3. Light source

This chapter explains how to set the light source in SGL. By using a light source, objects displayed in 3D graphics are shaded, making it possible to draw more realistic images.

3-1. Light source

Light sources generally contain the following information:

・ Position of light source

・ Light intensity

・ Color of light

However, it takes an enormous amount of time to calculate all of these as light source information and draw them accurately. Therefore, in SGL, only the light source direction is selected from these light source information and reflected in the polygon drawing.
Also, regarding the direction of light, light is usually defined as radiation from a certain point, which is also too time-consuming to calculate. Therefore, assuming that the light source position is infinity, the direction of the light is treated as if it is always emitted from the same direction regardless of the coordinate position of the object.

<Fig. 3-1 Light source image model>

Light source changes on the surface of an object are determined by the direction of the rays and the orientation of the normal vector on each polygonal surface. The image below shows this image.

Figure 3-2 Shadow image model Shadow of polygonal surface by light source

The polygon surface changes the polygon color according to the angle of incidence of light.

The closer the angle of incidence is to vertical, the brighter it is, and the closer it is to horizontal, the darker it is.

It is darkest when the angle of incidence is negative and vertical.

3-2. Light source setting

Use the function "slLight" to set the light source in SGL. The light source function has only a direction vector (light source vector) that represents the direction of the light ray, and does not include light intensity or color information.

[Void slLight (VECTOR light);]

Set the light source in SGL.
Substitute a VECTOR type variable (light source vector) that indicates the direction of the ray as a parameter.
Information such as light source intensity and color information is not included in the parameters.
In SGL, a ray is defined as always shining from the direction of the specified direction vector.
Also, unless you turn on light source calculation in the surface attributes of the object, setting the light source does not reflect the shadows caused by the light source (see “ Chapter 7 Polygon Surface Attributes ” for details).

The polygon surface determines the polygon color from the angle of incidence of light on the surface.
The closer the angle of incidence is to vertical, the brighter it is, and the closer it is to horizontal, the darker it is. Also, the negative vertical direction is the darkest.

Precautions when setting the light source

If the current matrix is subjected to various conversion operations such as scale, the light source calculation may not be performed normally.
This happens because the user uses a matrix operation to calculate the light source vector specified by the function “ slLight”. Therefore, when setting the light source, it is necessary to initialize the current matrix.
Use the function “ slPushUnitMatrix ” or “ slUnitMatrix ” to initialize the current matrix.

[Bool slPushUnitMatrix (void)]

Allocate the identity matrix on the stack and use it as the current matrix.
The previous current matrix is temporarily stored at a higher level in the stack.

[Void slUnitMatrix (MATRIX mtptr)]

Makes the matrix specified by the parameter an identity matrix.
Assign the specified MATRIX type variable to the parameter.
If "CURRENT" is specified for the parameter, the conversion target matrix will be the current matrix, and the current matrix can be initialized.

For details on the current matrix and matrix functions, refer to " Chapter 5 Matrix".

Listing 3-1 below shows the changes in the shadows on the cube polygon plane when the direction vector of the light source is changed one after another for a cube placed in space.

<List 3-1 sample_3_2: Changes in the object surface due to the movement of the light source>

/ * ------------------------------------------------ ---------------------- * /
/ * Cube & A Light Source * /
/ * ------------------------------------------------ ---------------------- * /
#include "sgl.h"

extern PDATA PD_PLANE1;

void ss_main (void)
{
	static ANGLE ang [XYZ];
	static FIXED pos [XYZ];
	static FIXED light [XYZ];
	static ANGLE tmp = DEGtoANG (0.0);

	slInitSystem (TV_320x224, NULL, 1);

	ang [X] = DEGtoANG (30.0);
	ang [Y] = DEGtoANG (45.0);
	ang [Z] = DEGtoANG (0.0);
	pos [X] = toFIXED (0.0);
	pos [Y] = toFIXED (0.0);
	pos [Z] = toFIXED (190.0);

	light [X] = toFIXED (-1.0);
	light [Y] = toFIXED (0.0);
	light [Z] = toFIXED (0.0);

	slPrint ("Sample program 3.2", slLocate (9,2));

	while (-1) {
		slPushMatrix ();
		{
			slRotY (tmp);
			slRotX (DEGtoANG (15.0));
			slRotZ (DEGtoANG (15.0));
			slCalcPoint (toFIXED (0.0), toFIXED (0.0), toFIXED (1.0), light);
		}
		slPopMatrix ();
		slLight (light);
		tmp + = DEGtoANG (1.0);

		slPushMatrix ();
		{
			slTranslate (pos [X], pos [Y], pos [Z]);
			slRotX (ang [X]);
			slRotY (ang [Y]);
			slRotZ (ang [Z]);
			slPutPolygon (& PD_PLANE1);
		}
		slPopMatrix ();

		slSynch ();
	}
}

Flow 3-1 sample_3_2: Light source movement flowchart

Appendix SGL library functions that appeared in this chapter

In this chapter, the functions in the following table are explained.

Table 3-1 SGL library functions appearing in this chapter

Functional type	Function name	Parameters	function
void	slLight	VECTOR light	Light source vector settings
Bool	slPushUintMatrix	void	Secure the identity matrix on the stack
void	slUintMatrix	MATRIX mptrt	Make the specified matrix an identity matrix