Un apunte telegráfico de parte de www.nicolasgenette.com
Generales de la ley.
HDRI images (High Dynamic Range Image) are in fact a combination of several shots with different exposures, authorizing levels for brightness and chrominance higher than 255. For example, where a white wall will have an intensity of 1.0 (255,255,255 in RGB space), the sun, also white in space RVB, will have an intensity of 4.5.
The raw, recorded in 12bits, (14bits on last Canon DSLRs and 16bits on the digital backs) makes it possible to get the complete dynamic range of the sensor, already higher than 8bits, but does not completely solve the problem. HDR Formats are indeed much wider, reaching usually 32bpc.
Ejemplos:
8bpc = 2^8 = 256 tones/channel (monitor o impresora)
12bpc = 2^12 = 4096 tones/channel (imagen en crudo, raw)
14bpc = 2^14 = 16384 tones/channel (raw de última Canon DSRL; 08/2007)
16bpc = 2^16 = 65536 tones/channel
32bpc = 2^32 = 4294 million tones/channel (lo que se consigue con HDRI)
A pixel of an image is composed of three layers of colors, Red Blue Green, each layer being able to contain a certain number of tones.
In 8bits, each layer is thus coded on 8bits (or 8 bpc, for Bits Per Channel), to say 256 tones of red, green, and blue.
Indeed, data processing being binary, one has 8 "0" or "1" to code information, so 2^8 = 256 possibilities. On three layers, one thus obtains 256^3 = 16 777 216 colors, in other words 24 bpp (Bit Per Pixel).
A 32bits file is in fact a 8bpc file (so 24bpp) but with a fourth layer, called alpha layer, containing information of transparency. For example, a pixel 100,100,100,128 will be a dark gray pixel, at 50% of transparency. This just in order to clarify things, as we won't be interested here in the alpha layer.
HDR en si
You can shoot in jpg or raw, but in raw, takes cares to develop them at neutral exposure (0 EV compensation during development). On the other hand, we see that such technique is doable only in certain conditions. That's why manufacturers works a lot to improve their sensors dynamic range. Indeed, requiring multiple exposures of the same scene, it can be considered only on tripod, with fixed subjects. Even for landscape, you will have to avoid any wind that would ruins sharpness of foliages since each exposure would be different not only in exposure...
Shooting
Ejemplo
se mide en Av (diafragma fijo) el punto mas claro y el mas obscuro de la escena. Supongamos que la velocidad da 1/1600 en las altas luces y 1/8 en la parte mas oscura. Ese rango tonal se cubre en 8 pasos de un punto, a saber:
1/1600 1/800 1/400 1/200 1/100 1/50 1/25 1/12 1/16.
Si saltamos de a 2 puntos tenemos que hacer 5 tomas para cubrirlo, a saber:
1/1600, 1/400, 1/100, 1/25, 1/6
El sensor de una camara digital tiene un rango dinamico de 6 pasos (1 paso = 1 cambio de diafragma o de velocidad, pasar de 1/400 a 1/800 es un paso) pero puede sacarse una escena de 8 pasos en modo crudo (raw) con algun sacrificio de detalles en las sombras.
Procesamiento
Tenemos informacion de toda la gama de luces en las cinco tomas que hicimos con la cámara, cada foto esta en 8 bpc (JPG) o 12 bpc (raw) o 14 bpc (raw con la mejor Canon de agosto/2007) El objetivo ahora es mezclar esas cinco imagenes de 8, 12 o 14bpc en una sola de 32bpc y luego reducir la de 32bpc a una de 8bpc.
Conseguir una de 32bpc para pasarla a una de 8pbc parece una boludez pero no lo es: la inmensa mayoria de impresoras y pantallas son incapaces de manejar 32bpc, es decir no es posible ver o imprimir una imagen de 32 bpp como realmente es.
El paso de 32bpc a 8bpc no se hace con una transferencia lineal. La tecnica de reducir los 32 bpp a 24 bpp mediante funciones no-lineales se conoce como "Tone-mapping".
Software tools for HDR.
Once realized your shots, you will have to merge them to form only one full image. This stage is done with some specific softwares able to handle such large dynamic range :
- Photomatix pro
- Photoshop which has this function since version CS2.
- easyHDR pro, which also has a free version.
- Photogenics HDR
Glosario:
RVB space es RGB (RED, GREEN, BLUE)
bpp: bit per pixel (normalmente 24 bit)
bpc: bit per canal (normalmente 8 bit)
canal: 3 canales, RGB (Red, Green Blue)
Generales de la ley.
HDRI images (High Dynamic Range Image) are in fact a combination of several shots with different exposures, authorizing levels for brightness and chrominance higher than 255. For example, where a white wall will have an intensity of 1.0 (255,255,255 in RGB space), the sun, also white in space RVB, will have an intensity of 4.5.
