Jump to content

  • Log In with Google      Sign In   
  • Create Account

[DeferredLighting+iPad/iPhone tested]Compress specular power and specular scale value into 8-bit alpha channel


Old topic!
Guest, the last post of this topic is over 60 days old and at this point you may not reply in this topic. If you wish to continue this conversation start a new topic.

  • You cannot reply to this topic
1 reply to this topic

#1 liuzewei   Members   -  Reputation: 124

Like
1Likes
Like

Posted 02 January 2012 - 04:25 PM

Hi,
My english is bad, but I just wanna share something here, because I learned alot in this forum. Maybe these stuff will help someone.

Nowadays, I wanna to implement defered rendering on mobile device(ipad/iphone), because my project needs geometry-independent ligths, so many lights...

After some searching and hard trying, I figured out these devices are not support MRT, and even not support half or single presicion float render target/texture.

So, G-BUFFER is the first and biggest problem. It must use multi-pass technique to generate G-BUFFER.
Have no choice is not a problem, so keep going.
I use three RT and rendering pass to generate G-BUFFER.

[PASS ONE]
POSITION-RT: GL_RGBA GL_UNSIGNED_BYTE
Compressed the position's depth value(float32 in projection space) into this RGBA8 RT, then unpacked/trans back to view-space in lighting stage.
GOOGLE: pack 32-bit float into RGBA(8-bit per channel) render target

[PASS TWO]
NORMAL-RT: GL_RGBA GL_UNSIGNED_BYTE
RGB stored the normal, alpha channel store shadow mask(main directional light's shadow)

[PASS THREE]
COLOR-RT: GL_RGBA GL_UNSIGNED_BYTE
RGB stored the albedo. and alpha channel is free till now.

It's just one 8-bit alpha channel remain, but still have two elements to store.
These are specluar power and specular scale value.
So, Here have two choice:
1. add a new RT and rendering pass, means rendering the entire scene more one time.
2. compress these two value in the free 8-bit space.
Which is the better choice? problem come.

I just cant accept rendering the entire scene four times, so compress is my choice.

[MAIN CONCEPT]
give scale value a [0, 2] range.
give it 3-bit can represent 2^3=8 values in theory.
precision is 2/8=0.25(0, 0.25, 0.5, 0.75 ... 2.0), this is acceptable.

give power value a [0, 200] range.
give it 5-bit can represent 2^5=32 value.
precision is 200/32=6.25(0, 6, 12, 18...200), this "average-kind" of distribution is not acceptable.

[KEY CONCEPT]
I wanna more precision on small power values, and less precision on large ones, when the power value become larger and larger.
someting like this:
0, 0.4, 1.2, 2.4, 4, 6, 8.4, 11.2, 14.4, 18, 22, 26.4 ...... 200
yes, the key-idea for this compression is "arithmetic progression".


----------------------------------------------------------------------
-- below is implement code snippet
----------------------------------------------------------------------
[deferred_gbuffer_normal.fsh]

uniform highp float uSpcPower;
uniform highp float uSpcScale;

varying highp vec3 vNormal;

void main()
{
	// normal
	gl_FragColor.rgb = 0.5 * (normalize(vNormal) + 1.0);
  
	// compress specular power value and specular scale value into 8-bit alpha channel
	//
	// compress
	// d: 0.43 step length
	// s(n) = s_n = (n^2*d - n*d) * 0.5			   range: [0, 199]
	// n(s) = n_s = (d + sqrt(d^2 + 8*d*s_n)) / (2*d) range: [0, 31]
	//
	// power value range: [0, 199] aka s_n, precision: n*d(in other words, precision decreasing when power value become larger and larger)
	// scale value range: [0, 2)   precision: 2/8 = 0.25
	// final value range: [0, 1)
	//
	// output
	//	integer part: floor(n_s+0.5)			range: [0, 31]
	//	float   part: fract(uSpcScale*0.5)	  range: [0, 1)
	//	final  value: (integer_part + float_part) / 32  range: [0, 1)
	//
	// uncompress
	//	alpha channel -> val
	//	power value: (floor(val*32)^2*d - floor(val*32)*d) * 0.5
	//	scale value: fract(val*32) * 2.0
	//
	// for example
	//	 original power value: 0.5 -> integer part: 2
	//	 original scale value: 1.6 -> float   part: 0.8
	//	 final output   value: 0.0875
	//
	//	 uncompressed power value: 0.43
	//	 uncompressed scale value: 1.44
	//
	/* conceptual version
	const highp float d = 0.43;
  
