If you're in my situation... with a overall good graphics card but just lacking floating-point target support (what makes HDR), and you want to both achieve and be able to see HDR-ish effects, then you must get creative.

First step into faking HDR, is understanding how HDR works.

True HDR

Quickly, HDR works by rendering to an offscreen buffer in higher precision, higher range pixel formats - like floating point formats. The objective is to not require saturation of supra-white pixels. Rather, precise lighting information is retained for post-processing.

Thus, objects can be as dark or as bright as they need to, without having to "look all black" or "look all white". Why don't they? Because after the whole frame is rendered, a post-processing phase will compute an appropriate exposure level, and adjust lighting that way.

After exposure control, though, things may look all black, or all white. With high exposure levels, this excessive contrast may look unnatural. How does one fix that problem? Well...

Emulation of lens aberrations

When taking a picture, lens aberrations produce a very subtle but present bloom effect on the pictures taken. If you apply such effect to a saturated picture, you get the classic oniric look - dreamish. Now... get inside things, and apply the bloom effect before saturating the image, when there are whiter than white pixels. What do you get? A realistic "glow". This, is HDR, when used with taste.

So... the process

Simple.

  • Render to a floating point framebuffer, with a mixture of ultra-bright and dark colors, as realistic as possible.
  • Select an appropriate exposure level, and adjust the framebuffer accordingly (multiply by an appropriate scalar).
  • Apply bloom.
  • Transfer the floating point framebuffer to the main framebuffer (the backbuffer), converting formats and saturating on the way (usually done automatically by the hardware).




You're done.

Faking it

Let's se...

  • Render to a floating point framebuffer, with a mixture of ultra-bright and dark colors, as realistic as possible.




Oops... I can't use floating point frambuffers.
So... No HDR? Wrong!

The previous line should be read, actually, as:

  • Render to a temporary framebuffer, with extended range, and with a mixture of ultra-bright and dark colors, as realistic as possible.




There are ways to accomplish that without floating point framebuffers. The way I decided to use is a mockup of the sRGB standard, tweaked for my own requirements.

Basically, instead of storing alpha in the alpha channel, alpha will now store a scaling factor. So, finally, the tuple r,g,b,s translates to r,g,b*(1+4*s^2) - this gives us the ability to represent color components in the range 0,5, with a maximum contrast ratio of 1280:1 - not that bad. Of course... lots of precision issues arise in reality, so those 1280:1 are of academic interest only. But still, in practice, the extra range does make a difference.

All shaders in materials used to render to such a buffer will have to be modified, so that:

  • They do not saturate the specular component - the one most likely to end up in the extended range
  • They perform a final translation pass to the output prior to any kind of saturation, that encodes the r,g,b,s tuple. Ideally, they would select the lowest s component that prevents saturation of the rgb part... but since that is difficult on pixel shaders (without table lookups - which are possible, have fun, this is left as an excercise to the reader), approximate solutions will have to suffice.
  • They take equally encoded textures and colors (both diffuse and specular) - this is crucial to be able to model any kind of visually appealing scene.




After this, the rest of the HDR process goes on as usual, with some interesting implementations though on the bloom section (which must process r,g,b,s-encoded data).

Avoiding excessive bloom

However, due to the reduced range of the data, it is very useful to do some preprocessing of the picture before applying the bloom. Thus, the bloom is performed in a strange way... in an additive way - instead of blooming the picture and using the bloomed picture as replacement, the picture gets its low-intensity data removed (to avoid excessive blooming), bloom-filtered, and then added to the original picture. This effectively avoids excessive blooming and produces utterly good-looking results ;-)
See the example implementation for more details on this part.

Back to floating-point?

Now... keep all things equal, and go back to using floating-point textures (if you can) - removing, of course, all things related to sRGB-ish conversion. You'll see remarkable improvement over standard HDR. Why? Basically, because you retained the measures taken to avoid excessive bloom - and are indeed, avoiding it. Although it is possible with floating point textures to use standard HDR techniques and achieve a similar, well-balanced effect, it is much harder. By applying the transfer procedure mentioned earlied that removes low-intensity content, any image content within a normal level of exposure will look sharp and clean.

Without floating-point, though, I'm having lots of precision issues

Inevitable... but you may want to try using fixed-point formats like A2R10G10B10 in temporary textures used for the bloom filter stages, though, to avoid those.

