《OpenGL ES 2.0 Programming Guide》第9章“最简单的MipMap”示例代码【C语言版】
由于《OpenGL ES 2.0 Programming Guide》原书第9章的示例代码使用的纹理是程序生成的，作者还自己实现了Mipmap的过程，对于理解Mipmap的原理很有帮助，但是并不实用，遂自己实现了一份C语言版本的（加载本地纹理+调用glGenerateMipmap），希望能够帮助到同样喜欢OpenGL ES 2.0的同学。
废话不多说，直接上代码：
// MipMap2D.c
//
// This is a simple example that demonstrates generating a mipmap chain
// and rendering with it
//
#include
#include "esUtil.h"
typedef struct
{
// Handle to a program object
GLuint programObject;
// Attribute locations
GLint positionLoc;
GLint texCoordLoc;
// Sampler location
GLint samplerLoc;
// Offset location
GLint offsetLoc;
// Texture handle
GLuint textureId;
} UserData;
///
// From an RGB8 source image, generate the next level mipmap
//
GLboolean GenMipMap2D( GLubyte *src, GLubyte **dst, int srcWidth, int srcHeight, int *dstWidth, int *dstHeight )
{
int x,
y;
int texelSize = 3;
*dstWidth = srcWidth / 2;
if ( *dstWidth <= 0 )
*dstWidth = 1;
*dstHeight = srcHeight / 2;
if ( *dstHeight <= 0 )
*dstHeight = 1;
*dst = malloc ( sizeof(GLubyte) * texelSize * (*dstWidth) * (*dstHeight) );
if ( *dst == NULL )
return GL_FALSE;
for ( y = 0; y < *dstHeight; y++ )
{
for( x = 0; x < *dstWidth; x++ )
{
int srcIndex[4];
float r = 0.0f,
g = 0.0f,
b = 0.0f;
int sample;
// Compute the offsets for 2x2 grid of pixels in previous
// image to perform box filter
srcIndex[0] =
(((y * 2) * srcWidth) + (x * 2)) * texelSize;
srcIndex[1] =
(((y * 2) * srcWidth) + (x * 2 + 1)) * texelSize;
srcIndex[2] =
((((y * 2) + 1) * srcWidth) + (x * 2)) * texelSize;
srcIndex[3] =
((((y * 2) + 1) * srcWidth) + (x * 2 + 1)) * texelSize;
// Sum all pixels
for ( sample = 0; sample < 4; sample++ )
{
r += src[srcIndex[sample]];
g += src[srcIndex[sample] + 1];
b += src[srcIndex[sample] + 2];
}
// Average results
r /= 4.0;
g /= 4.0;
b /= 4.0;
// Store resulting pixels
(*dst)[ ( y * (*dstWidth) + x ) * texelSize ] = (GLubyte)( r );
(*dst)[ ( y * (*dstWidth) + x ) * texelSize + 1] = (GLubyte)( g );
(*dst)[ ( y * (*dstWidth) + x ) * texelSize + 2] = (GLubyte)( b );
}
}
return GL_TRUE;
}
///
// Generate an RGB8 checkerboard image
//
GLubyte* GenCheckImage( int width, int height, int checkSize )
{
int x,
y;
GLubyte *pixels = malloc( width * height * 3 );
if ( pixels == NULL )
return NULL;
for ( y = 0; y < height; y++ )
for ( x = 0; x < width; x++ )
{
GLubyte rColor = 0;
GLubyte bColor = 0;
if ( ( x / checkSize ) % 2 == 0 )
{
rColor = 255 * ( ( y / checkSize ) % 2 );
bColor = 255 * ( 1 - ( ( y / checkSize ) % 2 ) );
}
else
{
bColor = 255 * ( ( y / checkSize ) % 2 );
rColor = 255 * ( 1 - ( ( y / checkSize ) % 2 ) );
}
pixels[(y * height + x) * 3] = rColor;
pixels[(y * height + x) * 3 + 1] = 0;
pixels[(y * height + x) * 3 + 2] = bColor;
}
return pixels;
}
///
// Load texture from disk
//
GLuint LoadTexture ( char *fileName )
{
int width, height;
char *buffer = esLoadTGA ( fileName, &width, &height );
GLuint texId;
if ( buffer == NULL )
{
esLogMessage ( "Error loading (%s) image.\n", fileName );
return 0;
}
glGenTextures ( 1, &texId );
glBindTexture ( GL_TEXTURE_2D, texId );
glTexImage2D ( GL_TEXTURE_2D, 0, GL_RGB, width, height, 0, GL_RGB, GL_UNSIGNED_BYTE, buffer );
glTexParameteri ( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR );
glTexParameteri ( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR );
glTexParameteri ( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE );
glTexParameteri ( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE );
free ( buffer );
return texId;
}
///
// Create a mipmapped 2D texture image
//
GLuint CreateMipMappedTexture2D( )
{
// Texture object handle
GLuint textureId;
#if 0
int width = 256,
height = 256;
int level;
GLubyte *pixels;
pixels = GenCheckImage( width, height, 8 );
if ( pixels == NULL )
return 0;
// Generate a texture object
glGenTextures ( 1, &textureId );
// Bind the texture object
glBindTexture ( GL_TEXTURE_2D, textureId );
// Load mipmap level 0
glTexImage2D ( GL_TEXTURE_2D, 0, GL_RGB, width, height,
0, GL_RGB, GL_UNSIGNED_BYTE, pixels );
GLubyte *prevImage;
GLubyte *newImage;
level = 1;
prevImage = &pixels[0];
while ( width > 1 && height > 1 )
{
int newWidth,
newHeight;
// Generate the next mipmap level
GenMipMap2D( prevImage, &newImage, width, height,
&newWidth, &newHeight );
// Load the mipmap level
glTexImage2D( GL_TEXTURE_2D, level, GL_RGB,
newWidth, newHeight, 0, GL_RGB,
GL_UNSIGNED_BYTE, newImage );