	highp float s_n = uSpcPower;
	highp float n_s = (d + sqrt(d*d + 8.0*d*s_n)) / (2.0*d);
	highp float integer_part = floor(n_s + 0.5);
	highp float float_part = fract(uSpcScale * 0.5);
	highp float final_value = (integer_part + float_part) / 32.0;
  
	gl_FragColor.a = final_value;*/
  
	// optimized version
	highp float n_s = (0.43 + sqrt(0.1849 + 3.44*uSpcPower)) * 1.1627907;
	highp float integer_part = floor(n_s + 0.5);
	highp float float_part = fract(uSpcScale * 0.5);
	gl_FragColor.a = (integer_part + float_part) * 0.03125;
}


[deferred_lighting_point.fsh]

uniform highp mat4 uInvPMat;
uniform highp vec3 uLightPos;
uniform highp vec3 uLightClr;
uniform highp float uLightRadius;
uniform lowp sampler2D uSamplePosition;
uniform lowp sampler2D uSampleNormal;

varying highp vec4 vTexCoord;

const highp vec4 unpackFactors = vec4(1.0 / (256.0 * 256.0 * 256.0), 1.0 / (256.0 * 256.0), 1.0 / 256.0, 1.0);

void main()
{
	highp vec2 screenPos = vTexCoord.xy / vTexCoord.w;
	highp vec2 texCoord = (screenPos + 1.0) * 0.5;

	// unpack pos
	highp float depthVal = dot(texture2D(uSamplePosition, texCoord), unpackFactors);
	depthVal = 2.0 * depthVal - 1.0;
	highp vec4 texPos;
	texPos.xy = screenPos;
	texPos.z = depthVal;
	texPos.w = 1.0;
	texPos = uInvPMat * texPos;
	texPos /= texPos.w;

	mediump vec4 texNorm = texture2D(uSampleNormal, texCoord);

	// dif lgt
	highp vec3 lgtDir = uLightPos - texPos.xyz;
	highp float lenLgtDir = length(lgtDir);
	lgtDir /= lenLgtDir;
	highp float attLgt = max(0.0, 1.0 - lenLgtDir/uLightRadius);

	highp vec3 norm = 2.0 * texNorm.rgb - 1.0;
	highp float NdL = max(0.0, dot(norm, lgtDir));
	highp vec3 difLgt = attLgt * NdL * uLightClr;

	// spc lgt

	// uncompress specular power value and specular scale value from 8-bit alpha channel
	//	alpha channel -> val
	//	power value: (floor(val*32)^2*d - floor(val*32)*d) * 0.5
	//	scale value: fract(val*32) * 2.0
	highp float scaleBackValue = texNorm.a * 32.0 + 0.004;
	highp float integer_part = floor(scaleBackValue);
	highp float float_part = fract(scaleBackValue);
  
	highp float spcPower = (integer_part*integer_part*0.43 - integer_part*0.43) * 0.5;
	highp float spcScale = float_part * 2.0;

	highp vec3 rflDir = normalize(reflect(-lgtDir, norm));
	highp vec3 dirToCam = normalize(-texPos.rgb);
	highp float spcLgt = attLgt * spcScale * pow(max(0.0, dot(rflDir, dirToCam)), spcPower);
	gl_FragColor.rgb = difLgt;
	gl_FragColor.a = spcLgt;
}


----------------------------------------------------------------------
-- and a screen shot
-- this is captured on windows but I have tested on ipad, it's ok also.
----------------------------------------------------------------------
shot.jpg

zewei, liu

Sponsor:

#2 bzroom   Members   -  Reputation: 646

Like
0Likes
Like

Posted 02 January 2012 - 08:10 PM

Thank you!




Old topic!
Guest, the last post of this topic is over 60 days old and at this point you may not reply in this topic. If you wish to continue this conversation start a new topic.



PARTNERS