Loading a PNG as texture in OpenGL using libPNG

When learning texture mapping OpenGL most example uses targa (.tga) or PPM (.ppm). Both of these formats are easy to parse, but they are not well supported in modern image editors and they do not compress the data well. An attractive alternative is PNG images. PNG provides lossless image compression and is well supported in all image editors. In this post I’ll show how to load PNG images as OpenGL textures using libPNG.

libPNG is the official PNG reference library you can find the sourcecode here: (http://www.libpng.org/pub/png/libpng.html). For my example I use version 1.2.44. libPNG depends on zlib verion 1.0.4 or later (http://www.zlib.net/).

The PNG file format supports of different modes; roughly speaking the models are: palette modes and RGB modes. In palette mode each pixel contains a pointer (a byte) to a palette table with the RGB color. In RGB mode each pixel contains a RGB (A) value. When loading the texture we are only interested in the RGB mode, and PNG_TRANSFORM_EXPAND flag convert the image to RGB(A).

PNG also supports different bit depths. We are usually only interested in 8 bit and we can use the flags PNG_TRANSFORM_STRIP_16 and PNG_TRANSFORM_PACKING to convert into 8 bit per channel when loading.

Png texture loaded

A last but important thing. The PNG format stores the pixels left-to-right-top-to-bottom (first pixel is in upper left corner), but the OpenGL expects it left-to-right-bottom-to-top (first pixel is in lower left corner). So we must reorder rows.

The program simply loads a texture and render it as a quad. This quad can be rotated using mouse drag. The texture width and height must be a power of 2.

Note: most of the comments in the loadPngImage function is copied from the example.c file from the libPNG source code.

#ifdef _WIN32
#include <GL/glut.h>
#else
#include <GLUT/glut.h>
#endif
#include <stdlib.h>
#include <stdio.h>
#include <png.h>
#include <iostream>

GLubyte *textureImage;
float rotateX = 0;
float rotateY = 0;

int mouseX;
int mouseY;

bool loadPngImage(char *name, int &outWidth, int &outHeight, bool &outHasAlpha, GLubyte **outData) {
    png_structp png_ptr;
    png_infop info_ptr;
    unsigned int sig_read = 0;
    int color_type, interlace_type;
    FILE *fp;

    if ((fp = fopen(name, "rb")) == NULL)
        return false;

    /* Create and initialize the png_struct
     * with the desired error handler
     * functions.  If you want to use the
     * default stderr and longjump method,
     * you can supply NULL for the last
     * three parameters.  We also supply the
     * the compiler header file version, so
     * that we know if the application
     * was compiled with a compatible version
     * of the library.  REQUIRED
     */
    png_ptr = png_create_read_struct(PNG_LIBPNG_VER_STRING,
            NULL, NULL, NULL);

    if (png_ptr == NULL) {
        fclose(fp);
        return false;
    }

    /* Allocate/initialize the memory
     * for image information.  REQUIRED. */
    info_ptr = png_create_info_struct(png_ptr);
    if (info_ptr == NULL) {
        fclose(fp);
        png_destroy_read_struct(&png_ptr, png_infopp_NULL, png_infopp_NULL);
        return false;
    }

    /* Set error handling if you are
     * using the setjmp/longjmp method
     * (this is the normal method of
     * doing things with libpng).
     * REQUIRED unless you  set up
     * your own error handlers in
     * the png_create_read_struct()
     * earlier.
     */
    if (setjmp(png_jmpbuf(png_ptr))) {
        /* Free all of the memory associated
         * with the png_ptr and info_ptr */
        png_destroy_read_struct(&png_ptr, &info_ptr, png_infopp_NULL);
        fclose(fp);
        /* If we get here, we had a
         * problem reading the file */
        return false;
    }

    /* Set up the output control if
     * you are using standard C streams */
    png_init_io(png_ptr, fp);

    /* If we have already
     * read some of the signature */
    png_set_sig_bytes(png_ptr, sig_read);