The raw, recorded in 12bits, (14bits on last Canon DSLRs and 16bits on the digital backs) makes it possible to get the complete dynamic range of the sensor, already higher than 8bits, but does not completely solve the problem. HDR Formats are indeed much wider, reaching usually 32bpc.
Ejemplos:
8bpc = 2^8 = 256 tones/channel (monitor o impresora)
12bpc = 2^12 = 4096 tones/channel (imagen en crudo, raw)
14bpc = 2^14 = 16384 tones/channel (raw de última Canon DSRL; 08/2007)
16bpc = 2^16 = 65536 tones/channel
32bpc = 2^32 = 4294 million tones/channel (lo que se consigue con HDRI)
A pixel of an image is composed of three layers of colors, Red Blue Green, each layer being able to contain a certain number of tones.
8 bpc x 3 canales (RGB) = 24 bpp
In 8bits, each layer is thus coded on 8bits (or 8 bpc, for Bits Per Channel), to say 256 tones of red, green, and blue.
Indeed, data processing being binary, one has 8 "0" or "1" to code information, so 2^8 = 256 possibilities. On three layers, one thus obtains 256^3 = 16 777 216 colors, in other words 24 bpp (Bit Per Pixel).
A 32bits file is in fact a 8bpc file (so 24bpp) but with a fourth layer, called alpha layer, containing information of transparency. For example, a pixel 100,100,100,128 will be a dark gray pixel, at 50% of transparency. This just in order to clarify things, as we won't be interested here in the alpha layer.
HDR en si
You can shoot in jpg or raw, but in raw, takes cares to develop them at neutral exposure (0 EV compensation during development). On the other hand, we see that such technique is doable only in certain conditions. That's why manufacturers works a lot to improve their sensors dynamic range. Indeed, requiring multiple exposures of the same scene, it can be considered only on tripod, with fixed subjects. Even for landscape, you will have to avoid any wind that would ruins sharpness of foliages since each exposure would be different not only in exposure...
Shooting
- Poner camara en manual Av y determinar el rango de luces de la escena: medir (puntualmente) lo mas luminoso y lo mas oscuro (se puede usar el zoom para medir la parte que interesa).
- Si se cubre el rango medido en 6 o menos pasos no se necesita HDR. Si lo cubre en 8 pasos el sacrificio (sacando en modo raw) es aceptable en general (pero se pierde un poco de detalle en sombras) salvo que uno quiera la foto perfecta.
- Sacar tantas fotos como sea necesario para cubrir el rango con un bracketing de 2.
Ejemplo
se mide en Av (diafragma fijo) el punto mas claro y el mas obscuro de la escena. Supongamos que la velocidad da 1/1600 en las altas luces y 1/8 en la parte mas oscura. Ese rango tonal se cubre en 8 pasos de un punto, a saber:
1/1600 1/800 1/400 1/200 1/100 1/50 1/25 1/12 1/16.
Si saltamos de a 2 puntos tenemos que hacer 5 tomas para cubrirlo, a saber:
1/1600, 1/400, 1/100, 1/25, 1/6
El sensor de una camara digital tiene un rango dinamico de 6 pasos (1 paso = 1 cambio de diafragma o de velocidad, pasar de 1/400 a 1/800 es un paso) pero puede sacarse una escena de 8 pasos en modo crudo (raw) con algun sacrificio de detalles en las sombras.
Procesamiento
Tenemos informacion de toda la gama de luces en las cinco tomas que hicimos con la cámara, cada foto esta en 8 bpc (JPG) o 12 bpc (raw) o 14 bpc (raw con la mejor Canon de agosto/2007) El objetivo ahora es mezclar esas cinco imagenes de 8, 12 o 14bpc en una sola de 32bpc y luego reducir la de 32bpc a una de 8bpc.
Conseguir una de 32bpc para pasarla a una de 8pbc parece una boludez pero no lo es: la inmensa mayoria de impresoras y pantallas son incapaces de manejar 32bpc, es decir no es posible ver o imprimir una imagen de 32 bpp como realmente es.
El paso de 32bpc a 8bpc no se hace con una transferencia lineal. La tecnica de reducir los 32 bpp a 24 bpp mediante funciones no-lineales se conoce como "Tone-mapping".
Software tools for HDR.
Once realized your shots, you will have to merge them to form only one full image. This stage is done with some specific softwares able to handle such large dynamic range :
- Photomatix pro
- Photoshop which has this function since version CS2.
- easyHDR pro, which also has a free version.
- Photogenics HDR
Glosario:
RVB space es RGB (RED, GREEN, BLUE)
bpp: bit per pixel (normalmente 24 bit)
bpc: bit per canal (normalmente 8 bit)
canal: 3 canales, RGB (Red, Green Blue)
--
LolaMento pero una vuelta por mi blog http://bari2k.blogspot.com
casi siempre sienta bien.
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