GLSL hue shader producing odd results, IOS only?

Question

I have a cross-platform LibGDX app. This particular GLSL shader code is used to shift the hue of a particular texture.

It works great on Android and when debugging on Desktop, but on an iPad this is the result (excuse photos of screen, easiest way to get data from this device).

Code:

const mat3 rgb2yiq = mat3(0.299, 0.595716, 0.211456, 0.587, -0.274453, -0.522591, 0.114, -0.321263, 0.311135);
const mat3 yiq2rgb = mat3(1.0, 1.0, 1.0, 0.9563, -0.2721, -1.1070, 0.6210, -0.6474, 1.7046);

vec4 outColor = texture2D(u_texture, v_texCoord) * v_color;
float alpha = outColor.a;

// Hue shift
if (u_hueAdjust > 0.0 && u_hueAdjust < 1.0 && alpha > 0.0)
{
  vec3 unmultipliedRGB = outColor.rgb / alpha;

  vec3 yColor = rgb2yiq * unmultipliedRGB;

  float originalHue = atan(yColor.b, yColor.g);
  float finalHue = originalHue + u_hueAdjust * 6.28318; //convert 0-1 to radians

  float chroma = sqrt(yColor.b * yColor.b + yColor.g * yColor.g);

  vec3 yFinalColor = vec3(yColor.r, chroma * cos(finalHue), chroma * sin(finalHue));
  outColor.rgb = (yiq2rgb * yFinalColor) * alpha;
}

Obviously there's some really weird artifacts that seem to affect certain areas, in particular black/white colors. But also in general a subtle change in color is noted that isn't attributable to a desired hue-change effect.

Overall this shader is wonky on IOS (but working fine on Android/Desktop), but after playing with it for a while I'm completely out of ideas, anyone lead me in the right direction?

score 2 · Accepted Answer · answered Nov 21 '19 at 15:19

2

In the documentation for atan, it says The result is undefined if x=0..

Is it possible that yColor.g is zero on the greyscale?

The issue is discussed here: Robust atan(y,x) on GLSL for converting XY coordinate to angle

answered Nov 21 '19 at 15:19

Columbo

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if i could hug you i would, that was it, thanks a lot – yesbutmaybeno Nov 21 '19 at 20:28

GLSL hue shader producing odd results, IOS only?

1 Answers1