Example implementation

Here's a RenderMonkey project implementing a very dumbed-down version of the approach outlined above. Even so it is simple, it still achieves a good, realistic look. Simplifications include a very simple colorspace translation phase in the pixel shader of rendered objects (fixed s component that allows only a mild range extension, and only for the specular highlight), sRGB-ish decoding prior to bloom effects (so that the bloom filter can be applied as normal, instead of having to work natively with sRGB-ish data - this introduces some precision issues, but nothing too worrysome), and who knows what else (I sure don't ;-) ).

HDR.rfx

NOTE: It uses standard resources that come with RenderMonkey itself.

Shader code dump

Notes:

  • If you don't understand how those are used, see the RenderMonkey project for enlightment.
  • Don't forget to pay close attention to minification/magnification settings - they do matter a lot.
  • Also pay attention to intermediate texture resolution - you can play with it, but not much.
  • Only the first convolution stage will be fully dumped... the rest involve only replacing the convolution matrix, and updating the texture dimension constant.
  • Just in case you don't notice, they're all GLSL shaders.



Fragment program for pre-bloom processing

uniform float Exposure;
 uniform sampler2D SrcColor;
 uniform sampler2D SrcHDR;
 
 varying vec2 texCoord;
 
 const vec4 gloomStart = vec4(0.95,0.95,0.95,0.95);
 
 float sqr(float x) { return x*x; }
 vec4 sqr(vec4 x) { return x*x; }
 
 vec4 expand_Hdr(vec4 color)
 {
    return color*(sqr(color.a*2.0)+1.0);
 }
  
 void main(void)
 {
    vec4 color  = texture2D(SrcColor,texCoord);
    gl_FragColor = expand_Hdr(color*Exposure)-gloomStart;
 }

Fragment program for final composition stage


uniform float Exposure;
 uniform sampler2D SrcColor;
 uniform sampler2D SrcHDR1;
 uniform sampler2D SrcHDR2;
 uniform sampler2D SrcHDR3;
 uniform sampler2D SrcHDR4;
 uniform sampler2D Measure;
 
 uniform vec4 MipMix;
 
 float gloomIntensity=1.0;
 
 varying vec2 texCoord;
 
 float sqr(float x) { return x*x; }
 vec4 sqr(vec4 x) { return x*x; }
 
 vec4 expand_Hdr(vec4 color)
 {
    return color*(sqr(color.a*2.0)+1.0);
 }
 
 void main(void)
 {
    vec4 color  = texture2D(SrcColor,texCoord);
    vec4 gloom  = mat4(
       texture2D(SrcHDR1,texCoord),
       texture2D(SrcHDR2,texCoord),
       texture2D(SrcHDR3,texCoord),
       texture2D(SrcHDR4,texCoord) ) * MipMix;
    gl_FragColor = (expand_Hdr(color*Exposure)+gloom*16.0*gloomIntensity)*Exposure;
 }

Vertex/Fragment program for rendered objects



Vertex program 

uniform vec3 LightDir;
 uniform vec4 vViewPosition;
 uniform mat4 matViewProjection;
 
 varying vec2 texCoord;
 varying vec3 normal;
 varying vec3 lightDirInTangent;
 varying vec3 viewDirInTangent;
 
 attribute vec3 rm_Tangent;
 attribute vec3 rm_Binormal;
 
 void main(void)
 {
    texCoord = gl_MultiTexCoord0.xy;
    
    mat3 tangentMat = mat3(rm_Tangent,
                           rm_Binormal,
                           gl_Normal);
    lightDirInTangent = normalize(LightDir) * tangentMat;
    viewDirInTangent  = normalize(vViewPosition-gl_Position).xyz * tangentMat;
    
    gl_Position = ftransform();
 }

Fragment program 

uniform sampler2D BumpMap;
 uniform sampler2D ObjectMap;
 uniform sampler2D SpecMap;
 
 uniform float Shininess;
 uniform float SpecularIntensity;
 
 varying vec2 texCoord;
 varying vec3 lightDirInTangent;
 varying vec3 viewDirInTangent;
 
