//
// Simple passthrough vertex shader
//
attribute vec3 in_Position;                  // (x,y,z)
//attribute vec3 in_Normal;                  // (x,y,z)     unused in this shader.
attribute vec4 in_Colour;                    // (r,g,b,a)
attribute vec2 in_TextureCoord;              // (u,v)

varying vec2 v_vTexcoord;
varying vec4 v_vColour;

void main()
{
 vec4 object_space_pos = vec4( in_Position.x, in_Position.y, in_Position.z, 1.0);
 gl_Position = gm_Matrices[MATRIX_WORLD_VIEW_PROJECTION] * object_space_pos;
 
 v_vColour = in_Colour;
 v_vTexcoord = in_TextureCoord;
}

// Simple passthrough fragment shader
//
varying vec2 v_vTexcoord;
varying vec4 v_vColour;

uniform sampler2D Palette;

//size of palette image, in pixels.
uniform vec2 PaletteImageSize;

void main()
{

//Use 255 if not intel.
//texture2D() returns colors in range [0,1].  Put them back into the range[0,255],
//where the value refers to the exact pixel X,Y.
vec4 indexPixel = texture2D( gm_BaseTexture, v_vTexcoord) * vec4(256);

//Comment out these conditionals if not intel.
if(indexPixel.r >= 128.0)indexPixel.r--;
if(indexPixel.g >= 128.0)indexPixel.g--;
if(indexPixel.b >= 128.0)indexPixel.b--;
if(indexPixel.a >= 128.0)indexPixel.a--;

 //convert the pixel coords into UV coords in the range [0,1],
 // as expected by texture2D()
 vec2 paletteCoords = vec2(indexPixel.x,indexPixel.y) / PaletteImageSize;
 
 //issue: gamemaker stores all images into a "Texture page",
 //thus, the above coords are incorrect.
 //fix: externally loaded palette images
 gl_FragColor =  texture2D( Palette, paletteCoords);
}