I've tried to load the same texture (which represents a gear) in two different squares, but the alpha channel of the PNG image used as texture works only for the background color meanwhile the first gear alpha part (colored like the background) cover the second one:
here is my display func, where I draw the squares and place the texture:
void display(void) {
glLoadIdentity();
glTranslatef(0.0, 0.0, -1.6);
glRotatef(rotateX, 0,0,1);
glRotatef(rotateY, 0,0,1);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(60.0, 1.0 * (GLfloat) 250 / (GLfloat) 250, 1.0, 10.0);
glMatrixMode(GL_MODELVIEW);
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
glEnable(GL_BLEND);
glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glColor4f(0.0f, 0.0f, 0.0f, 0.5);
//glDisable(GL_BLEND);
//create first square
glBegin(GL_QUADS);
glTexCoord2f(0.0, 0.0);
glVertex3f(-0.5, -0.5, 0.0);
glTexCoord2f(0.0, 1.0);
glVertex3f(-0.5, 0.5, 0.0);
glTexCoord2f(1.0, 1.0);
glVertex3f(0.5, 0.5, 0.0);
glTexCoord2f(1.0, 0.0);
glVertex3f(0.5, -0.5, 0.0);
glEnd();
//create second square
glBegin(GL_QUADS);
glTexCoord2f(0.0, 0.0);
glVertex3f(-1.0, 0.0, 0.0);
glTexCoord2f(0.0, 1.0);
glVertex3f(-1.0, 1.0, 0.0);
glTexCoord2f(1.0, 1.0);
glVertex3f(0.0, 1.0, 0.0);
glTexCoord2f(1.0, 0.0);
glVertex3f(0.0, 0.0, 0.0);
glEnd();
glutSwapBuffers();
}
EDIT: Here is my init func:
void init(void) {
glClearColor(1.0, 0.0, 0.0, 0.0);
glEnable(GL_DEPTH_TEST);
glEnable(GL_BLEND);
glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
int width, height;
bool hasAlpha;
char filename[] = "/home/alberto/Scaricati/Ruota_1-ext-int-small-32.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);
}
And this is the func that I use to load the png (I found this one on the net) it uses libpng:
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, NULL, 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, 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, NULL);
png_uint_32 width, height;
int bit_depth;
png_get_IHDR(png_ptr, info_ptr, &width, &height, &bit_depth, &color_type,
&interlace_type, NULL, NULL);
outWidth = width;
outHeight = height;
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, NULL);
/* Close the file */
fclose(fp);
/* That's it */
return true;
}