 void main(void)
 {
    vec3  n_lightDirInTangent = -normalize(lightDirInTangent);
    vec3  n_viewDirInTangent = normalize(viewDirInTangent);
    vec3  bump     = normalize(texture2D(BumpMap,texCoord).xyz * 2.0 - 1.0);
    float lighting = dot(bump,n_lightDirInTangent);
    float blighting= n_lightDirInTangent.z;
    float specular = dot(-reflect(n_lightDirInTangent,bump),n_viewDirInTangent);
    
    vec4 texColor = texture2D(ObjectMap,texCoord);
    vec4 specColor = texture2D(SpecMap, texCoord); 
    vec4 color = texColor * lighting 
               + float(lighting>0.0)*float(blighting>0.0) 
                 * SpecularIntensity*pow(specular,Shininess)*specColor;
    gl_FragColor = vec4(color.xyz*0.5,0.414);
 }

Fragment programs for first separable convolution stage



Horizontal 

uniform sampler2D Src;
 
 varying vec2 texCoord;
 
 const float texDimension = 512.0;
 const float texScaler =  1.0/texDimension;
 const float texOffset = -0.5/texDimension;
 
 void main(void)
 {
    vec4 color = vec4(0.0,0.0,0.0,0.0);
    
    const float gauss0 = 1.0/32.0;
    const float gauss1 = 5.0/32.0;
    const float gauss2 =15.0/32.0;
    const float gauss3 =22.0/32.0;
    const float gauss4 =15.0/32.0;
    const float gauss5 = 5.0/32.0;
    const float gauss6 = 1.0/32.0;   
    
    vec4 gaussFilter[7];
    gaussFilter[0]  = vec4( -3.0*texScaler , 0.0, 0.0, gauss0); 
    gaussFilter[1]  = vec4( -2.0*texScaler , 0.0, 0.0, gauss1); 
    gaussFilter[2]  = vec4( -1.0*texScaler , 0.0, 0.0, gauss2); 
    gaussFilter[3]  = vec4(  0.0*texScaler , 0.0, 0.0, gauss3);
    gaussFilter[4]  = vec4( +1.0*texScaler , 0.0, 0.0, gauss4);
    gaussFilter[5]  = vec4( +2.0*texScaler , 0.0, 0.0, gauss5);
    gaussFilter[6]  = vec4( +3.0*texScaler , 0.0, 0.0, gauss6);
   
    int i;
    for (i=0;i<7;i++)
       color += texture2D(Src, texCoord + gaussFilter[i].xy) * gaussFilter[i].w;
    
    gl_FragColor = color*0.5;
 }

Vertical 

uniform sampler2D Src; 
 
 varying vec2 texCoord;
 
 const float texDimension = 512.0;
 const float texScaler =  1.0/texDimension;
 const float texOffset = -0.5/texDimension;
 
 void main(void)
 {
    vec4 color = vec4(0.0,0.0,0.0,0.0);
    
    const float gauss0 = 1.0/32.0;
    const float gauss1 = 5.0/32.0;
    const float gauss2 =15.0/32.0;
    const float gauss3 =22.0/32.0;
    const float gauss4 =15.0/32.0;
    const float gauss5 = 5.0/32.0;
    const float gauss6 = 1.0/32.0;   
   
    vec4 gaussFilter[7];
    gaussFilter[0]  = vec4( -3.0*texScaler , 0.0, 0.0, gauss0).yxzw;
    gaussFilter[1]  = vec4( -2.0*texScaler , 0.0, 0.0, gauss1).yxzw;
    gaussFilter[2]  = vec4( -1.0*texScaler , 0.0, 0.0, gauss2).yxzw;
    gaussFilter[3]  = vec4(  0.0*texScaler , 0.0, 0.0, gauss3).yxzw;
    gaussFilter[4]  = vec4( +1.0*texScaler , 0.0, 0.0, gauss4).yxzw;
    gaussFilter[5]  = vec4( +2.0*texScaler , 0.0, 0.0, gauss5).yxzw;
    gaussFilter[6]  = vec4( +3.0*texScaler , 0.0, 0.0, gauss6).yxzw;
   
    for (int i=0;i<7;i++)
       color += texture2D(Src, texCoord + gaussFilter[i].xy) * gaussFilter[i].w;
    
    gl_FragColor = color*0.5;
 }

Convolution matrix for second separable convolution stage


const float gauss0 = 1.0/32.0;
   const float gauss1 = 5.0/32.0;
   const float gauss2 =15.0/32.0;
   const float gauss3 =22.0/32.0;
   const float gauss4 =15.0/32.0;
   const float gauss5 = 5.0/32.0;
   const float gauss6 = 1.0/32.0;

