#version 150 uniform sampler2D img; uniform float m_stretch; uniform float m_alpha; in vec2 m_cord; out vec4 fragColor; vec2 stretch(vec2 pos) { // our transform should map [0..1] to itself, with f(0) = 0, f(1) = 1, f(0.5) = 0.5, and f'(0.5) = b. // a simple curve to do this is g(x) = b(x-0.5) + (1-b)2^(n-1)(x-0.5)^n + 0.5 // where the power preserves sign. n = 2 is the simplest non-linear case (required when b != 1) float x = pos.x - 0.5; return vec2(mix(x * abs(x) * 2.0, x, m_stretch) + 0.5, pos.y); } void main() { fragColor.rgb = texture(img, stretch(m_cord)).rgb; fragColor.a = m_alpha; }