// Free the previous image
free ( prevImage );
// Set the previous image for the next iteration
prevImage = newImage;
level++;
// Half the width and height
width = newWidth;
height = newHeight;
}
free ( newImage );
#else
char *fileName = "D:/Projects/Visual Studio 2012/OpenGL_Demo/opengles-book-samples-master/Windows/Chapter_9/Simple_Texture2D/Fieldstone.tga";
textureId = LoadTexture (fileName);
glGenerateMipmap(GL_TEXTURE_2D);
#endif
// Set the filtering mode
glTexParameteri ( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST_MIPMAP_NEAREST );
glTexParameteri ( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR );
return textureId;
}
///
// Initialize the shader and program object
//
int Init ( ESContext *esContext )
{
UserData *userData = esContext->userData;
GLbyte vShaderStr[] =
"uniform float u_offset; \n"
"attribute vec4 a_position; \n"
"attribute vec2 a_texCoord; \n"
"varying vec2 v_texCoord; \n"
"void main() \n"
"{ \n"
" gl_Position = a_position; \n"
" gl_Position.x += u_offset;\n"
" v_texCoord = a_texCoord; \n"
"} \n";
GLbyte fShaderStr[] =
"precision mediump float; \n"
"varying vec2 v_texCoord; \n"
"uniform sampler2D s_texture; \n"
"void main() \n"
"{ \n"
" gl_FragColor = texture2D( s_texture, v_texCoord );\n"
"} \n";
// Load the shaders and get a linked program object
userData->programObject = esLoadProgram ( vShaderStr, fShaderStr );
// Get the attribute locations
userData->positionLoc = glGetAttribLocation ( userData->programObject, "a_position" );
userData->texCoordLoc = glGetAttribLocation ( userData->programObject, "a_texCoord" );
// Get the sampler location
userData->samplerLoc = glGetUniformLocation ( userData->programObject, "s_texture" );
// Get the offset location
userData->offsetLoc = glGetUniformLocation( userData->programObject, "u_offset" );
// Load the texture
userData->textureId = CreateMipMappedTexture2D ();
glClearColor ( 0.0f, 0.0f, 0.0f, 0.0f );
return TRUE;
}
///
// Draw a triangle using the shader pair created in Init()
//
void Draw ( ESContext *esContext )
{
UserData *userData = esContext->userData;
GLfloat vVertices[] = { -0.5f, 0.5f, 0.0f, 1.5f, // Position 0
0.0f, 0.0f, // TexCoord 0
-0.5f, -0.5f, 0.0f, 0.75f, // Position 1
0.0f, 1.0f, // TexCoord 1
0.5f, -0.5f, 0.0f, 0.75f, // Position 2
1.0f, 1.0f, // TexCoord 2
0.5f, 0.5f, 0.0f, 1.5f, // Position 3
1.0f, 0.0f // TexCoord 3
};
GLushort indices[] = { 0, 1, 2, 0, 2, 3 };
// Set the viewport
glViewport ( 0, 0, esContext->width, esContext->height );
// Clear the color buffer
glClear ( GL_COLOR_BUFFER_BIT );
// Use the program object
glUseProgram ( userData->programObject );
// Load the vertex position
glVertexAttribPointer ( userData->positionLoc, 4, GL_FLOAT,
GL_FALSE, 6 * sizeof(GLfloat), vVertices ); // 偏移量 4+2
// Load the texture coordinate
glVertexAttribPointer ( userData->texCoordLoc, 2, GL_FLOAT,
GL_FALSE, 6 * sizeof(GLfloat), &vVertices[4] );
glEnableVertexAttribArray ( userData->positionLoc );
glEnableVertexAttribArray ( userData->texCoordLoc );
// Bind the texture
glActiveTexture ( GL_TEXTURE0 ); // 激活 texture unit 0
glBindTexture ( GL_TEXTURE_2D, userData->textureId );
// Set the sampler texture unit to 0
glUniform1i ( userData->samplerLoc, 0 );
// Draw quad with nearest sampling (LEFT)
glTexParameteri ( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST );
glUniform1f ( userData->offsetLoc, -0.6f );
glDrawElements ( GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, indices );
// Draw quad with trilinear filtering (RIGHT)
glTexParameteri ( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR );
glUniform1f ( userData->offsetLoc, 0.6f );
glDrawElements ( GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, indices );
eglSwapBuffers ( esContext->eglDisplay, esContext->eglSurface );
}
///
// Cleanup
//
void ShutDown ( ESContext *esContext )
{
UserData *userData = esContext->userData;
// Delete texture object
glDeleteTextures ( 1, &userData->textureId );
// Delete program object
glDeleteProgram ( userData->programObject );
}
int main ( int argc, char *argv[] )
{
ESContext esContext;
UserData userData;
esInitContext ( &esContext );
esContext.userData = &userData;
esCreateWindow ( &esContext, "MipMap 2D", 640, 480, ES_WINDOW_RGB );
if ( !Init ( &esContext ) )
return 0;
esRegisterDrawFunc ( &esContext, Draw );
esMainLoop ( &esContext );
ShutDown ( &esContext );
}
效果如图：