Convolution matrix for third separable convolution stage


const float gauss0 = 1.0/32.0;
   const float gauss1 = 5.0/32.0;
   const float gauss2 =15.0/32.0;
   const float gauss3 =22.0/32.0;
   const float gauss4 =15.0/32.0;
   const float gauss5 = 5.0/32.0;
   const float gauss6 = 1.0/32.0;

Convolution matrix for fourth separable convolution stage


const float gauss0 = 1.0/18.0;
   const float gauss1 = 2.0/18.0;
   const float gauss2 = 4.0/18.0;
   const float gauss3 = 4.0/18.0;
   const float gauss4 = 4.0/18.0;
   const float gauss5 = 2.0/18.0;
   const float gauss6 = 1.0/18.0;

Screenshots



These were taken with an exposure level of 1. See how the sun, part of the cubic texture actually, creates an intense glow much like a lens flare. Also see how the specular highlights glow slightly, in a very pleasant way.

Hdr_ss_1.jpg

Hdr_ss_2.jpg

Hdr_ss_3.jpg

Hdr_ss_4.jpg

Hdr_ss_5.jpg

These were taken with an increased exposure. See how the specular highlights now glow in the final picture, due to being much more intense than what meets the eye.

Hdr_ss_6.jpg

Hdr_ss_7.jpg

Hdr_ss_8.jpg

Hdr_ss_9.jpg

Alias: Faking_HDR

<HR>
Creative Commons Copyright -- Some rights reserved.


THE WORK (AS DEFINED BELOW) IS PROVIDED UNDER THE TERMS OF THIS CREATIVE COMMONS PUBLIC LICENSE ("CCPL" OR "LICENSE"). THE WORK IS PROTECTED BY COPYRIGHT AND/OR OTHER APPLICABLE LAW. ANY USE OF THE WORK OTHER THAN AS AUTHORIZED UNDER THIS LICENSE OR COPYRIGHT LAW IS PROHIBITED.

BY EXERCISING ANY RIGHTS TO THE WORK PROVIDED HERE, YOU ACCEPT AND AGREE TO BE BOUND BY THE TERMS OF THIS LICENSE. THE LICENSOR GRANTS YOU THE RIGHTS CONTAINED HERE IN CONSIDERATION OF YOUR ACCEPTANCE OF SUCH TERMS AND CONDITIONS.

1. Definitions

  • "Collective Work" means a work, such as a periodical issue, anthology or encyclopedia, in which the Work in its entirety in unmodified form, along with a number of other contributions, constituting separate and independent works in themselves, are assembled into a collective whole. A work that constitutes a Collective Work will not be considered a Derivative Work (as defined below) for the purposes of this License.
  • "Derivative Work" means a work based upon the Work or upon the Work and other pre-existing works, such as a translation, musical arrangement, dramatization, fictionalization, motion picture version, sound recording, art reproduction, abridgment, condensation, or any other form in which the Work may be recast, transformed, or adapted, except that a work that constitutes a Collective Work will not be considered a Derivative Work for the purpose of this License. For the avoidance of doubt, where the Work is a musical composition or sound recording, the synchronization of the Work in timed-relation with a moving image ("synching") will be considered a Derivative Work for the purpose of this License.
  • "Licensor" means the individual or entity that offers the Work under the terms of this License.
  • "Original Author" means the individual or entity who created the Work.
  • "Work" means the copyrightable work of authorship offered under the terms of this License.
  • "You" means an individual or entity exercising rights under this License who has not previously violated the terms of this License with respect to the Work, or who has received express permission from the Licensor to exercise rights under this License despite a previous violation.
  • "License Elements" means the following high-level license attributes as selected by Licensor and indicated in the title of this License: Attribution, ShareAlike.

2. Fair Use Rights

Nothing in this license is intended to reduce, limit, or restrict any rights arising from fair use, first sale or other limitations on the exclusive rights of the copyright owner under copyright law or other applicable laws.

