OpenGL paint program based on Apple's "glPaint" on white background - how to combine?

I had the same problem. The edges of the brush were darker than they should be. It turns out that apple api pre multiplies alpha into rgb channels.

So, I objected by doing a grayscale in Photoshop with only rgb and alpha values. This should look like you want your brush to be white, which is full color pigment and black, which is not color pigmentation. I load this brush as in the apple glPaint example code. Then I copy the R channel (or G or B channels as they are all equal) to the alpha channel of the texture. After that I set the RGB values ​​to the maximum for all pixels of the brush texture.

So now your brush has an alpha channel with information about how your brush looks. and RGB are all 1.

Finally, I used the mixing function:

glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);

      

        
        
        
      

    

And don't forget to set the color before painting.

glColor4f(1.0f,0.0f,0.0f,1.0f); //red color

      

        
        
        
      

    

Check the code below, see if it works for you:

-(GLuint) createBrushWithImage: (NSString*)brushName
{

    GLuint            brushTexture;
    CGImageRef      brushImage;
CGContextRef    brushContext;
GLubyte         *brushData,*brushData1;
size_t          width, height;

//initialize brush image
brushImage = [UIImage imageNamed:brushName].CGImage;

// Get the width and height of the image
width = CGImageGetWidth(brushImage);
height = CGImageGetHeight(brushImage);

//make the brush texture and context
if(brushImage) {
    // Allocate  memory needed for the bitmap context
    brushData = (GLubyte *) calloc(width * height *4, sizeof(GLubyte));
    // We are going to use brushData1 to make the final texture
    brushData1 = (GLubyte *) calloc(width * height *4, sizeof(GLubyte));
    // Use  the bitmatp creation function provided by the Core Graphics framework. 

    brushContext = CGBitmapContextCreate(brushData, width, height, 8, width *4 , CGImageGetColorSpace(brushImage), kCGImageAlphaPremultipliedLast);
    // After you create the context, you can draw the  image to the context.
    CGContextDrawImage(brushContext, CGRectMake(0.0f, 0.0f, (CGFloat)width, (CGFloat)height), brushImage);
    // You don't need the context at this point, so you need to release it to avoid memory leaks.
    CGContextRelease(brushContext);

    for(int i=0; i< width*height;i++){
                    //set the R-G-B channel to maximum
        brushData1[i*4] = brushData1[i*4+1] =brushData1[i*4+2] =0xff;
                    //store originally loaded brush image in alpha channel
        brushData1[i*4+3] = brushData[i*4];
    }

    // Use OpenGL ES to generate a name for the texture.
    glGenTextures(1, &brushTexture);
    // Bind the texture name. 
    glBindTexture(GL_TEXTURE_2D, brushTexture);
    // Set the texture parameters to use a minifying filter and a linear filer (weighted average)
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
    // Specify a 2D texture image, providing the a pointer to the image data in memory
    glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, brushData1);
    // Release  the image data; it no longer needed
    free(brushData1);
    free(brushData);
}

      

        
        
        
      

    

return brushTexture; }