    /*
     * If you have enough memory to read
     * in the entire image at once, and
     * you need to specify only
     * transforms that can be controlled
     * with one of the PNG_TRANSFORM_*
     * bits (this presently excludes
     * dithering, filling, setting
     * background, and doing gamma
     * adjustment), then you can read the
     * entire image (including pixels)
     * into the info structure with this
     * call
     *
     * PNG_TRANSFORM_STRIP_16 |
     * PNG_TRANSFORM_PACKING  forces 8 bit
     * PNG_TRANSFORM_EXPAND forces to
     *  expand a palette into RGB
     */
    png_read_png(png_ptr, info_ptr, PNG_TRANSFORM_STRIP_16 | PNG_TRANSFORM_PACKING | PNG_TRANSFORM_EXPAND, png_voidp_NULL);

    outWidth = info_ptr->width;
    outHeight = info_ptr->height;
    switch (info_ptr->color_type) {
        case PNG_COLOR_TYPE_RGBA:
            outHasAlpha = true;
            break;
        case PNG_COLOR_TYPE_RGB:
            outHasAlpha = false;
            break;
        default:
            std::cout << "Color type " << info_ptr->color_type << " not supported" << std::endl;
            png_destroy_read_struct(&png_ptr, &info_ptr, NULL);
            fclose(fp);
            return false;
    }
    unsigned int row_bytes = png_get_rowbytes(png_ptr, info_ptr);
    *outData = (unsigned char*) malloc(row_bytes * outHeight);

    png_bytepp row_pointers = png_get_rows(png_ptr, info_ptr);

    for (int i = 0; i < outHeight; i++) {
        // note that png is ordered top to
        // bottom, but OpenGL expect it bottom to top
        // so the order or swapped
        memcpy(*outData+(row_bytes * (outHeight-1-i)), row_pointers[i], row_bytes);
    }

    /* Clean up after the read,
     * and free any memory allocated */
    png_destroy_read_struct(&png_ptr, &info_ptr, png_infopp_NULL);

    /* Close the file */
    fclose(fp);

    /* That's it */
    return true;
}

void init(void) {
    glClearColor(0.0, 0.0, 0.0, 0.0);
    glEnable(GL_DEPTH_TEST);
    // The following two lines enable semi transparent
    glEnable(GL_BLEND);
    glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

    int width, height;
    bool hasAlpha;
    char filename[] = "logo.png";
    bool success = loadPngImage(filename, width, height, hasAlpha, &textureImage);
    if (!success) {
        std::cout << "Unable to load png file" << std::endl;
        return;
    }
    std::cout << "Image loaded " << width << " " << height << " alpha " << hasAlpha << std::endl;
    glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
    glTexImage2D(GL_TEXTURE_2D, 0, hasAlpha ? 4 : 3, width,
            height, 0, hasAlpha ? GL_RGBA : GL_RGB, GL_UNSIGNED_BYTE,
            textureImage);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_LINEAR);
    glTexParameterf(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    glEnable(GL_TEXTURE_2D);
    glShadeModel(GL_FLAT);
}

void display(void) {
    glLoadIdentity();
    glTranslatef(0.0, 0.0, -3.6);
    glRotatef(rotateX, 0,1,0);
    glRotatef(rotateY, 1,0,0);

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glBegin(GL_QUADS);
    glTexCoord2f(0.0, 0.0);
    glVertex3f(-2.0, -1.0, 0.0);
    glTexCoord2f(0.0, 1.0);
    glVertex3f(-2.0, 1.0, 0.0);
    glTexCoord2f(1.0, 1.0);
    glVertex3f(0.0, 1.0, 0.0);
    glTexCoord2f(1.0, 0.0);
    glVertex3f(0.0, -1.0, 0.0);

    glEnd();
    glutSwapBuffers();
}

void myReshape(int w, int h) {
    glViewport(0, 0, w, h);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluPerspective(60.0, 1.0 * (GLfloat) w / (GLfloat) h, 1.0, 30.0);
    glMatrixMode(GL_MODELVIEW);
}

void mousePassive(int x, int y){
    mouseX = x;
    mouseY = y;
}

void mouseMotion(int x, int y){
    const float SPEED = 2;

    rotateX += (mouseX-x)/SPEED;
    rotateY += (mouseY-y)/SPEED;
    mousePassive(x, y);
    glutPostRedisplay();
}

int
main(int argc, char** argv) {
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_RGB | GLUT_DEPTH);
    glutCreateWindow("PNG texture");
    glutMotionFunc(mouseMotion);
    glutPassiveMotionFunc(mousePassive);
    init();
    glutReshapeFunc(myReshape);
    glutDisplayFunc(display);
    std::cout << "Use mouse drag to rotate." << std::endl;
    glutMainLoop();
    return 0;
}

About these ads