shithub: ft²

ref: cf174b4c8c75858b067f2072ffac8336c6ede9c1
dir: /src/mixer/ft2_quadratic_spline.c/

// 3-point quadratic spline interpolation LUT generator

#include <stdint.h>
#include <stdbool.h>
#include <stdlib.h>
#include "ft2_quadratic_spline.h"
#include "../ft2_video.h" // showErrorMsgBox()

float *fQuadraticSplineLUT = NULL; // globalized

bool setupQuadraticSplineTable(void)
{
	fQuadraticSplineLUT = (float *)malloc(QUADRATIC_SPLINE_WIDTH * QUADRATIC_SPLINE_PHASES * sizeof (float));
	if (fQuadraticSplineLUT == NULL)
	{
		showErrorMsgBox("Not enough memory!");
		return false;
	}

	float *fPtr = fQuadraticSplineLUT;
	for (int32_t i = 0; i < QUADRATIC_SPLINE_PHASES; i++)
	{
		const float x1 = (float)i * (1.0f / QUADRATIC_SPLINE_PHASES);
		const float x2 = x1 * x1; // x^2

		const float t1 = (x1 * -1.5f) + (x2 *  0.5f) + 1.0f;
		const float t2 = (x1 *  2.0f) + (x2 * -1.0f);
		const float t3 = (x1 * -0.5f) + (x2 *  0.5f);

		*fPtr++ = t1; // tap #1 at sample offset 0 (center)
		*fPtr++ = t2; // tap #2 at sample offset 1
		*fPtr++ = t3; // tap #3 at sample offset 2
	}

	return true;
}

void freeQuadraticSplineTable(void)
{
	if (fQuadraticSplineLUT != NULL)
	{
		free(fQuadraticSplineLUT);
		fQuadraticSplineLUT = NULL;
	}
}