precision highp float;

varying vec2      outTex;	// texture coords of row/column to calculate
uniform sampler2D X;		// texture with data from padded A
uniform int       N;		// number of columns
uniform int       pad;		// additional columns to nearest multiple of four
uniform float     a; 		// lower bound
uniform float     b; 		// upper bound

#pragma glslify: encode_float = require(../encode_float)
#pragma glslify: select_index = require(../select_index)


void main(void) {

	// get the implied row and column from .y and .x of passed (output)
	// texture coordinate. These map directly to input texture space when
	// the relevant dimensions are the same.
	float row = outTex.y;
	float col = outTex.x;

	// return 0.0 if in padded region of output texture
	if(col * float(N + pad) > float(N) ) {
		gl_FragColor = vec4(0.,0.,0.,0.);
		return;
	}

	// direct usage of col requires output be padded exactly like input
	vec4 x = texture2D( X, vec2(col, row));
	vec4 val = clamp(x, a, b);

	// select and output channel (standalone version only)
	int channel = int(mod(col * float(N + pad), 4.0));
	float sum = select_index(val, channel);

	if (sum == 0.) {
		gl_FragColor = vec4(0.,0.,0.,0.);
		return;
	}

	// output vec4 with bytes for an IEEE754 32-bit floating point number
	gl_FragColor = encode_float(sum);
}