《OpenGL ES 2.0 Programming Guide》第9章“最简单的MipMap”示例代码【C语言版】
【摘要】
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...
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：
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// MipMap2D.c
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//
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// This is a simple example that demonstrates generating a mipmap chain
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// and rendering with it
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//
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#include <stdlib.h>
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#include "esUtil.h"
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typedef struct
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{
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// Handle to a program object
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GLuint programObject;
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// Attribute locations
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GLint positionLoc;
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GLint texCoordLoc;
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// Sampler location
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GLint samplerLoc;
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// Offset location
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GLint offsetLoc;
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// Texture handle
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GLuint textureId;
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} UserData;
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///
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// From an RGB8 source image, generate the next level mipmap
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//
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GLboolean GenMipMap2D( GLubyte *src, GLubyte **dst, int srcWidth, int srcHeight, int *dstWidth, int *dstHeight )
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{
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int x,
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y;
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int texelSize = 3;
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*dstWidth = srcWidth / 2;
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if ( *dstWidth <= 0 )
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*dstWidth = 1;
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*dstHeight = srcHeight / 2;
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if ( *dstHeight <= 0 )
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*dstHeight = 1;
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*dst = malloc ( sizeof(GLubyte) * texelSize * (*dstWidth) * (*dstHeight) );
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if ( *dst == NULL )
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return GL_FALSE;
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for ( y = 0; y < *dstHeight; y++ )
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{
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for( x = 0; x < *dstWidth; x++ )
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{
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int srcIndex[4];
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float r = 0.0f,
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g = 0.0f,
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b = 0.0f;
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int sample;
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// Compute the offsets for 2x2 grid of pixels in previous
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// image to perform box filter
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srcIndex[0] =
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(((y * 2) * srcWidth) + (x * 2)) * texelSize;
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srcIndex[1] =
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(((y * 2) * srcWidth) + (x * 2 + 1)) * texelSize;
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srcIndex[2] =
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((((y * 2) + 1) * srcWidth) + (x * 2)) * texelSize;
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srcIndex[3] =
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((((y * 2) + 1) * srcWidth) + (x * 2 + 1)) * texelSize;
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// Sum all pixels
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for ( sample = 0; sample < 4; sample++ )
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{
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r += src[srcIndex[sample]];
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g += src[srcIndex[sample] + 1];
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b += src[srcIndex[sample] + 2];
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}
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// Average results
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r /= 4.0;
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g /= 4.0;
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b /= 4.0;
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// Store resulting pixels
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(*dst)[ ( y * (*dstWidth) + x ) * texelSize ] = (GLubyte)( r );
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(*dst)[ ( y * (*dstWidth) + x ) * texelSize + 1] = (GLubyte)( g );
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(*dst)[ ( y * (*dstWidth) + x ) * texelSize + 2] = (GLubyte)( b );
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}
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}
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return GL_TRUE;
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}
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///
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// Generate an RGB8 checkerboard image
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//
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GLubyte* GenCheckImage( int width, int height, int checkSize )
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{
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int x,
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y;
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GLubyte *pixels = malloc( width * height * 3 );
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if ( pixels == NULL )
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return NULL;
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for ( y = 0; y < height; y++ )
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for ( x = 0; x < width; x++ )
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{
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GLubyte rColor = 0;
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GLubyte bColor = 0;
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if ( ( x / checkSize ) % 2 == 0 )
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{
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rColor = 255 * ( ( y / checkSize ) % 2 );
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bColor = 255 * ( 1 - ( ( y / checkSize ) % 2 ) );
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}
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else
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{
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bColor = 255 * ( ( y / checkSize ) % 2 );
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rColor = 255 * ( 1 - ( ( y / checkSize ) % 2 ) );
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}
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pixels[(y * height + x) * 3] = rColor;
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pixels[(y * height + x) * 3 + 1] = 0;
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pixels[(y * height + x) * 3 + 2] = bColor;
