Convert an UIImage in a texture

Question

In my opengl project I need to convert an UIImage in texture; what's the way to do it? Can you help me?

Answer 1

I haven't test the following but i will decompose the conversion in 3 steps:

1. Extract info for your image:

   UIImage* image = [UIImage imageNamed:@"imageToApplyAsATexture.png"];
   CGImageRef imageRef = [image CGImage];
   int width = CGImageGetWidth(imageRef);
   int height = CGImageGetHeight(imageRef);
   

2. Allocate a textureData with the above properties:

   GLubyte* textureData = (GLubyte *)malloc(width * height * 4); // if 4 components per pixel (RGBA)
   
   CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
   NSUInteger bytesPerPixel = 4;
   NSUInteger bytesPerRow = bytesPerPixel * width;
   NSUInteger bitsPerComponent = 8;  
   CGContextRef context = CGBitmapContextCreate(textureData, width, height,
                                                bitsPerComponent, bytesPerRow, colorSpace,
                                                kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
   
   CGColorSpaceRelease(colorSpace);
   
   CGContextDrawImage(context, CGRectMake(0, 0, width, height), imageRef);
   CGContextRelease(context);
   

3. Set-up your texture:

   GLuint textureID;    
   glPixelStorei(GL_UNPACK_ALIGNMENT, 1);
   glGenTextures(1, &textureID);
   
   glBindTexture(GL_TEXTURE_2D, textureID);
   glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, textureData);
   

EDIT:

Read this tut; everything is explained from the conversion of one image to a texture and applying a texture in an iOS environment.

Answer 2

Here is swift version of getting texture out of an UIImage

func setupTexture(sourceImage: UIImage) -> GLuint {

guard let textureImage = sourceImage.cgImage else {
    print("Failed to load image")
    return 0
}

let width = textureImage.width
let height = textureImage.height

/*
 it will write one byte each for red, green, blue, and alpha – so 4 bytes in total.
 */

let textureData = calloc(width * height * 4, MemoryLayout<GLubyte>.size) //4 components per pixel (RGBA)
let spriteContext = CGContext(data: textureData,
                              width: width,
                              height: height,
                              bitsPerComponent: 8,
                              bytesPerRow: width * 4,
                              space: textureImage.colorSpace!,
                              bitmapInfo: CGImageAlphaInfo.premultipliedLast.rawValue)

spriteContext?.draw(textureImage, in: CGRect(x: 0, y: 0, width: width, height: height))

var textName = GLuint()
glGenTextures(1, &textName)
glBindTexture(GLenum(GL_TEXTURE_2D), textName)
glTexParameteri(GLenum(GL_TEXTURE_2D), GLenum(GL_TEXTURE_MIN_FILTER), GL_NEAREST)
glTexImage2D(GLenum(GL_TEXTURE_2D), 0, GL_RGBA, GLsizei(width),
             GLsizei(height), 0, GLenum(GL_RGBA), GLenum(GL_UNSIGNED_BYTE), textureData)

return textName

}

Note: Need to keep in mind that Core Graphics flips images when we load them in.