
gl_FragColor = (2.0 * here + up1 + down1) / 4.0; \
}\
";
int main()
{
sf::RenderWindow window(sf::VideoMode(800, 600), "SFML works!");
sf::RenderTexture renderTex;
if (!renderTex.create(200, 150))
return -1;
sf::CircleShape shape(10.f);
shape.setFillColor(sf::Color::Black);
shape.setOrigin(shape.getRadius(), shape.getRadius());
sf::Shader upscaleShader;
upscaleShader.loadFromMemory(FRAG_SHADER, sf::Shader::Fragment);
//set up the 'texture' variable in the shader
upscaleShader.setParameter("texture", renderTex.getTexture());
sf::RenderStates upscaleRenderStates(&upscaleShader);
//FIXME: hardcoding
sf::Vertex winRect[4] = {
sf::Vector2f(0,0),
sf::Vector2f(800,0),
sf::Vector2f(0,600),
sf::Vector2f(800,600)};
while(window.isOpen()) {
sf::Event event;
while(window.pollEvent(event)) {
if(event.type == sf::Event::Closed)
window.close();
}
sf::Vector2i mousePos = sf::Mouse::getPosition(window);
//shape.setPosition(sf::Vector2f(static_cast<float>(mousePos.x)/4.f, static_cast<float>(mousePos.y)/4.f));
shape.setPosition(sf::Vector2f(static_cast<float>(mousePos.x/4), static_cast<float>(mousePos.y/4)));
//Draw everything
renderTex.clear(sf::Color(200,200,200));
renderTex.draw(shape);
renderTex.display();
window.clear();
//this is where the upscaling happens
window.draw(winRect, 4, sf::TrianglesStrip, upscaleRenderStates);
window.display();
}
return 0;
}
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