3. License Grant

Subject to the terms and conditions of this License, Licensor hereby grants You a worldwide, royalty-free, non-exclusive, perpetual (for the duration of the applicable copyright) license to exercise the rights in the Work as stated below:

  • to reproduce the Work, to incorporate the Work into one or more Collective Works, and to reproduce the Work as incorporated in the Collective Works;
  • to create and reproduce Derivative Works;
  • to distribute copies or phonorecords of, display publicly, perform publicly, and perform publicly by means of a digital audio transmission the Work including as incorporated in Collective Works;
  • to distribute copies or phonorecords of, display publicly, perform publicly, and perform publicly by means of a digital audio transmission Derivative Works.
  • For the avoidance of doubt, where the work is a musical composition:
    • Performance Royalties Under Blanket Licenses. Licensor waives the exclusive right to collect, whether individually or via a performance rights society (e.g. ASCAP, BMI, SESAC), royalties for the public performance or public digital performance (e.g. webcast) of the Work.
    • Mechanical Rights and Statutory Royalties. Licensor waives the exclusive right to collect, whether individually or via a music rights society or designated agent (e.g. Harry Fox Agency), royalties for any phonorecord You create from the Work ("cover version") and distribute, subject to the compulsory license created by 17 USC Section 115 of the US Copyright Act (or the equivalent in other jurisdictions).
    • Webcasting Rights and Statutory Royalties. For the avoidance of doubt, where the Work is a sound recording, Licensor waives the exclusive right to collect, whether individually or via a performance-rights society (e.g. SoundExchange), royalties for the public digital performance (e.g. webcast) of the Work, subject to the compulsory license created by 17 USC Section 114 of the US Copyright Act (or the equivalent in other jurisdictions).


The above rights may be exercised in all media and formats whether now known or hereafter devised. The above rights include the right to make such modifications as are technically necessary to exercise the rights in other media and formats. All rights not expressly granted by Licensor are hereby reserved.

4. Restrictions

The license granted in Section 3 above is expressly made subject to and limited by the following restrictions:

  • You may distribute, publicly display, publicly perform, or publicly digitally perform the Work only under the terms of this License, and You must include a copy of, or the Uniform Resource Identifier for, this License with every copy or phonorecord of the Work You distribute, publicly display, publicly perform, or publicly digitally perform. You may not offer or impose any terms on the Work that alter or restrict the terms of this License or the recipients' exercise of the rights granted hereunder. You may not sublicense the Work. You must keep intact all notices that refer to this License and to the disclaimer of warranties. You may not distribute, publicly display, publicly perform, or publicly digitally perform the Work with any technological measures that control access or use of the Work in a manner inconsistent with the terms of this License Agreement. The above applies to the Work as incorporated in a Collective Work, but this does not require the Collective Work apart from the Work itself to be made subject to the terms of this License. If You create a Collective Work, upon notice from any Licensor You must, to the extent practicable, remove from the Collective Work any credit as required by clause 4(c), as requested. If You create a Derivative Work, upon notice from any Licensor You must, to the extent practicable, remove from the Derivative Work any credit as required by clause 4(c), as requested.
  • You may distribute, publicly display, publicly perform, or publicly digitally perform a Derivative Work only under the terms of this License, a later version of this License with the same License Elements as this License, or a Creative Commons iCommons license that contains the same License Elements as this License (e.g. Attribution-ShareAlike 2.5 Japan). You must include a copy of, or the Uniform Resource Identifier for, this License or other license specified in the previous sentence with every copy or phonorecord of each Derivative Work You distribute, publicly display, publicly perform, or publicly digitally perform. You may not offer or impose any terms on the Derivative Works that alter or restrict the terms of this License or the recipients' exercise of the rights granted hereunder, and You must keep intact all notices that refer to this License and to the disclaimer of warranties. You may not distribute, publicly display, publicly perform, or publicly digitally perform the Derivative Work with any technological measures that control access or use of the Work in a manner inconsistent with the terms of this License Agreement. The above applies to the Derivative Work as incorporated in a Collective Work, but this does not require the Collective Work apart from the Derivative Work itself to be made subject to the terms of this License.
  • If you distribute, publicly display, publicly perform, or publicly digitally perform the Work or any Derivative Works or Collective Works, You must keep intact all copyright notices for the Work and provide, reasonable to the medium or means You are utilizing: (i) the name of the Original Author (or pseudonym, if applicable) if supplied, and/or (ii) if the Original Author and/or Licensor designate another party or parties (e.g. a sponsor institute, publishing entity, journal) for attribution in Licensor's copyright notice, terms of service or by other reasonable means, the name of such party or parties; the title of the Work if supplied; to the extent reasonably practicable, the Uniform Resource Identifier, if any, that Licensor specifies to be associated with the Work, unless such URI does not refer to the copyright notice or licensing information for the Work; and in the case of a Derivative Work, a credit identifying the use of the Work in the Derivative Work (e.g., "French translation of the Work by Original Author," or "Screenplay based on original Work by Original Author"). Such credit may be implemented in any reasonable manner; provided, however, that in the case of a Derivative Work or Collective Work, at a minimum such credit will appear where any other comparable authorship credit appears and in a manner at least as prominent as such other comparable authorship credit.