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}
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return pixels;
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}
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///
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// Load texture from disk
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//
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GLuint LoadTexture ( char *fileName )
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{
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int width, height;
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char *buffer = esLoadTGA ( fileName, &width, &height );
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GLuint texId;
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if ( buffer == NULL )
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{
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esLogMessage ( "Error loading (%s) image.\n", fileName );
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return 0;
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}
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glGenTextures ( 1, &texId );
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glBindTexture ( GL_TEXTURE_2D, texId );
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glTexImage2D ( GL_TEXTURE_2D, 0, GL_RGB, width, height, 0, GL_RGB, GL_UNSIGNED_BYTE, buffer );
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glTexParameteri ( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR );
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glTexParameteri ( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR );
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glTexParameteri ( GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE );
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glTexParameteri ( GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE );
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free ( buffer );
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return texId;
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}
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///
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// Create a mipmapped 2D texture image
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//
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GLuint CreateMipMappedTexture2D( )
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{
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// Texture object handle
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GLuint textureId;
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#if 0
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int width = 256,
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height = 256;
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int level;
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GLubyte *pixels;
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pixels = GenCheckImage( width, height, 8 );
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if ( pixels == NULL )
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return 0;
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// Generate a texture object
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glGenTextures ( 1, &textureId );
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// Bind the texture object
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glBindTexture ( GL_TEXTURE_2D, textureId );
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// Load mipmap level 0
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glTexImage2D ( GL_TEXTURE_2D, 0, GL_RGB, width, height,
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0, GL_RGB, GL_UNSIGNED_BYTE, pixels );
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GLubyte *prevImage;
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GLubyte *newImage;
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level = 1;
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prevImage = &pixels[0];
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while ( width > 1 && height > 1 )
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{
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int newWidth,
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newHeight;
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// Generate the next mipmap level
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GenMipMap2D( prevImage, &newImage, width, height,
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&newWidth, &newHeight );
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// Load the mipmap level
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glTexImage2D( GL_TEXTURE_2D, level, GL_RGB,
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newWidth, newHeight, 0, GL_RGB,
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GL_UNSIGNED_BYTE, newImage );
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// Free the previous image
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free ( prevImage );
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// Set the previous image for the next iteration
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prevImage = newImage;
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level++;
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// Half the width and height
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width = newWidth;
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height = newHeight;
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}
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free ( newImage );
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#else
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char *fileName = "D:/Projects/Visual Studio 2012/OpenGL_Demo/opengles-book-samples-master/Windows/Chapter_9/Simple_Texture2D/Fieldstone.tga";
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textureId = LoadTexture (fileName);
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glGenerateMipmap(GL_TEXTURE_2D);
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#endif
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// Set the filtering mode
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glTexParameteri ( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST_MIPMAP_NEAREST );
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glTexParameteri ( GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR );
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return textureId;
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}
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///
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// Initialize the shader and program object
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//
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int Init ( ESContext *esContext )
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{
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UserData *userData = esContext->userData;
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GLbyte vShaderStr[] =
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"uniform float u_offset; \n"
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"attribute vec4 a_position; \n"
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"attribute vec2 a_texCoord; \n"
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"varying vec2 v_texCoord; \n"
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"void main() \n"
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"{ \n"
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" gl_Position = a_position; \n"
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" gl_Position.x += u_offset;\n"
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" v_texCoord = a_texCoord; \n"
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"} \n";
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GLbyte fShaderStr[] =
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"precision mediump float; \n"
