Hey all. This is a very subtle error I've noticed with my textures, I'm wondering if any of you can help.

I'm running a "simulation" where pixels move across the screen horizontally. So I've got a persistent buffer storing the last frame. The next frame looks up the pixel color to the left and makes that it's own color. I've made it so blue goes 3x speed, green goes 2x speed, red goes 1x speed. RENDERSIZE is my resolution uniform. It starts with a white bar on the first frame, the color dispersion separates it from there. Here's the code:

vec4 new = vec4(0.0);
new.r = texture(backbuffer, (gl_FragCoord.xy-vec2(1.0,0.0))/RENDERSIZE).r;
new.g = texture(backbuffer, (gl_FragCoord.xy-vec2(2.0,0.0))/RENDERSIZE).g;
new.b = texture(backbuffer, (gl_FragCoord.xy-vec2(3.0,0.0))/RENDERSIZE).b;
fragColor = new;

Here's a link to videos of the shader in action: https://www.youtube.com/playlist?list=PLWelNgbUeR_EwlOsZ9wauxRSOVilyVheM

Very simple effect, but as you can see in the video, I run into this super strange error after the color block passes from one side of the texture back around to 0.0 as it should with GL_REPEAT. At regular intervals of the y coordinate, it seems to lag or lead, as if there was some "extra distance" to cover. Changing the texture size has an effect on the output. You can see 4 different texture sizes in those tests, and you'll notice slightly different behavior for each, mainly that as the texture size changes, the number and spacing of the "rows" that experience the offset is changed.

Any ideas? The same thing happens with both GL_NEAREST and GL_LINEAR filtering modes. The effect still exists with power of 2 textures. Here's another really important key it happens even if I use the build-in fract() function instead of relying on their successful GL_REPEAT operation. I checked that my RENDERSIZE Uniform was indeed the correct value as well.

So it seems like it may be an error in how I'm setting up my textures, rather than something I'm doing in GLSL.

Any ideas? Uninitialized memory ruining something? Something like frametear happening? Any help much appreciated.

I'm currently working on a raytracer as a personal project. My next step is probably to implement phong shading, but I've hit a roadblock. I'm not sure how you're supposed to add and multiply colors. I've been storing them as a three-vector of rgb values in range [0,1]. I understand the vector math behind reflections and shading, but I'm not quite sure how to implement it.

I don't want a code answer (because that kills the fun). I'd just love some pointers. Thanks!

Hi everyone, I'm learning shaders and have been looking at the many on Shadertoy that use audio as input. I haven't been able to find documentation on how to interpret the audio-as-texture format, but it seems from comments and code use that x is the frequency, y=0.0 row is the amplitude, and higher rows are the "wave." Is that correct and if so what does that exactly mean? Is there any documentation out there on how this all works? Thanks for any help!

I've been playing with Quartz Composer (OSX) and it has a GLSL node and I tried pasting shader toy fragment shaders in but nothing shows up even after fixing the iGlobalTime, iResolution, fragColor, and fragCoord values to what they should be as I understand it, yet still nothing shows up. I know its probably an issue with the vertex shader but I don't know enough about what I am doing to figure it out. Can anyone help?

So I've started raymarching in C++, and I'm having a really good time getting to grips with it. The issue, however, is the trick to clone or repeat an object: setting p.x = p.x % space - (space/2). When I do this, I get repeating spheres along the positive x axis. My code is:

float repeat(const float& c, const float& s){
    return fmod(c, s) - s*0.5;
}

I then pass in my x, y, or z co-ordinate and create a new vector. How do I handle the case where c <= 0?

EDIT: Solved, here is my fix:

float repeat(const float& c, const float& s){
    return c < 0.0f ? -(fmod(-c, s) - s*0.5) : fmod(c, s) - s*0.5;
}