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I have this correctly-working OpenGL shader producing Perlin noise:

 float left = lerp(fade(v),  downleft,  topleft);
 float right = lerp(fade(v), downright, topright);
 float result = lerp(fade(u), left, right);

Then I tried plugging the definitions of right and left into result:

float result = lerp(fade(u),
                    lerp(fade(v), downleft,  topleft),
                    lerp(fade(v), downright, topright));

Surprisingly, this behaves completely differently, giving visible edges in my Perlin noise. Below are both results:

enter image description here

My whole 30-line shader is here.

What is the difference between those?

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    \$\begingroup\$ Some trivia notes -- your lerp() is equivalent to the built-in function mix(). And, it may be handy to put your shader into ShaderToy, as a convenient way for others to easily run/debug it. (That all said, I'm so far mystified why the substitution would do what it seems to be doing.) \$\endgroup\$ – david van brink Oct 23 '15 at 18:47
  • \$\begingroup\$ I bet that you're taking coordinates from each render in one of them and screen in the other. \$\endgroup\$ – Lolums Nov 1 '15 at 10:43
  • \$\begingroup\$ @Lolums 34 can you rephrase? I did not quite get you. \$\endgroup\$ – hungry91 Nov 1 '15 at 11:20
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    \$\begingroup\$ @hungry91 ignore that comment, it's invalid. I'll try to answer the question later though when I get some time :) \$\endgroup\$ – Lolums Nov 1 '15 at 14:53
  • \$\begingroup\$ I'm unable to recreate this. Are you sure you didn't accidentally plug left in twice or something? \$\endgroup\$ – Yousef Amar Nov 24 '15 at 22:00
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It seems like the processing system is loosing floating point precision because you're not declaring memory for the variable. Normally this isn't a problem, however it seems that your lerp function requires a good bit of precision. Imagine this: computers cannot know irrational numbers. So if X is the square root of 5, and Y is the square root of 3. Saying X + Y is different than sqrt(5) + sqrt(3)

This is because on the lower level the data in the first operation goes

data > memory > cpu > memory

And in the second operation it's more like

Data > cpu > memory

While faster, this operation results in precision loss, especially in 32 bit systems.

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