wavefront OBJ file format parsing with bash

Recently, I needed to extract some vertices from an OBJ file and drop them into my code. Rather than writing a OBJ file parser, I used bash to process the text. Here’s the OBJ file for a simple cube exported from Blender:

# Blender v2.71 (sub 0) OBJ File: ''
# www.blender.org
v -1.000000 -1.000000 1.000000
v -1.000000 -1.000000 -1.000000
v 1.000000 -1.000000 -1.000000
v 1.000000 -1.000000 1.000000
v -1.000000 1.000000 1.000000
v -1.000000 1.000000 -1.000000
v 1.000000 1.000000 -1.000000
v 1.000000 1.000000 1.000000
s off
f 6 2 1
f 7 3 2
f 8 4 3
f 5 1 4
f 2 3 4
f 7 6 5
f 5 6 1
f 6 7 2
f 7 8 3
f 8 5 4
f 1 2 4
f 8 7 5

In bash, I cd to the relevant file and run:

$ cat cube.obj | grep "^v " | cut -c 3- | xargs printf '%.1f,' | xargs printf 'static const float positions[] = { %s };' | pbcopy

Then I go to my source code and Cmd+V to paste:

static const float positions[] = { -1.0,-1.0,1.0,-1.0,-1.0,-1.0,1.0,-1.0,-1.0,1.0,-1.0,1.0,-1.0,1.0,1.0,-1.0,1.0,-1.0,1.0,1.0,-1.0,1.0,1.0,1.0, };

Nifty. I’ll likely extend it to extract additional data, compile to a custom binary format, and save it out to a shell script. After that I can call my make binary obj from either the command line or Xcode:

$ mbo cube.obj

Update:

Full script to create a binary OBJ file with an interleaved vertex buffer (v/n/uv).

#!/bin/bash
obj=$(<$1)
vertices=($(echo "$obj" | grep "^v " | cut -c 3- | xargs printf '%f '))
normals=($(echo "$obj" | grep "^vn " | cut -c 4- | xargs printf '%f '))
uvs=($(echo "$obj" | grep "^vt " | cut -c 3- | xargs printf '%f '))
faces=($(echo "$obj" | grep "^f" | cut -c 3- | tr '/' ' ' | xargs -n 1 expr -1 +))
buffer=()
for (( i = 0 ; i < ${#faces[@]}; i+=3 )) do
vi=$((${faces[$(($i+0))]}*3))
ui=$((${faces[$(($i+1))]}*2))
ni=$((${faces[$(($i+2))]}*3))
buffer+=("${vertices[$(($vi+0))]}")
buffer+=("${vertices[$(($vi+1))]}")
buffer+=("${vertices[$(($vi+2))]}")
buffer+=("${normals[$(($ni+0))]}")
buffer+=("${normals[$(($ni+1))]}")
buffer+=("${normals[$(($ni+2))]}")
buffer+=("${uvs[$(($ui+0))]}")
buffer+=("${uvs[$(($ui+1))]}")
done
vertexFormat=$(printf "%s," "${buffer[@]}")
vertexFormat=${vertexFormat%?}
main="#import <Foundation/Foundation.h>
static float buffer[] = { $vertexFormat };
int main(int argc, const char* argv[]) {
NSData* data = [NSData dataWithBytes:&buffer length:sizeof(buffer)];
[data writeToFile:@\"$2.mbo\" atomically:YES];
return 0;
}"
echo "${main}" > main.m
clang -fobjc-arc main.m -o main_app
./main_app
rm main.m main_app
echo now add the file to xcode

To use the script:

  1. Copy the text into a file called mbo.sh
  2. chmod +x  mbo.sh
  3. ./mbo.sh cube.obj cube

Then load it into a vertex buffer. Using Metal in this example:

NSURL* modelUrl = [[NSBundle mainBundle] URLForResource:@”cube” withExtension:@”mbo”];

        NSData* modelBinData = [NSData dataWithContentsOfURL:modelUrl];

        _vertexBuffer = [_device newBufferWithBytes:modelBinData.bytes length:modelBinData.length options:MTLResourceOptionCPUCacheModeDefault];