Question

I want to render an NSString to a monochrome OpenGL texture. I found many examples on how to to that on iOS, but I’m struggling to find a code snippet that works on OSX.

I know how to do the last bit, uploading a texture to the GPU, but how do I get the raw (8 bit per pixel) data?

Answer 1

Take a look at GLString.m in Apple’s Cocoa OpenGL code sample. You can easily modify it to generate a monochrome texture. The relevant modifications result in code that looks like so:

NSBitmapImageRep *bitmap = [[NSBitmapImageRep alloc] initWithBitmapDataPlanes:NULL pixelsWide:frameSize.width pixelsHigh:frameSize.height
    bitsPerSample:8 samplesPerPixel:1 hasAlpha:NO isPlanar:NO colorSpaceName:NSDeviceWhiteColorSpace bitmapFormat:0 bytesPerRow:frameSize.width bitsPerPixel:8];

NSGraphicsContext *context = [NSGraphicsContext graphicsContextWithBitmapImageRep:bitmap];
[NSGraphicsContext saveGraphicsState];
[NSGraphicsContext setCurrentContext:context];

…    

[string drawAtPoint:NSMakePoint(marginSize.width, marginSize.height)]; // draw at offset position

…

[NSGraphicsContext restoreGraphicsState];

if (0 == texName)
    glGenTextures (1, &texName);

glBindTexture (GL_TEXTURE_RECTANGLE_EXT, texName);
glTexSubImage2D(GL_TEXTURE_RECTANGLE_EXT, 0, 0, 0, texSize.width, texSize.height, GL_LUMINANCE, GL_UNSIGNED_BYTE, [bitmap bitmapData]);

The key parts are:

1. Allocate your own NSBitmapImageRep with the desired format.
2. Allocate an NSGraphicsContext that writes in to your NSBitmapImageRep.
3. Draw your string.
4. Restore the graphics context to its original state.
5. Call glTexSubImage2D with a pixel format that is compatible with the format you initialized your NSBitmapImageRep to.