small explanatory code for OpenGL vertex array draw modes

print_arrays.c

#include <stdio.h>

typedef long MYenum;

enum {
	SINGLES,
	PAIRS,
	TRIPLES,
};

int *array = NULL;

/* mimic the call to glVertexAttribPointer */
void array_pointer(void *p)
{
	array = p;
}

/* mimicks the behavior of glDrawArrays */
void print_arrays(MYenum mode, int first, size_t count)
{
	size_t N, i, j;

	if( !array ) return;

	switch(mode) {
	default: return;

	case SINGLES: N = 1; break;
	case PAIRS:   N = 2; break;
	case TRIPLES: N = 3; break;
	}

	for(i = 0; i <= count-N;) {
		printf("(");
		for(j = 0; j < N; ++j, ++i) {

			printf("%d", array[ first + i ] );

			if( j + 1 < N ) printf(", ");
		}
		printf(")");
		if( i + 1 <= count-N+1 ) printf(", ");
	}
	printf("\n");
}

/* mimics the behavior of glDrawElements */
void print_elements(MYenum mode, size_t count, int const *indices)
{
	size_t N, i, j;

	if( !array 
	 || !indices ) return;

	switch(mode) {
	default: return;

	case SINGLES: N = 1; break;
	case PAIRS:   N = 2; break;
	case TRIPLES: N = 3; break;
	}

	for(i = 0; i <= count-N;) {
		printf("(");
		for(j = 0; j < N; ++j, ++i) {

			printf("%d", array[ indices[i] ] );

			if( j + 1 < N ) printf(", ");
		}
		printf(")");
		if( i + 1 <= count-N+1 ) printf(", ");
	}
	printf("\n");
}

int main(int argc, char *argv[])
{
	int singles[4] = {
		11,
		12,
		13,
		14
	};
	int single_elements[10] = {
		0, 1, 0, 2, 3,
		3, 0, 0, 2, 1
	};

	int pairs[6] = {
		21, 22,
		31, 32,
		41, 42
	};
	int pairs_elements[12] = {
		0, 1,
		0, 2,
		2, 3,
		2, 4,
		4, 5,
		4, 0
	};

	int triples[9] = {
		51, 52, 53,
		61, 62, 63,
		71, 72, 73
	};
	int triple_elements[15] = {
		0, 1, 2,
		3, 4, 5,
		6, 7, 8,
		7, 4, 1,
		0, 3, 6
	};

	printf("== a few chains of singles ==\n");
	array_pointer(singles);
	printf("print array starting with 0, 3 consecutive elements:\n");
	print_arrays(SINGLES, 0, 3);
	printf("print array starting with 1, 3 consecutive elements:\n");
	print_arrays(SINGLES, 1, 3);
	printf("print 10 elements by index:\n");
	print_elements(SINGLES, 10, single_elements);

	printf("\n== a few chains of pairs ==\n");
	array_pointer(pairs);
	printf("print array starting with 0, 6 consecutive elements:\n");
	print_arrays(PAIRS, 0, 6);
	printf("print array starting with 1, 3 consecutive elements (should drop one element at the end since 3%%2 = 1):\n");
	print_arrays(PAIRS, 1, 3);
	printf("print 12 elements by index:\n");
	print_elements(PAIRS, 12, pairs_elements);
	
	printf("\n== a few chains of triples ==\n");
	array_pointer(triples);
	printf("print array starting with 0, 9 consecutive elements:\n");
	print_arrays(TRIPLES, 0, 9);
	printf("print array starting with 2, 6 consecutive elements:\n");
	print_arrays(TRIPLES, 2, 6);
	printf("print 15 elements by index:\n");
	print_elements(TRIPLES, 15, triple_elements);

	return 0;
}