5. Representations, Warranties and Disclaimer

UNLESS OTHERWISE AGREED TO BY THE PARTIES IN WRITING, LICENSOR OFFERS THE WORK AS-IS AND MAKES NO REPRESENTATIONS OR WARRANTIES OF ANY KIND CONCERNING THE MATERIALS, EXPRESS, IMPLIED, STATUTORY OR OTHERWISE, INCLUDING, WITHOUT LIMITATION, WARRANTIES OF TITLE, MERCHANTIBILITY, FITNESS FOR A PARTICULAR PURPOSE, NONINFRINGEMENT, OR THE ABSENCE OF LATENT OR OTHER DEFECTS, ACCURACY, OR THE PRESENCE OF ABSENCE OF ERRORS, WHETHER OR NOT DISCOVERABLE. SOME JURISDICTIONS DO NOT ALLOW THE EXCLUSION OF IMPLIED WARRANTIES, SO SUCH EXCLUSION MAY NOT APPLY TO YOU.

6. Limitation on Liability.

EXCEPT TO THE EXTENT REQUIRED BY APPLICABLE LAW, IN NO EVENT WILL LICENSOR BE LIABLE TO YOU ON ANY LEGAL THEORY FOR ANY SPECIAL, INCIDENTAL, CONSEQUENTIAL, PUNITIVE OR EXEMPLARY DAMAGES ARISING OUT OF THIS LICENSE OR THE USE OF THE WORK, EVEN IF LICENSOR HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

7. Termination

  • This License and the rights granted hereunder will terminate automatically upon any breach by You of the terms of this License. Individuals or entities who have received Derivative Works or Collective Works from You under this License, however, will not have their licenses terminated provided such individuals or entities remain in full compliance with those licenses. Sections 1, 2, 5, 6, 7, and 8 will survive any termination of this License.
  • Subject to the above terms and conditions, the license granted here is perpetual (for the duration of the applicable copyright in the Work). Notwithstanding the above, Licensor reserves the right to release the Work under different license terms or to stop distributing the Work at any time; provided, however that any such election will not serve to withdraw this License (or any other license that has been, or is required to be, granted under the terms of this License), and this License will continue in full force and effect unless terminated as stated above.

8. Miscellaneous

  • Each time You distribute or publicly digitally perform the Work or a Collective Work, the Licensor offers to the recipient a license to the Work on the same terms and conditions as the license granted to You under this License.
  • Each time You distribute or publicly digitally perform a Derivative Work, Licensor offers to the recipient a license to the original Work on the same terms and conditions as the license granted to You under this License.
  • If any provision of this License is invalid or unenforceable under applicable law, it shall not affect the validity or enforceability of the remainder of the terms of this License, and without further action by the parties to this agreement, such provision shall be reformed to the minimum extent necessary to make such provision valid and enforceable.
  • No term or provision of this License shall be deemed waived and no breach consented to unless such waiver or consent shall be in writing and signed by the party to be charged with such waiver or consent.
  • This License constitutes the entire agreement between the parties with respect to the Work licensed here. There are no understandings, agreements or representations with respect to the Work not specified here. Licensor shall not be bound by any additional provisions that may appear in any communication from You. This License may not be modified without the mutual written agreement of the Licensor and You.