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"varying vec2 v_texCoord; \n"
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"uniform sampler2D s_texture; \n"
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"void main() \n"
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"{ \n"
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" gl_FragColor = texture2D( s_texture, v_texCoord );\n"
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"} \n";
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// Load the shaders and get a linked program object
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userData->programObject = esLoadProgram ( vShaderStr, fShaderStr );
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// Get the attribute locations
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userData->positionLoc = glGetAttribLocation ( userData->programObject, "a_position" );
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userData->texCoordLoc = glGetAttribLocation ( userData->programObject, "a_texCoord" );
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// Get the sampler location
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userData->samplerLoc = glGetUniformLocation ( userData->programObject, "s_texture" );
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// Get the offset location
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userData->offsetLoc = glGetUniformLocation( userData->programObject, "u_offset" );
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// Load the texture
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userData->textureId = CreateMipMappedTexture2D ();
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glClearColor ( 0.0f, 0.0f, 0.0f, 0.0f );
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return TRUE;
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}
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///
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// Draw a triangle using the shader pair created in Init()
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//
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void Draw ( ESContext *esContext )
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{
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UserData *userData = esContext->userData;
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GLfloat vVertices[] = { -0.5f, 0.5f, 0.0f, 1.5f, // Position 0
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0.0f, 0.0f, // TexCoord 0
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-0.5f, -0.5f, 0.0f, 0.75f, // Position 1
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0.0f, 1.0f, // TexCoord 1
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0.5f, -0.5f, 0.0f, 0.75f, // Position 2
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1.0f, 1.0f, // TexCoord 2
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0.5f, 0.5f, 0.0f, 1.5f, // Position 3
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1.0f, 0.0f // TexCoord 3
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};
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GLushort indices[] = { 0, 1, 2, 0, 2, 3 };
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// Set the viewport
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glViewport ( 0, 0, esContext->width, esContext->height );
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// Clear the color buffer
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glClear ( GL_COLOR_BUFFER_BIT );
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// Use the program object
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glUseProgram ( userData->programObject );
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// Load the vertex position
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glVertexAttribPointer ( userData->positionLoc, 4, GL_FLOAT,
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GL_FALSE, 6 * sizeof(GLfloat), vVertices ); // 偏移量 4+2
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// Load the texture coordinate
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glVertexAttribPointer ( userData->texCoordLoc, 2, GL_FLOAT,
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GL_FALSE, 6 * sizeof(GLfloat), &vVertices[4] );
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glEnableVertexAttribArray ( userData->positionLoc );
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glEnableVertexAttribArray ( userData->texCoordLoc );
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// Bind the texture
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glActiveTexture ( GL_TEXTURE0 ); // 激活 texture unit 0
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glBindTexture ( GL_TEXTURE_2D, userData->textureId );
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// Set the sampler texture unit to 0
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glUniform1i ( userData->samplerLoc, 0 );
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// Draw quad with nearest sampling (LEFT)
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glTexParameteri ( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST );
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glUniform1f ( userData->offsetLoc, -0.6f );
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glDrawElements ( GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, indices );
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// Draw quad with trilinear filtering (RIGHT)
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glTexParameteri ( GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR_MIPMAP_LINEAR );
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glUniform1f ( userData->offsetLoc, 0.6f );
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glDrawElements ( GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, indices );
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eglSwapBuffers ( esContext->eglDisplay, esContext->eglSurface );
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}
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///
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// Cleanup
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//
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void ShutDown ( ESContext *esContext )
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{
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UserData *userData = esContext->userData;
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// Delete texture object
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glDeleteTextures ( 1, &userData->textureId );
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// Delete program object
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glDeleteProgram ( userData->programObject );
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}
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int main ( int argc, char *argv[] )
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{
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ESContext esContext;
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UserData userData;
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esInitContext ( &esContext );
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esContext.userData = &userData;
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esCreateWindow ( &esContext, "MipMap 2D", 640, 480, ES_WINDOW_RGB );
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if ( !Init ( &esContext ) )
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return 0;
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esRegisterDrawFunc ( &esContext, Draw );
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esMainLoop ( &esContext );
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ShutDown ( &esContext );
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}
效果如图：
文章来源: panda1234lee.blog.csdn.net，作者：panda1234lee，版权归原作者所有，如需转载，请联系作者。
原文链接：panda1234lee.blog.csdn.net/article/details/52409090
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